Fundamentals of Power Generation PG V1.2.pdf

Last updated: Dec-2011

Fundamentals of Power Generation

Classroom Training

Participants Guide Version 1.2

CMT 0177

Fundamentals of Power Generation (CMT 0177) 12/1/2011

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This product is for TRAINING PURPOSES only. Do not use this material in place of the current revision of controlled documents such as technical manuals, operator’s manuals or other work instructions.


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Contents

Course Introduction and Objectives ............................................................................................................. 4

Course Lessons .............................................................................................................................................. 5

Lesson 1 - Brief Overview of Gensets ........................................................................................................... 7

Lesson 2: Prime Mover ................................................................................................................................. 9

Lesson 3: Principles of Electricity ................................................................................................................ 17

Lesson 4: Principles of Magnetism .............................................................................................................. 34

Lesson 5: Electrical Components ................................................................................................................ 42

Lesson 6: Alternating Current ..................................................................................................................... 53

Lesson 7 : Alternators ................................................................................................................................. 65

Lesson 8: Transfer Switches ........................................................................................................................ 85

Lesson 9: Genset Controls ........................................................................................................................... 93

Lesson 10: Paralleling of Gensets................................................................................................................ 96

Lesson 11: Transmission and Distribution ................................................................................................ 102

Conclusion ................................................................................................................................................. 103

Knowledge Check ...................................................................................................................................... 104


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Course Introduction and Objectives

Main Goal

The primary purpose of this course is to teach you the basic principles of electricity and magnetism, and

then to apply those principles to the types of power-generating equipment that Cummins manufactures.

Along the way we will also discuss some of the main components that are used in power-generation

equipment.

Taking Notes

It really is a good idea to take notes and make your own little sketches during the class. For most

people, the act of writing and drawing helps you remember what you’re hearing better – even if you

never look at the notes again after class is over.

Making this an Active and Interactive Course

This should be an active + interactive course: This course is not about memorizing facts, formulas or

laws. This course is about absorbing information and getting a general feel and appreciation for the

technology and science behind Cummins products. Even if you don’t learn anything that you’re actually

going to apply in your job, we really hope this course gives you reason to think in broad terms about

what it is Cummins actually does. This is probably a Humanities course as much as it is a Technical

course.

Note that there will be frequent stopping points for discussion, that questions are encouraged and

expected throughout the course, and that participant interaction and participation is key to the success

of the course.

Asking Questions

One of the most important parts of this course is for you to ask any question you want and for the

instructor to give you a good answer. Feel free to ask a question any time you like.

Different Participants Have Different Objectives + Goals

We recognize that this Fundamentals course draws people from all areas of Cummins. We are going to

cover a very wide range of topics here. It might be inevitable that some of the material will be “old hat”

to some of you, and that some material will seem over some of your heads.

Our goal is to make the entire course fun, interesting and informative for everyone, regardless of your

reasons for being here. Even if some of the things we talk about seem like more than you want to know,

I hope the entire course seems accessible and non-intimidating to all of you.


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Course Lessons

1. Brief overview

a. Types of Power Generation

b. Key Components of a Genset

2. Prime mover

a. Understand what a prime mover is.

b. Have a very basic understanding of how gasoline and diesel engines work.

c. Understand the components of an engine

d. Understand the major differences between a gasoline engine and a diesel engine.

e. Understand the role of gasoline and diesel engines as the prime mover in power

generation systems.

3. Principles of Electricity

a. Basic atomic structure and its application to electricity

b. Understand Current flow and how it occurs.

c. Understand the basic units of electrical measurement: volts, amperes, ohms, and watts.

d. Understand series, parallel, and series/parallel circuits

4. Principles of Magnetism

a. Types of magnets

b. Understand the relationship between electricity and magnetism.

c. Understand what properties make a substance more or less magnetic.

d. Understand Magnetic and Mutual Induction

e. How an electromagnet operates

f. How to induce a current in a conductor using a magnet

5. Electrical Components

a. Be familiar with practical circuit components namely: resistors, diodes, inductors,

capacitors, switches, fuses, circuit breakers, relays, solenoids, transformers.

b. Have a general knowledge of how relays and solenoids operate.

6. Alternating Current

a. Understanding the definition of Alternating Current(AC)

b. Learn how to generate AC

c. Properties of an AC Sine wave

d. How a transformer works

e. Rectification

f. Filtering

7. Alternator

a. Understand the construction and function of an Alternator

b. Understand the difference between a rotating field vs. rotating armature design

c. Understand Brushed vs. Brushless Alternators


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d. Understand Self-excited vs. Separately-excited Alternators

8. Transfer Switches

a. Understand the operation of a typical Automatic Transfer Switch (ATS)

b. Know the different types of transfer switches

c. Familiarization with typical TS components

d. Understand transfer switch operation

e. Understand the importance of time delays

9. Genset Controls

a. Introduction to types of Genset controls

b. Get a basic understanding of control functionalities

10. Paralleling of Gensets

a. Know the conditions required for parallel operation

b. Understand synchronisation

c. Know the types of paralleling

d. Understand Peak shaving vs. Base loading

11. Transmission and Distribution

a. Understand how utility power gets from the power plant to a typical residential

application

b. Understand the importance of transformers in this process.


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Lesson 1 - Brief Overview of Gensets

Objectives Types of Power Generation Key Components of a Genset

Lesson Length : 20 mins

Types of Power Generation systems

Standby power systems

– Provides power if the utility power

fails

Prime power systems

– The primary source of power, the

utility is not available

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Standby systems A standby system provides electricity when main electrical power is partially or completely interrupted.

Generator sets can also be used to reduce the cost of electricity at locations with high electricity usage.

Generator sets can also reduce the burden on the grid in areas where the grid is highly burdened.

Prime Power systems Sometimes, a generator acts as the main source of electrical power in areas where utility power is unreliable or unstable.

Key components of a Genset

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Turbo ChargerAir Handling

Filtration

Control System

Engine

Transfer Switch

Alternator

Cooling system

Key Components of a Genset

Prime Mover

Alternator

Transfer Switch

Control

Supporting equipment o Fuel delivery o Exhaust o Air Handling/Filtration

Notes :


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Most power systems include the hardware listed above. A few points to clarify are:

1. Generator and Genset are common terms for the source of electrical power. To the purist, any machine that produces Alternating Current (AC) is an Alternator. In this course, we will mostly use the term Alternator.

2. Transfer switches may be either manual or automatic. We will focus on Automatic Transfer Switches (ATS). They perform the task of transferring loads to emergency power and re-transferring loads back to commercial (utility / mains) power.

3. Not all generating systems are used for standby power. There are many instances where the genset is the only source of power. This type of application is known as a Prime Power installation and probably wouldn’t include a transfer switch (because there is no other source of power to transfer to).

4. Any conventional generator needs mechanical effort to spin the alternator. The mechanical effort is supplied by a Prime Mover. The prime mover may be in the form of a windmill (or wind turbine), hydro (in the form of a water turbine), steam (via a steam turbine) or via an internal combustion engine. We will focus on engine driven alternators in this class.

Notes :


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Lesson 2: Prime Mover

Objectives Understand what a prime mover does. Have a very basic understanding of how

gasoline and diesel engines work. Understand the components of an engine Understand the major differences between a

gasoline engine and a diesel engine. Understand the role of gasoline and diesel

engines as the prime mover in power generation systems.

Lesson Length : 1 hour

Prime Mover

The prime mover provides a means to rotate the alternator of a

genset

Wind turbine

Water turbine

Steam turbine

Internal combustion

engines

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Prime Mover The prime mover provides a means to rotate the alternator of a genset which in turn generates electricity. It should be noted that the prime mover may be in the form of a

Wind turbine (Wind energy)

Water turbine (Hydropower plants)

Steam turbine (Coal, Nuclear etc.)

Internal combustion engine (at Cummins we mainly use internal combustion engines as our prime mover)

Wind Power The rotor-blade assembly spins by harnessing the energy stored in naturally occurring wind patterns. The rotor hub is connected to the gearbox. The gearbox gears up the rotational speed provided by the rotor-hub. The output shaft from the gearbox is connected to the generator which generates electricity.


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Steam power Water is heated and converted into highly pressurized steam by burning fuel (coal/oil) or via heat released in a nuclear fission reaction. The steam is supplied to a steam turbine. This turbine spins by harnessing the energy stored in the pressurized steam. The output shaft from the turbine is connected to a generator which generates electricity.

Hydro power In a hydro power plant, a large water turbine is located at the base of the dam (generally). There’s a lot of potential energy stored in the water. Deeper the water level the higher the potential energy is at that level. Water is allowed to run through the water turbine which rotates. This turbine is connected to the generator via a common shaft and the generator in turn generates electricity.

Internal Combustion Engines In most Cummins generators, gasoline and diesel engines are the prime movers used to run the generator. Cummins have also started introducing engines that are powered off of natural gas and LPG. But we will concentrate mainly on Diesel and Gasoline Engines. The engine is one-half of a Cummins Genset. Both gasoline and diesel engines contain an electrical system that needs to be maintained and serviced. By burning fuel the engine generates rotational energy which is connected to the generator.

Notes:


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Internal Combustion Engines

Uses fuel(chemical energy) as its input, and it produces mechanical

energy as its output

Internal Combustion Engines are available in many forms

Type of Fuel Mechanism Control

Gasoline Two stroke Hydro-mechanical

Diesel Four stroke Fully authority electronic (FAE)

Natural Gas Rotary

LP Gas

# of Pistons Piston Arrangement

2 V

4 Straight

6 Etc.

Etc.

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Internal Combustion Engines An internal combustion engine uses chemical energy as its input, and it produces mechanical energy as its output. By chemical energy we generally refer to the energy stored in Gasoline, Diesel, Natural Gas and other fuel sources. Gasoline and Diesel engines work in much the same way. The most significant differences between the two are the type of ignition employed and the type of fuel used. The fuel is burned inside the engine – hence the term, internal combustion engine. As opposed to say a rocket engine, where the actual combustion takes place outside of the engine. There are many popular types of internal combustion engines such as:

two stroke,

four stroke,

Rotary (wankel type) engines. In the following sections we will look at a four stroke engine, the participants are encouraged to do their own research on the other types of engines if it interests them.

Typical Engine Components

Valves

Piston

Cylinder

Connecting Rod

Camshaft

Crankshaft

Engine Block

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Typical Engine Components The figure shows an engine with four pistons in an inline-straight configuration, where the pistons are all arranged in a straight line in a single row. There are many other piston configurations, such as V-engine, with two banks of cylinders at an angle where the pistons are aligned such that it creates the letter V if you’re looking down the line of crankshaft. An internal combustion engine consists of the following basic parts: Cylinder Valves Piston Connecting Rod Camshaft Flywheel (can’t be seen in figure) Spark Plug (can’t be seen in figure)


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Camshaft and Crankshaft, and spark plug

Camshaft

Crankshaft

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Camshaft – The cam shaft provides a means to time the opening and closing of the intake and exhaust valves. If you look at an individual cam. The point of rotation (between 0 and 360 degrees) of the camshaft, and the shape of the Cam dictate when the valve will be open. Crankshaft – The crank shaft converts the up and down motion of the pistons into rotational motion of the crankshaft. In power generation applications the engine crankshaft is the main conveyor of rotary motion to the main generator shaft.

Sequence of Operation Most modern internal combustion engines operate on a four-stroke cycle. That means for each complete cycle the engine performs, the piston moves down, then up, then back down, and then back up again. These four motions of the piston are known as the Intake, Compression, Power, and Exhaust strokes. Before looking at the four strokes of an engine in detail, it’s beneficial to visualize a single piston and its connected components in simple up and down motions. If the piston is pushed downwards, the connecting rod, which connects the piston to the crankshaft, pivots as the piston moves. Since the connecting rod is connected to the crankshaft, this translates the piston up and down motion into the rotary motion of the crankshaft. Each time the piston moves down and then back up, the crankshaft completes one full 360o rotation, so the crankshaft makes two full revolutions for each four-stroke cycle completed by the engine.

Notes:

Q: What is the chemical energy that pushes the

pistons in an internal combustion engine?

A: A petroleum product (gasoline or diesel fuel)

mixed with air and burned very rapidly – almost

exploding – inside the engine cylinder creates the

force that pushes the piston downward and

turning the crankshaft. This combustion releases

the energy stored in the fuel into a form of

mechanical energy pushing the piston up.


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Intake Stroke

The piston starts out at the top of the cylinder.

The intake valve opens and as the piston begins its downward stroke, a mixture of fuel and air is drawn into the cylinder through the intake valve

When the piston approaches the bottom of its stroke, the intake valve closes, sealing in the fuel/air mixture.

And the compression stroke begins.

Compression Stroke:

The inertia of the

counterweights in the

crankshaft and

flywheel tends to

maintain the rotation

of the crankshaft.

The piston and the

crankshaft is coupled

through the

connecting rod, thus

the piston will start to

travel upwards.

As the piston travels

upward, it compresses

the fuel/air mixture

inside the cylinder

space.

And the power stroke begins.

Power Stroke:

Near the top of the stroke the spark plug fires

This ignites the fuel/air mixture

The resulting pressure force starts driving the piston downward.

And the exhaust stroke begins.

Exhaust Stroke:

With the piston near the bottom of the cylinder, the exhaust valve opens.

As the piston moves upward, it forces the combustion gases out of the cylinder and into the engine’s exhaust system through the exhaust valve.

When the piston reaches the top of the cylinder, the exhaust valve closes

And the intake stroke begins again.

In all the strokes the pistons up and down motion results in the crankshaft rotation. As long as the engine is running, it performs this four-stroke cycle over and over, in rapid succession, providing bursts of power to the crankshaft. The prime mover speed must be set very precisely since the speed of the generator’s input shaft determines the voltage and frequency of the electricity produced by the generator. The majority of Cummins gensets require a prime mover speed of 1800 rpm.


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Increasing Engine Power

Increase cylinder volume

Running the engine faster

Burning higher quality fuel

After-cooling and inter-cooling

Increasing the compression ratio

Turbo- or super-charging the engineTurbo charger cutout

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Increasing Engine Power Increased engine power can be achieved through a few ways, some of the more popular options used among engine designers are discussed below.

Increase cylinder volume The larger the cylinder volume, the more gas-air can be burned during the power stroke and, thus, more power is developed. Larger cylinder volume can be achieved through :

o Increase the piston diameter (also known as bore)

o Increasing the stroke: the distance from piston top dead center (TDC) to bottom dead center (BDC).

Running the engine faster

Burning higher quality fuel (more stored chemical energy)

Turbo- or super-charging the engine A turbocharger is a centrifugal compressor powered by a small turbine which is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the pressure of air entering the engine thus resulting in greater performance, since more air-fuel mixture is packed into the cylinder.

Increasing the compression ratio This ratio signifies how much the pistons compress the air-fuel mixture within the cylinder.

All of those methods come at a cost, however. Mainly, they are physically hard on the engine components, causing the engine to run hotter and harder. Many of Cummins engines are turbo charged (and after-cooled) but careful design has lead to producing an engine that is able to withstand thousands of hours running with little ill effect.

Notes :

Top Dead Center (TDC) is the highest level inside

the cylinder that the top of the piston reaches at

the when it’s extended all the way to the top.

Bottom Dead Center (BDC) is the lowest level

inside the cylinder that the top of the piston

reaches when it’s extended all the way to the

bottom of the cylinder.


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Diesel Engine (4 strokes)

Intake Compression Power Exhaust

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Diesel Engine The diesel engine operates much like a gasoline engine in that it has an operating cycle in which a combustible fuel and air mixture is burned to produce a piston stroke. Diesel Fuel Diesel fuel is a less refined and heavier fuel that contains more sulphur compounds than gasoline creating a dirtier and more polluting exhaust. Improvements in the fuel and engine technology have reduced emissions considerably.

Diesel Engines vs. Gasoline engines

Difference Diesel Gasoline

Ignition Heat of Compression Spark

Compression

RatioHigher Lower

Components Glowplug Sparkplug

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Diesel Engines vs. Gasoline Engines Diesel engines are different from gasoline engines: Uses a different fuel – diesel oil, instead of

gasoline. Uses heat of compression to ignite the fuel,

not an electric spark plug. The fuel is injected into the cylinder at the end

of the compression stroke. Since the air is very hot after compression the

fuel that is injected is immediately ignited. Therefore there are no sparkplugs Preheating may be required before starting the

engine. (glowplugs are optional)

The Ideal Gas Law The Ideal Gas Law describes the behavior of a gas under varying temperature, pressure and volume. Basically, if you enclose a gas inside a container, such as an engine cylinder, and manipulate one of the three variables, the other two variables are affected. For example, if you heat a fixed volume of gas while keeping the volume steady, the pressure inside the container will increase. If you compress the gas by decreasing the volume, its’ temperature and pressure will rise.

Q: With no spark plug to ignite the fuel-air mixture,

how does diesel ignition take place?

A: Any time a gas (air in this case) is quickly

compressed, it heats up as the molecules are

pressed together. Simply, the molecules hitting each

other create friction in the same way that rubbing

your hands together creates heat.

(Remember, when sprayed in a fine mist, gasoline or

diesel fuel behaves as if it were a gas.)When the

diesel fuel is sprayed into the cylinder, the heat of

the compressing gas causes the fuel to ignite and

create the diesel engine’s power stroke. The gas

heats up when it is compressed according to the

Ideal Gas Law.


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Energy Conversion in Power Generation

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Energy Conversion in Power Generation

The chemical energy stored in fuel (diesel, gasoline, natural gas, LPG, etc.) is converted to mechanical power using an engine.

This mechanical energy when supplied to a generator (alternator) generates electric power.

This conversion of mechanical energy (rotation of the engine-generator coupled shaft) into electrical energy by the generator will be discussed in detail in the lesson on Alternators.

Notes:


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Lesson 3: Principles of Electricity

Objectives

Basic atomic structure and its application to electricity

Understand Current flow and how it occurs.

Understand the basic units of electrical measurement: volts, amperes, ohms, and watts.

Understand series, parallel, and series/parallel circuits

Lesson Length : 1.5 hours

Electricity

‖

―Fundamental form of energy observable in

positive and negative forms that occurs

naturally (as in lightning) or is produced (as in a

generator) and that is expressed in terms of the

movement and interaction of electrons.

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Definition of Electricity Fundamental form of energy observable in positive and negative forms that occurs naturally (as in lightning) or is produced (as in a generator) and that is expressed in terms of the movement and interaction of electrons. Advantages of Electric Power Electricity offers the following advantages over other forms of power: It’s transmittable It’s portable Wiring is flexible and can be easily installed and

replaced in permanent or temporary locations It can be generated on-the-spot We can control/manipulate it and use it for very

many useful and necessary purposes

Composition of an Atom

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Electrons

Neutrons

Protons

Composition of an Atom The atom is the smallest unit of a chemical element. All atoms are made of three basic particles:

Protons are positively charged particles that are part of the nucleus of an atom.

Neutrons are the other particle in the atomic nucleus it has a neutral charge.

Electrons are negatively charged particles; they travel in a cloud around the nucleus.

Chemical elements differ from each other due to the number of protons and neutrons in the nucleus of the atom. The composition of the nucleus defines the atomic mass of the chemical element. The number of electrons orbiting the nucleus is usually equal to the number of protons in the nucleus. Electrons orbit the nucleus of an atom because of

Electrons

Neutrons

Protons


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the attraction of the positively charged protons and certain other nuclear attractive forces. Electrons have specific orbits or “shells” surrounding the nucleus of an atom. An atom’s electrons are distributed in the shells according to a formula from quantum physics. It is important to remember that the orbits or shells are not neatly defined, but exist as a kind of electron cloud.

Conductors

An Electrical Conductor a substance that can conduct electricity

because it has mobile electrons

Have an outermost "valence" shell with electron/s that is/are loosely

bound to the nucleus.

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Conductors A conductor is a material that provides passage for electrons wanting to move from one (electron surplus) area to another (electron deficit) area. The most common type of conductor of electricity is a wire. If you connect a wire between two objects that have a potential difference between them, electrons will pass through the conductor and try to equalize the two potentials. In the study of electricity, copper is one of the most important chemical elements. Copper is commonly used because it exhibits properties that lend themselves to the conduction of electricity. There are other elements that conduct better and have more desirable properties, but the relatively economical cost of copper makes it a popular choice in most applications. Copper has the additional benefit of having a higher melting point than other good conductors. Heat is a byproduct of electrical power flowing through a conductor.

Notes:


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Even though silver is the best metal conductor of electricity, it is obviously too expensive to be used for electric wire. Note: in the next lesson we will also quantify and define a unit of measure for conductivity.

Valence Shell : the outermost shell of electrons in an atom; these electrons take part in bonding with other atoms

Conductor Properties

Conductors are elements whose atoms have few

electrons in their valence shell.

Electrons can readily attach and then detach from a

valence shell.

Good conductors have their atoms in very close proximity

to each other.

Solid conductors such as copper wire tend to be the best

electrical conductors.

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Conductor Properties Conductors are elements whose atoms have few electrons in their valence shell. If an abundance of electrons is presents at one end of a conductor, these electrons can readily detach from the valence band of “atom A” and attach to the nearby valence shell of the “atom B” conductor, thereby, moving from atom to atom along the conductor. Good conductors have their atoms in very close proximity to each other. That way, electrons don’t have far to jump as they move from atom to atom. Solid conductors such as copper wire tend to be the best electrical conductors. In the same way that a large diameter pipe can carry more water than a small pipe, a large wire can normally carry more electric energy than a small wire can. List of Good Conductors (in order of conductivity) 1. Silver 2. Copper 3. Gold In electrical applications wires are the main conducting material between components. Let’s take a closer look at some types of wires.

Single conductors

Wires There are many types of wire. They vary from the type of conducting material to the type of insulation and the core layup. solid core or multi-strand insulated or non-insulated single conductor or multi-conductor

Notes:


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Multi-conductor

In the figure (last page), all four wires have insulation material covering the conducting core. Whilst the first two wires have a solid copper core, the other two has multi-stranded copper core. All the four wires are single conductors. Stranded wires have more air gaps within its cross section, while the solid core is in fact solid (no air gaps). Thus a solid core wire has more current carrying capacity. An example for a multi-conductor wire would be if you take any of the single conductor wires shown in the earlier page and stranded them together inside another insulating material as seen on the image on the left. Wires are selected for specific purposes based on a number of factors: Conductivity Flexibility and bend radius Weight Cost Temperature performance

The gage of the wire determines the diameter of the wire. The standard used for measuring this is the American wire gauge (AWG), a table comprising of different AWG values is attached on the right. Magnet wire or enameled copper wire is a copper or aluminum wire covered with thin insulation. It is used in the construction of generators (windings), transformers, inductors, motors, headphones, loudspeakers, hard drive head positioners, potentiometers, and electromagnets, among other applications. However, it is not itself magnetized.


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Insulators

Materials that are good electrical insulators, like glass,

plastic, porcelain, wood, rubber and air, have the

outermost electrons of their atoms tightly bound to the

nucleus, so there are no free electrons to carry an electric

current.

Common insulators

– Glass + braided glass fibers

– Porcelain

– Rubber compounds

– Fabric materials

– Enamel or paint

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Insulators If conductors are materials that allow the flow of electrons, it follows that other materials block the flow of free electrons through them. These substances are called Insulators. Electrical insulators have a full complement of electrons in the valence shells of their atoms. Thus, when an abundance of free electrons are present at the surface of an insulator, the electrons can not readily attach themselves to any atoms and cannot make their way through the insulating material. Many elements are good insulators, but they tend to lack the physical properties needed for practical use. Therefore, various chemical compounds and man-made materials are generally used. Commonly used insulators: Glass + braided glass fibers Porcelain Rubber compounds Fabric materials Enamel or paint

Wires are normally covered with insulation. Insulation serves an important safety function by preventing bare wires from being a shock or electrocution hazard. Materials that are normally insulators can become conductors in some circumstances and have other physical limitations that engineers must consider when choosing insulating materials. For example, if a high voltage power transmission wire was too close to the ground, it could conduct through the air complete a circuit with ground. Air is generally a good insulator but it has a dielectric strength of about 1000V/mm, meaning if a potential difference of 1000V was applied between too conductors(or a conductor and ground), and the distance between the two was about 1mm it would conduct through the air.

Batteries Every type of battery contains chemicals that react with each other and, as a by-product of that reaction, produces a surplus of electrons that accumulate at the negative terminal. The figure in the next page shows a typical dry cell DC battery cross section, to show internal structure. Batteries are constructed so that the chemical reaction takes place in a controlled way, over a long period of time. This makes batteries a practical and portable source of DC power. When the chemical reaction has been exhausted, the battery is “dead.” Rechargeable batteries (i.e., your cell phone battery.) can have the chemical reaction reversed, restoring the battery to a near-new condition. When a battery is sitting unused, the chemical reaction is held in check. The reaction only takes place when the electrons have someplace to go. When the battery’s terminals are connected to each other by a conductor, electrons flow from one terminal, through the conductor, to the next terminal. This completed path allows the chemical reaction to take place inside the battery. A complete path from a power source’s negative terminal to its positive terminal is called a Circuit. Electricity (electrons) is said to flow through the circuit as Current.


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DC Battery : Dry Cell

Metal cap (+)

Carbon Rod

Zinc Case

Manganeese Oxide

Moist paste of ammonium chloride

Metal bottom (-)

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Dry Cell Battery Batteries for household appliances are generally dry cells. This type of battery is completely sealed and contains chemicals in solid form.

Battery : Wet Cell

Terminals

CasingSulphuric

Acid

Lead oxide (+ plate)

Lead

(- plate)

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Wet Cell Battery Wet cells consist of a liquid acid bath that reacts with lead plates to create the flow of electrons. Sometimes these batteries can be opened up to check and add liquid, but most often they are also sealed. Automotive batteries are generally wet cells

Theories of Current flow

Electron flow

theory

Conventional current

flow theory

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Theories of Current Flow We discussed how a battery produces a surplus of electrons that accumulate at the negative terminal. We also mentioned that these electrons flow from one terminal to the other when a conductor is connected between them. The figure shows a closer look of the conductor connecting the two terminals of the battery; electrons are flowing from the negative side of the circuit (negative terminal of the battery) to the positive side (positive terminal of the battery). This direction of flow agrees with the Electron flow theory of current.


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Electron Flow Theory The theory of electrons moving through a conductor, atom by atom, is the most widely accepted theory of current flow. The idea is that the electrons physically move along a conductor, much like water flows in a stream, providing a sort of physical force as they pass through electrical components (like a light bulb). According to this theory, negatively charged electrons are attracted to the positive terminal of a battery, and so electricity is said to flow from negative to positive.

Conventional Flow Theory The other current flow theory is that current is created by the “holes” left behind as each electron moves through a conductor, and that current flows from positive to negative. This idea is reinforced by the very terms “positive” and “negative.” For example, in the atmosphere, if there is one area of positive pressure and another area of negative pressure, the wind blows from positive to negative. Also, in automobile electrical systems, and in the way schematic drawings are made, it is often easier to think of current flowing from positive to negative.

Notes:

Q: So which theory is the correct one, Electron flow theory or conventional flow theory?

A: Both theories have valid arguments for them, the important point to remember is that current flow is

driven by a difference in potential between two points. And that negatively charged particles and positively

charged particles are attracted to each other, resulting in current movement. It’s also important to be

consistent in which theory you use, when analyzing a circuit or schematic. When we talk about an electron

moving through a conductor, there are actually many, many billions of atoms and electrons in even a very

short section of conductor.


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Circuits A power supply with high and low potential terminals.

A conductor making a path from the high to the low potential point.

Electrons will flow through the circuit only if both conditions are met.

Placing a load in the circuit allows the current to do some work.

D.C

BATTERY

+ -

Light

Bulb

Switch

Electron flow

Copper

Conductor

+

ON

Light

Bulb

OFF

9/28/201132

Circuits The point of the circuit concept is that if you place some electrical components in the circuit, the electrons will pass through those devices on their way from one battery terminal to the other, and will do some kind of work In the process of making the roundtrip. Measures of Electricity in a Circuit In a circuit, there are three basic measures to determine how much electricity is in the circuit: Voltage Resistance Current

Necessary Components of a Circuit: In order to make a complete circuit, the following components are needed:

A power supply with high and low potential terminals.

A conductor making a path from the high to the low potential point.

Electrons will flow through the circuit only if both these conditions are met.

Placing a load in the circuit allows the current to do some work.

Notes:

Circuit Analogy

Think of a hydraulic circuit – a pump draws oil from a reservoir and pumps it through pipelines to

hydraulically operated tools and devices (motors, cylinders, etc.). The pressure of the flowing oil makes

those devices do some work. After passing through the circuit completely, the oil winds up back in the

reservoir


Cummins Inc 25

Voltage

Voltage is defined by the

difference in electrical

potential between the

negative and positive

terminals of the battery.

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Voltage

Voltage is the amount of “pressure” available to push electrons through the circuit. Voltage is defined by the difference in electrical potential between the negative and positive terminals of the battery.

Voltage is a characteristic of the battery or power supply used in the circuit. Its unit of measure is volts. For example, a 1.5 volt battery, or a 9 or 12 volt battery are all supplies with different voltages.

Remember, electrons move from an area of high potential to an area of low potential. (I.e. a lightning bolt travels from a cloud to the ground.) In the same way, electrons at the negative terminal of a battery want to move to the positive terminal. The connected conductor provides a path for the electrons to flow from high to low potential.

So the voltage of the battery or power supply provides the push – represents the high potential in the electrons that makes them want to flow toward a low potential. The higher the voltage, the greater the potential energy or “pressure” there is.

Q: What is the Effect of Increasing Voltage in a

Circuit?

A: Increasing battery source voltage increases the

potential difference between the “high” and “low”

sides of the circuit. Like increasing the pressure of

water increases the water speed through a pipe.

Similarly, increasing the voltage in a DC circuit would

increase the rate of flow of electrons through circuit

components. Making motors run faster or a heating

element hotter. For this reason, electrical

components are manufactured with voltage ratings

that dictate the voltage (or voltage range) to be

used.

Q: Why Do Different Batteries Have Different

Voltages?

A: Voltage output for a common single chemical

battery cell is approximately 1.5 volts. Batteries that

power flashlights, portable radios and CD players,

toys, etc., (AAA through D cells) all have 1.5 volt

outputs. Each is a single battery “cell.” Higher

voltage batteries are made by connecting two or

more cells, in series, inside a single casing, thus

providing multiples of 1.5 volts. For example, a

common 9-volt battery contains six individual 1.5 volt

cells. There is nothing magical about 1.5 volts. It’s a

value that engineers and manufacturers have

adopted as a standard.

Voltage Analogy

Think of two waterfalls – one has a 10-foot drop

and the other has a 20-foot drop. The water at

the top of the 20-foot falls has more potential

than the water at the top of the 10-foot falls.

There’s nothing different about the water itself,

just as there’s no difference in the electrons in a 9

volt circuit or a 1.5 volt circuit. But the water in

the 20-foot falls carries more force because it falls

further – it has more potential to do work than

the water falling a shorter distance.


26 | Cummins Inc

Resistance

Resistance is just what

the word implies in

everyday life. It is the

amount of resistance to

the flow of electrons

through a conductor

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Resistance

Resistance is just what the word implies in everyday life. It is the amount of resistance to the flow of electrons through a conductor. For example, we said that some materials are better conductors of electricity than other materials. What we meant was that better conductors allow the flow of electricity more easily – they have less resistance.

Many electrical components such as lamps are resistive in nature. The unit of measure of resistance is Ohms, its shown as R or the Greek letter Omega (Ω)

Think about relatively thin pipes carrying water from one place to another. If the inside of the pipe is nice and smooth, without kinks, sharp bends or blockages, water will be able to flow through the tube much more easily.

Resistance is, literally, electrons banging into things as they make their way along a conductor or through a device. The electrons are entering and breaking out of the orbits of atoms.

Resistance is a form of friction, which always creates heat. How much heat will be created in conductors, components and circuits is a concern for engineers and can be a valuable troubleshooting point in the field.

Resistance in conductors is generally a bad thing. It results in efficiency losses and generates heat. The conductor’s job is to “conduct” the electricity from the source to where it is needed, without resisting the flow

Notes:

Q: Is there any use of having resistance at all?

A: Resistance isn’t all bad, it also what makes

electrical components work. For example, look at

the lamp in our little circuit. See how thin the

filament is? It’s also made of a special high-

resistance material (usually Tungsten) that can

stand up to very high temperatures.

As electrons flow through the filament, resistance

causes it to heat up and glow intensely. The

more electrons flowing through the lamp (the

higher the voltage), the brighter the lamp glows.

Resistance can also be manipulated to control

currents in circuits in certain applications.


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Current

Electric current is a flow

of electric charge

through a medium. This

charge is typically carried

by moving electrons in a

conductor such as wire.

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Current

Electric current is a flow of electric charge through a medium. This charge is typically carried by moving electrons in a conductor such as wire.

The result of a voltage being applied to a circuit is current draw. (Remember, we talked about electrons flowing through a conductor as being “current”?)

Let’s define current as the number of electrons (rate at which) flowing through a circuit.

Current is not an inherent quality of the power supply or of the circuit. Rather, it is a measure of the electron flow made possible by:

o the voltage “pushing” electrons through, and

o The resistance of the circuit opposing the electron flow.

We say current “draw” because the characteristics of the circuit pulls or draws the current out of the battery at the battery’s rated voltage.

Again, think of a pump pushing water through a pipe. If you increase or decrease the pump pressure, water flow will change as a result. If you place or remove restrictions inside the pipe, water flow will also change as a result.

In the same way, if you add or remove batteries or add or remove loads from the circuit, more or less current will flow as a result.

Measured value

Scientific symbols used

in this text

Other Accepted Scientific symbols

Measured in Example

Voltage V E Volts (V) 10V

Resistance R Ohms (Ω) 3Ω

Current I A Amperes (A) 20A

*These names for each type of unit is interchangeable : whether its E or V, its referring to voltage etc.


28 | Cummins Inc

Technical Training

Ohm’s Law

Ohm’s Law defines the mathematical relationship

between voltage, resistance and current.

12/1/201133

I

VR

R

VI

RIV

Ohm’s Law Ohm’s Law defines the mathematical relationship between voltage, resistance and current. If you know any two of these three variables you can calculate the third value using Ohm’s Law. Ohm’s Law is expressed mathematically by:

Where “I” represents the current in amperes, “V” the voltage in volts, and “R” equals resistance in ohms. When using this equation, it is important to pay attention to the units of measure of the three electrical variables. It’s apparent that current (or amperage) is directly proportional to voltage, while inversely proportional to resistance. The circular figure is a quick way to memorize Ohm’s law; if you cover the value you’re looking for with your hand you can get the equivalent equation. (I.e. for Voltage, if you cover “V” the figure shows that it’s equal to IxR). Applications of Ohm’s Law If Voltage “V” increases or Resistance “R” decreases, the current “I” draw increases as a result. Circuit current “I” is what engineers and product developers are most likely to be interested in. For a battery-powered device (e.g., a cell phone), the lower the current drawn from the battery, the longer the battery lasts. Now that we know the relationship between the measures of electricity let’s look at circuits.

Q: How Does Electricity Do Work?

A: Electricity is a form of energy which has the

potential to do work. The higher the voltage

potential (water pressure) on the current (water),

the more potential work can be done. In the next

section we talk about Power which a measure of

the rate of electrical energy used or produced.


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Technical Training

Power

Power is the rate at which electrical energy is used or

consumed as it does work.

12/1/201134

I

PV

V

PI

IVP

Power Power is the rate at which electrical energy is

used or consumed as it does work. Power is measured in Watts, has the symbol “P”

and is measured with a Wattmeter. Can be calculated by formulas One Watt is the power used when one Ampere

of current flows through a circuit with the potential difference of one Volt.

The electricity you use at home is given in kilowatt-hours which is power consumed over time.

Power is expressed mathematically by the following equations

You can also substitute in the equations from ohm’s law and find more ways to find power. For example if you plug-in (ohm’s law) for V in the power equaiton, you get , which is also a correct way of expressing Power. Thus, like with ohm’s law you only need to know two out of the three measures of electricity (voltage, current, resistance) to figure out the power of the circuit.

Current through different bulbs

From household applications we know that the higher

the wattage of a typical incandescent light bulb, the

brighter it is.

10/3/201138

60W

40W

100W

120V

household

supply

V

WA

_____

_____

_____

A

VR

_____

_____

_____

Applying Ohm’s law and the Power equation


30 | Cummins Inc

The nth term signifies that the circuit can have as

many components as necessary.

Series Circuits A series circuit consists of one loop leading directly from the battery’s negative terminal to the positive terminal, with one or more loads placed in the path.

A series circuit has components connected so current has only one path to travel. Thus, the current through each component is the same.

The voltage across all the components is the sum of the voltages across each component. There’s a voltage drop across each component. We will discuss voltage drop later on in the Lesson.

The overall resistance of a series circuit with two or more components is the sum of the individual resistance's.

The sum of the voltage drops equals applied voltage, and is comprised of the individual voltage drops that can be measured around each load in the circuit. Current draw from the battery is equal to battery voltage divided by total circuit resistance, and is the same throughout the circuit (obeying Ohm’s Law).

Series circuits - loads

There’s a voltage drop

corresponding to each of

the loads.

Current through all the

loads are the same

If there are too many loads

the lamps will light dimly or

not light at all

10/3/201139

Loads in series circuits

Each device placed in a circuit is a load – it uses some of the electricity flowing through the circuit. It might seem from Ohm’s Law that the more loads you place in a circuit, the higher the resistance will be, and the lower the current draw will be (battery will last longer), right? The catch is that each individual load in the circuit causes a voltage drop in proportion to the load’s resistance. The Ohm’s Law equation (V=A x R) tells us that there is an inverse relationship between resistance and load. That is, the higher the resistance of a circuit the lower the load on the power source.

Notes:


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Voltage drop in series circuits Because the negative terminal of a battery represents the high potential side of the circuit (using electron flow theory and not the conventional flow theory), and the positive terminal represents the low potential side, it’s correct to say that each of the loads uses up some of the available voltage in the circuit. At the positive terminal of the battery then, the potential is essentially zero – the same way that the water at the bottom of the falls has no potential, but at the top of the falls the potential is at a maximum. All the potential energy is given up during the fall. You might think that you should be able to put as many light bulbs in a circuit as you want and that the electricity will flow through and light all of them. Think of this hydraulic circuit: a pump, and a motor. Now suppose you add more and more motors to the circuit. Each motor represents a load, and at some point the load of all the motors is too great for the pump to operate. It’s the same with electricity. Each lamp uses up (drops) some of the voltage across it so that the potential of the electrons after passing through all the loads is essentially zero. If there’s only one lamp in the circuit, that lamp uses up the entire potential voltage drops across the lamp. If there are two or more lamps, each lamp uses a share of the voltage potential. Either way, at the positive terminal of the battery, the electrons entering the battery have essentially zero potential left. It’s actually the increased current that causes the lamp to burn out, but the reason the current increases is because of the increased voltage.

Let’s go back and look at DC batteries again. AAA, AA, C and D cells all put out 1.5 volts per cell. Different batteries you buy at hardware shop will have different voltage ratings such as: 1.5V, 3V 12V etc. this is achieved by connecting individual cells in series. The larger volume and internal surface area of the larger batteries gives a greater contact area for the chemicals to react in, allowing more electrons to be produced per second, when called for by the circuit. Battery Ratings You’ve probably noticed that car batteries come in many different sizes, even if each is a 12-volt battery. Automotive and industrial lead-acid batteries have ratings that specify how much current they can supply.

The standard rating method indicates the number of amps the battery can produce for 20 continuous hours at 80o F. and is listed in terms of amp-hours. A battery that can produce 4 amps for 20 hours has a rating of 80 amp-hours.

Another rating is called cold-cranking amps – the number of amps the battery can supply at 68o F (20o C).

Q: What happens if you put so many lamps together that the circuit’s current draw exceeds the maximum battery

amperage output?

A: Depending on how much current draw there is, the lights may turn on very dimly or not turn on at all (because the

batteries can’t supply enough current).


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Parallel circuits A parallel circuit consists of multiple loops leading directly from the battery’s negative terminal to the positive terminal, with one or more loads placed in each of the paths.

A parallel circuit has components connected to the voltage source as shown in the figure on the left.

In parallel circuits the voltage different is equal across each branch of the circuit. This is because there are only two sets of electrically common points in a parallel circuit, and voltage measured between sets of common points must always be the same at any given time. Therefore, in the above circuit, the voltage across all the light bulbs is equal and is also equal to the voltage across the battery.

By applying ohm's law to each component you can find the current in each branch of the circuit. The sum of these branch currents will equal the total current in the circuit.

You can find the total resistance of the circuit by using the second equation on the left.

Parallel circuits – loads and voltage drop

The voltage across each

lamp is 12V.

Current through all the

lamps doesn’t have to be

the same.

If there are too many loads

the lamps will light dimly or

not light at all

10/3/201141

Voltage Drop in Parallel Circuits The voltage drop is the same for all circuits in a parallel circuit network. The voltage drop across each of the light bulbs is 12V, since they are all in parallel. This is in contrast to the different voltage drops through each bulb that was observed in a series circuit. Current Draw of Parallel Circuits The current draw of each circuit in a parallel circuit network depends on the value of each load presented to the voltage source. High loads (low resistance) draw more current than low loads (high resistance). The third equation on the left can be used to figure out the current draw in a parallel circuit.

Christmas tree light bulb example

A string of Christmas tree lights can be an

example of either a series or parallel circuit.

If you remove a light bulb from a string of

lights that is plugged in and lit and the

whole string goes out, the lights are wired

in series. If the string of lights stays lit when

you remove one light bulb, the lights are

wired in parallel. Some strings are a

combination of both series and parallel.


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Resistance of Parallel Circuits The resistance of a parallel circuit is not equal to the sum of all resistances like it was for series circuits. The total resistance is found by using :

Series-Parallel Circuits Series-parallel circuits consist of sub circuits wired in series and in parallel. They show characteristics from both types of circuits. We will not be discussing series-parallel circuits in detail.

Notes:

Q: So what happens if you add more and more batteries in parallel in our little lamp circuit?

A: Connecting batteries together in parallel increases the amount of current that can be supplied for a particular

constant voltage. The voltage does not increase since it’s connected in parallel.


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Lesson 4: Principles of Magnetism

Objectives

Types of magnets

Understand the relationship between electricity and magnetism.

Understand what properties make a substance more or less magnetic.

Understand Magnetic and Mutual Induction

How an electromagnets operate

How to induce a current in a conductor using a magnet


Magnetism

Magnetism is basically the property of

an object being able to attract another

object. Now this definition is a simple

one. A more precise definition would

explain that the two substances that

attract must be magnetic.

– Permanent magnets

– Residual magnets

– Electro-magnets

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Overview of Magnets A magnet is a material, natural or man-made, whose atoms are lined up in an orderly fashion so that the tiny forces generated by spinning electrons add up and create a force field around the material magnet. The field around any magnet is called a magnetic field.

Iron Filings experiment (to show magnetic field lines) In the figure on the right we see the classic bar magnet and iron filings demonstration. If you place a bar magnet on a piece of paper with iron filings and tap it lightly the iron filings will be attracted along the field lines of the magnetic field. You can see that it’s the strongest at the two ends of the magnet. This would be the north and south pole. It’s not completely known how magnetic fields are generated, but it seems to have something to do with the spin of electrons in the atoms of certain materials. Besides orbiting the nucleus of an atom, electrons also spin on their axes – just like the earth spins on its axis as it orbits around the sun. Origins of Magnetism If you can imagine a bunch (billions + billions – remember how many atoms and electrons we said are present in any solid) of electrons spinning in unison maybe you can imagine each spinning electron having a tiny effect on neighboring electrons and extending out into space. This basic explanation of magnetism is sufficient for this course. Some natural metals “take” magnetism very well (iron, nickel, cobalt and manganese) as do some man-made materials (ceramic magnets). Other materials have very little ability to take or hold magnetism (wood, glass, copper). Once a permanent magnet is created no further energy input is required to maintain the force. However, the atoms and electrons that create the force are always in motion and, one way or another, this motion results in the magnetic field being generated.


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Definition of Poles Every magnet, including the earth, has a north pole and a south pole. The magnetic field is thought to emerge from the North Pole and re-enter the magnet at the South Pole, though this is not an established fact. (Kind of like the controversy over whether current flows from positive-to-negative or from negative-to-positive.)Permanent magnets can have many different shapes: horseshoe, bar, disc, flat, etc. Regardless of the shape, flux lines go directly from one pole to the other.

Types of Magnets

Permanent magnets

Residual magnets

Electro-magnets

9/28/201143

Permanent magnets Permanent Magnets are made from a material that tends to retain its magnetism. The problem is that you cannot increase or decrease the amount of magnetism in a permanent magnet generator. Residual magnets Residual Magnetism is the residue or leftover magnetism left in a piece of iron, like a screwdriver, after it has been stroked with a permanent magnet or attached to an electromagnetic field. Iron that has residual magnetism in it tends to lose it in a short period of time and will have to have it restored periodically. Electro-magnets Electro Magnetism is the third type of magnet and the only magnetism that can be turned off/on and also increased or decreased. An electromagnet can be made by wrapping a coil of wire around an iron core and passing a DC electric current through it. An electromagnet can only be manufactured with DC voltage.

Properties of a Magnet

The inherent properties of a material can be

evaluated with regard to magnetism by using the

following measures.

– Permeability

– Reluctance

– Retentivity

9/28/201144

Permeability Permeability is a measure of how easily a material can become magnetized – that is, how easily a magnetic field can flow through a material, magnetizing it in the process. For example, a magnet is attracted to a piece of iron. The magnet’s field extends through the iron, lining up its atoms and effectively extending the magnetic field. If you stick the North Pole to a piece of iron, the opposite end of the iron becomes the new north pole of the magnet.


36 | Cummins Inc

Reluctance Materials that are reluctant resist having their atoms lined up and becoming magnetic. Generally, a magnet’s field will extend through a piece of wood (for example), but the wood itself does not become magnetic as a result. Magnetic fields tend to flow around materials that are especially reluctant. Retentivity When the magnet is removed from the piece of iron, the iron does not act like a magnet any more. The iron does not retain much of the magnetism. In general, the softer the iron (or steel) the less ability it has to retain a magnetic field. The harder the steel (for example tool steel) the more ability it has to retain magnetism.

Attraction and Repulsion of Magnetic Poles

Like poles repel each other

Unlike poles attract each other

9/28/201145

Attraction and Repulsion of Magnetic Poles Remember that two positively charged particles repel each other. Also, two negatively charged particles (electrons) repel each other. Dissimilar charges attract each other. Likewise in a magnet, like poles repel each other and unlike poles attract each other (see figure 7-3).

The Earth is a magnet

The magnetic field of the earth is thought to be caused by the magnetic properties of the earth’s core. The

core is believed to be composed primarily of metals (iron, nickel, etc.) that exhibit a strong magnetic polarity

index.

The earth’s magnetic field is used for navigation purposes and may be used by migratory birds to find their

way north and south. The earth’s magnetic poles do not match up exactly with its geographic poles. No one

knows why this is, but there is evidence that the magnetic poles have moved over the course of billions of

years, and that the poles have reversed themselves repeatedly over time.


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Electromagnetism and Magnetic Flux Lines Magnetism and electricity were thought to have no relationship until the nineteenth century. A compass was placed over a current carrying wire in an experiment revealed the compass needle turned perpendicular to the wire. It was concluded only a magnetic field around the wire could affect the compass such a way. Thus, if a wire carries a current a magnetic field is generated around it. As seen from the two figures on the left, the magnetic field around the wire changes direction depending on which way the current flows through the wire. If a compass is held next to a current carrying wire, the compass needle will point in one direction. If the current is reversed through the wire, the compass needle will point in the opposite direction. Since the force of the magnetic field leaves the north of the compass and enters the south pole, it is concluded that magnetic flux lines have direction.

Strength of a Magnetic Field The strength of a magnetic field is defined by magnetic flux density. The more lines of flux passing through a square inch (for example), the stronger the force at that point. The stronger the magnet, the more lines of flux emerges from the North Pole. This is why any magnet is strongest at the poles. In the earlier slide this is indicated by the presence of higher amounts of iron filings at the poles. In a stronger magnet, the lines of flux also extend further out from the poles before arching around and surrounding the magnet. The strength of a magnetic field fades rapidly as distance from the pole increases. This is because the lines of flux spread out and get farther apart as they arc around and head toward the opposite pole of the magnet. Magnetic lines of flux repel each other and do not cross.

Q: What Are Magnetic Flux Lines Made of?

A: Lines of magnetic flux are pure force and there

are no electrons or other particles flowing from one

magnetic pole to the other.

Q: How strong can a magnet be?

A: If a permeable material is brought close to a

magnetic field, some of its atoms line up causing the

material to act as a magnet. The more atoms that

get lined up, the stronger the magnetic field will be.

If all atoms in the magnet are perfectly lined up, the

material is said to be saturated. A saturated magnet

cannot be made any stronger.


38 | Cummins Inc

Magnetic Induction If the molecules are attracted to a magnetic field and align themselves so all their north poles point in one direction and south poles in the opposite direction, the magnetic forces of each molecule combine to produce one strong magnetic effect. Some materials maintain their molecular alignment only while in a magnetic field. When the field is removed, the molecules rearrange themselves and magnetism is lost. But, materials with good magnetic retentivity such as iron, nickel and cobalt are called permanent magnets. Their molecules remain aligned when removed from the magnetic field unless heated or pounded.

Mutual induction If two coils of wire in two separate circuits are brought into close proximity with each other so that the changing (collapsing/expanding) magnetic field from one coil induces a magnetic field in the second, a voltage will be generated in the second coil (in the other circuit) as a result. This is called mutual inductance. When voltage impressed upon one coil induces a voltage in another. No physical connection is needed.

Keep in mind that this only works under a collapsing/expanding magnetic field. A device specifically designed to produce the effect of mutual inductance between two or more coils is called a transformer. We will talk more about transformers later on in lesson 5 and 7. If you apply DC voltage source to circuit on the left, it produces a constant magnetic field in the coil. Thus, you will not be able to maintain mutual inductance between two coils since there’s no changing magnetic field.

Due to this fact transformers only work if an Alternating Current (AC) I supplied to it, since AC produces a constantly changing magnetic field. We will discuss AC power generation in lesson 7. For now it’s adequate that you understand that continued mutual inductance occurs only with an AC voltage input.


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Inducing a voltage in a wire using magnetism or create a magnet using current You can induce a voltage in a wire using magnetism, or the opposite. It must be noted that the current flows only if the circuit is closed (load connected), otherwise only a voltage will be induced. You can also generate an electric current in a conductor by moving it through a magnetic field. These concepts are very important in power generation. 1. Any time an electric current passes through a conductor, a magnetic field is created around that

conductor. 2. Any time a conductor moves through a magnetic field, an electric current is created in that conductor.

The graphic above illustrates this concept. The three variables or necessary conditions to generate a current in the conductor are:

Existence of a magnetic field

Conductor

Relative motion of the conductor through the magnetic field

The two variables or necessary conditions to create magnetic field

Conductor

Continuous Current flow through the conductor

One way to make use of this relationship is to use electricity to make electromagnets. When a current passes through a permeable conductor it turns it into an electromagnet We will soon see that there are a number of common electronic components that use electromagnetism to work (e.g., relays, solenoids, transformers, etc). Coiling the conductor multiplies the strength of the magnetic field. In fact, doubling the number of turns in a coil doubles the strength of the field produced when current is run through the coil.


40 | Cummins Inc

Electromagnets

9/28/201150

Electromagnets A coil wound around a hollow form or around a core that is not permeable creates a magnetic field whose strength is proportional to the number of turns and the amount of current applied to the coil. The term used to describe the strength of this kind of electromagnet is ampere-turns. Multiply the number of turns by the number of amps to find the ampere-turns value. The strength of an electromagnet can be greatly increased by winding the coil around a material with high permeability, like iron. The atoms of the core line up just as they do when attached to a magnet, but in this case it is the current in the coil providing the magnetizing action. As a result, the iron core becomes the magnet through which flux lines emanate and extend from pole to pole. The more permeable the core material, the stronger the resulting magnet will be. A material whose permeability is 1 does not increase or decrease the magnetism of the coil. A material whose permeability is 10 increases the strength of the magnetic field by 10 times. Again, magnetic strength is defined by flux density – the number of flux lines created in the core.

Man-made Permanent Magnets

A very strong, over-saturation current is applied with

the desired magnet as the core.

When the current is removed, a high percentage of

the applied magnetism ―permanently‖ resides in the

core.

The percentage that remains is a measure of the

material’s Retentivity.

Depending on the use for an object, high receptivity

might be a desirable or an undesirable quality.

– Junkyard cranes, relays, solenoids

9/28/201151

Creating Man-made Permanent Magnets In the creation of permanent magnets a very strong, over-saturation current is applied with the desired magnet as the core. When the current is removed, a high percentage of the applied magnetism “permanently” resides in the core. The percentage that remains is a measure of the material’s Retentivity. You can make any retentive material into a permanent magnet by stroking it repeatedly with another magnet. Depending on the use for an object, high retentivity might be a desirable or an undesirable quality. For example, large electromagnets are used to pick up junked cars. When the operator turns off the current to the magnet, the idea is for the car to drop

Q: Can you increase the magnetic strength

continuously by increasing the amount of current

through the conductor?

A: No, Increasing the current in the conductor only

increases magnetic strength to a certain point of

saturation. Beyond the Saturation Point the

relationship between current increase and increase

in magnetic force starts to level off, and no further

increase in magnetic strength is possible.


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– not to remain held to the magnet. Thus high retentivity would not be useful in this case. Similarly, solenoids, relays and magnetic switches are electric devices whose operation depends on being able to turn magnetism on and off instantly. The creation of a magnetic field around a conductor when current is applied adds to its electrical resistance (induction – the magnetic field opposes the motion that creates it).

Notes:


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Lesson 5: Electrical Components

Objectives

Be familiar with practical circuit components namely: resistors, diodes, inductors, capacitors, switches, fuses, circuit breakers, relays, solenoids, transformers.

Have a general knowledge of how relays and solenoids transfer or switch large power by applying small amounts of electricity.

Note : We will revisit some of these components again after learning about Alternating Current (AC)


Wirewound Power resistors (8W-1325W)

Resistors Resistors are specifically built to resist the flow of current. If a voltage difference is applied between the two ends of the resistor, a current will flow across it and it will be proportional to the voltage difference across it. (obeying Ohm’s Law) There are many variations of resistors. Types of resistors Although the fixed carbon resistor is the most common type of resistor used in electronic applications, in some applications it’s important to have the ability to change the resistance depending, and thus vary the current through the circuit. The fixed wirewound (tapped) resistor offers a few discrete resistance values depending on the ohm rating of the resistor, the number of tap points in the resistor and how it is connected. The other three types (adjustable wirewound, potentiometer, and rheostat) mentioned in the table below can vary the resistance through a bigger range of values. We will only need to understand the application of a fixed resistor for the scope of this course, the participant is encouraged do his/her own research if they are interested in learning more on the other types of resistors.


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Fixed Carbon Resistors

Notes:


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Switches In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts. Each set of contacts can be in one of two states: either 'closed' meaning the contacts are touching and electricity can flow between them, or 'open', meaning the contacts are separated and the switch is non-conducting. The mechanism actuating the transition between these two states (open or closed) can be either a "toggle" (flip switch for continuous "on" or "off") or "momentary" (push-for "on" or push-for "off") type. The table below has an array of other types of switches that are commonly used in electronic applications.

Notes:


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Fuses

Fast acting

Slow blow

Fuse symbol

A fuse is a type of sacrificial overcurrent protection

device.

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Fuses A fuse is a type of sacrificial overcurrent protection device. Its essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected. Short circuit, overload or device failure is often the reason for excessive current. Once a fuse is burnt out, it needs to be replaced with a similar fuse otherwise the circuit will not function. Fuses are rated by amp capacity and are constructed of a thin (or not so thin) metal strip through which current flows when the circuit is energized. The fuse has very little resistance and does not influence circuit behavior. When designing a circuit, it is the engineer’s job to decide where fuses are needed and what size and type the fuses should be. For example, your car probably has eight or more fuses, each protecting a different part of the electrical system. Why so many? It’s so one blown fuse doesn’t affect the whole car. If a problem in your stereo causes a fuse to blow, you wouldn’t want that to prevent your headlights from working. Fuses are often constructed so you can visually determine whether a fuse has blown. A typical design is a glass tube with connectors at both ends. In addition to having an amp rating, there are different types of fuses – determining when they will blow: Fast-acting – the fuse should blow immediately

if the rated current flow is exceeded, even briefly.

Slow blow – the fuse should only blow when the rated current flow is exceeded for a longer period of time. How long it takes the fuse to blow depends on how big the current surge is.

As we will soon see, some devices (e.g., motors) draw a very high current at startup, with a reduced running current. Engineers and designers have to choose the proper fuse size and type for each application to provide both the desired protection while avoiding unnecessary fuse blowouts. Switches, resistors and fuses are all resistive loads.


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Diodes

Diode symbol

A diode is a device that only conducts electricity in

one direction

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Diode in a series circuit

Diode and lamp in series

Diodes Diodes basically allow current to flow through them in one direction only, and blocking current from passing in the opposite direction. Diodes are a type of semiconductor – materials that conduct electricity under some conditions, and block the flow of electricity under other conditions. LEDs or Light emitting diodes emit light when current is passed through them. The symbol for LEDs is similar to that of a normal diode but also has a few arrows pointing away from the diode to indicate that it emits light. The schematic symbol for a diode reminds us of the question we discussed earlier: Which way does current flow? When placed in a DC circuit, the diode must be placed as shown in figure on the left for current to flow through it. Conventional current, flowing from the positive to negative terminals of the battery, passes through the diode in the direction of the arrow. Electrons, flowing from the negative battery terminal to the positive terminal, flow against the arrow. This comes back to the argument between conventional current flow vs. electron flow theories. Let’s look at our little lamp circuit again. If we were concerned with preventing current from flowing backward through the lamp, we could insert a diode in the circuit: As shown above, the lamp will light as current flows through the circuit. The only noticeable effect of adding the diode is that the lamp will be a little dimmer, due to the voltage drop (usually 0.7 volts) across the diode. If we reverse the diode (or reverse the battery), essentially no current will flow through the circuit. Of course the only reason for putting a diode in a DC circuit is if the circuit is more complicated than ours, with the diodes used to guide current flow to the components – or if a component could be damaged by current flowing the wrong way through it. We will discuss more applications of diodes such as rectification in later lessons.


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Capacitors

Capacitor symbol

A capacitor (formerly known as condenser) is a passive

two-terminal electrical component used to store energy

in an electric field.

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Capacitors Capacitors come in many types, but the basic premise is that two metal plates separated by an insulator. When connected to a power supply, electrons are removed from the plate connected to the positive terminal, and deposited on the plate connected to the negative terminal. Suppose it was important that the lamp in our little circuit burn with the same brightness even if there was a temporary voltage drop from the power supply. For example, voltage might drop if other components in the circuit are suddenly turned on. One way of solving this problem would be to place a capacitor in the circuit. A capacitor is a device that stores electricity – it has a “capacity” to hold an electrical charge. The buildup continues until the voltage potential difference across the capacitor terminals = the voltage of the power supply. At that point the current stops flowing in the circuit. The “capacitance” of a capacitor is determined by the size of the plates. The larger the plates’ surface area, the more electrons can be stored on them. Capacitor size is measured in Farads. Capacitors are also rated by voltage. The voltage rating indicates the point at which the dielectric insulator material breaks down and becomes a conductor, causing the capacitor to be shorted out (current flows straight through).

Properties of capacitors

The thickness of the insulating layer (also called the dielectric) and the specific material used, determines how much voltage potential difference the capacitor can maintain across its terminals. Note that the potential difference is not the same thing as the number of electrons stored on the plate.

A capacitor can operate at any voltage at or below its rating. If it’s used at or near its maximum rated voltage, the life of the capacitor may be shortened.

Once a capacitor is charged up, it can be removed from the circuit and still retain its charge. Connecting a charged capacitor to a circuit is the same thing as connecting a battery of the same voltage – for as long as the capacitor charge lasts. The larger the capacitor’s physical size, the longer it can power a circuit at any given rate of current draw.


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Charge/Discharge rates of capacitors

Uses for Capacitors make up for voltage drops suppress voltage surges provide bursts of high voltage filter pulsating DC

A capacitor can’t hold its charge forever. Electrons gradually “leak” across the insulator until the potential difference is reduced to zero.

The electrons that build up on the capacitor plate are an electrostatic charge – very much like the static electricity we talked about earlier. Touching the two leads of a charged capacitor together is exactly like touching a doorknob after scuffing your feet on a carpet- but in some cases the charge in the capacitor may supply much more current than the typical household static discharge. Some capacitors can store a lethal charge, and extreme caution should be used when handling capacitors or circuits with capacitors in them.

Capacitors charge and discharge at an exponential rate. For engineering and design purposes, the charge/discharge rate of a capacitor is an important consideration.

Inductors

Inductor symbol

An inductor (or reactor or coil) is a passive two-terminal

electrical component used to store energy in a magnetic

field.

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Inductors An inductor is a device whose behavior is opposite to that of a capacitor (it “resists” changes in current while passing steady-state or DC current). An inductor resembles a spring like coil. While capacitors store energy in two plates inductors store its energy in the magnetic field that it generates. We will look at uses of inductors in smoothing circuits in Lesson 7. The figure shows some examples of low-value inductors that are commonly used.

Hydraulic Accumulator Analogy to a capacitor

Capacitors serve the same purpose as an accumulator does in a hydraulic system. When the hydraulic pump

starts working, oil not used by the system goes into an accumulator under pressure. An accumulator can store

a certain amount of energy, depending on its size, just as the size of a capacitor determines how much static

charge it can hold.

When the accumulator is full it just sits there, neither absorbing more energy nor outputting any energy. But

if the hydraulic system suddenly calls for more pressurized oil than the pump can deliver, the accumulator

discharges its stored energy at a rate demanded by the system.

When there is again low demand by the system, the accumulator will refill and await the next needed burst of

power.


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Circuit Breakers

CB symbol

A circuit breaker is an automatically operated electrical

switch designed to protect an electrical circuit from

damage caused by overload or short circuit.

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Circuit Breakers A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Relays A relay is basically an electro-magnetic switch that opens or closes, depending on whether the solenoid is energized or de-energized. When the coil is energized it will open/close the contacts related to the coil. There are normally-open relays, and normally- closed relays, the difference between the two being the response to a control voltage input energizing the coil: the first closes the circuit, and the second opens the circuit respectively.

Q: Why Bother Using Relays Instead of Simpler Switches?

A: One of the main uses of relays is to provide a measure of remote control. A low voltage – low current circuit can

control the solenoid, which may be located some distance from the user switch or control device. The circuit that is

controlled by the solenoid may be very high voltage – high current, and the use of the relay keeps dangerous voltages or

currents away from the human user.

For example, consider the headlight switch in your car. The headlights draw a lot of current and need pretty large

diameter wires. Besides being hard (and more expensive) to run many big diameter wires to the dashboard, it is safer to

use low current control voltages in the vicinity of the operator.


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Internal structure of a linear solenoid

Solenoids Another electromechanical device used to control systems or circuits is a solenoid. Like a relay, the solenoid uses electro-magnetism to create physical movement. It converts electrical energy to mechanical energy. There are two main types of these devices, namely linear and rotary solenoids. We will focus on the linear solenoid in this section. A linear solenoid consists of a conducting coil wrapped around a movable steel or iron slug (plunger). When the coil is energized it turns into an electromagnet, thus it pulls in the plunger. When the coil is de-energized and the magnetic field is turned off, the plunger is returned to the original position by a return spring. Relays and solenoids are electro-mechanical devices that operate by converting electrical energy to mechanical energy. Today, some of these devices are being replaced by semiconductor devices like transistors and microprocessors.

In the next section we talk about transformers. Transformers operate using the principal of mutual induction which is only sustained in Alternating Current (AC). We will be discussing AC in detail in Lesson 6. If you find any of the material confusing please revisit the transformers lesson again once you have learned about AC.


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Transformers

Transformer symbol

A transformer is a device that transfers electrical energy

from one circuit to another through inductively coupled

conductors—the transformer's coils

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Transformers Transformers use mutual induction to transform an input voltage into either a higher or a lower output voltage, as desired. Creating a transformer A transformer is usually created by winding two separate coils onto an iron core. The primary coil (winding) is connected to a source of AC power.

How a transformer operates When power is applied to the primary coil, it creates a magnetic field in the core. The core, which is generally built of laminated pieces of iron, becomes an electromagnet that goes “all the way around” without having a north or south pole. In a typical transformer there is no electrical connection between the primary and secondary coils. The voltage in the secondary winding is created strictly by mutual induction. We discussed mutual induction in lesson 4.

Factors that affect the strength of the transformers core magnetic field The strength of the magnetic field generated in the core is the result of:

The number of turns in the primary winding

The number of amps of current in the coil.

The permeability of the core material. The transformer core must be closed for maximum efficiency. If the core is open-ended (like an electromagnet), some of the magnetic flux lines created by the primary winding are lost. The amount of transformer core material must be sufficient to prevent it from being magnetically saturated at full load. Saturation Loss As current in the primary winding increases, the number of flux lines in the core also increases, up to the point where the core is magnetically saturated. (Remember this from the lesson on Magnetism.) Once saturation is reached, adding current to the primary winding will not result in a stronger current being induced in the secondary winding unless the core was made larger or a more permeable material were used. Efficiency of Transformers Depending on their construction and when operated within specifications, transformers are 90% to 98% efficient. That is 90% to 98% of the input energy to the transformer becomes output energy. The lost


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energy is mostly in the form of heat. When you see large transformers outside a building or at a utility substation the transformers have fins (and sometimes fans as well). Even at 90% efficiency, the 10% can generate a lot of heat- especially when there may be thousands and thousands of amps involved. Factors that affect the Output Voltage of the Secondary Winding As the magnetic field rises in the core, it induces a current in the coils of the secondary winding. This is the output current. The output voltage depends on the following factors: The strength of the magnetic field. The number of turns in the secondary winding. The speed with which the magnetic field rises and falls (the frequency of the AC input) (the rate of flux

change in the core).


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Lesson 6: Alternating Current

Objectives

Understanding Alternating Current(AC)

Learn how to generate AC

Properties of a Sine wave

How a transformer works

Rectification

Filtering


Alternating Current (AC)

Alternating current (AC) operates with current flowing

back and forth in two directions in a cyclical manner.

The voltage and current output of the terminals on an AC

power source alternate between positive and negative

polarity.

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Alternating Current A direct current (DC) circuit operates with current flowing in just one direction. A DC battery or power supply always has one positive terminal and one negative terminal. Electrons are driven through the circuit from the negative terminal to the positive terminal by the battery’s voltage.

Alternating current (AC) operates with current flowing back and forth in two directions in a cyclical manner. The voltage and current output of the terminals on an AC power source alternate between positive and negative polarity.

Throughout an AC circuit, current flows first in one direction, then reverses itself and flows in the opposite direction. This current reversal occurs continuously whenever the circuit is powered up. Remember what we learned about the transmission speed of a pulse of current in a conductor- it travels instantaneously. When current flows in the negative direction in a circuit, electrons move in the same direction throughout the circuit. When the pulse flows in the opposite (positive) direction, electrons flow in the opposite direction. Every component in the circuit continually has electrons flowing in first one direction, then the other.

The electrical service that comes into your home is AC. Your home appliances, electric lights, and everything you plug into a wall socket operate on AC. Small household items may use DC – especially devices such as Laptop computers, portable radios


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or CD players that you can run either by plugging into the wall or with batteries. Any device that you plug in via an AC adaptor (like a cell phone charger which converts ACDC) probably uses DC voltage to operate. Also, RVs (recreational vehicles) often have specially-made appliances designed to use direct current because the primary source of electric power inside an RV is often the vehicle’s battery system. It’s hard to see why alternating current is a good idea, isn’t it? It seems so much simpler to use direct current where you’ll always have a defined positive and negative side of the circuit, and you’ll always know which way current is flowing.

Benefits of AC

Alternating current is the natural output of rotating a

coil through a magnetic field, it makes sense that we

would use electricity in the form that we generate it

in.

Transmission & Distribution

Ease of voltage conversion

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Benefits of AC In the early years of electrical system development, George Westinghouse (with the aid of Nicola Tesla) were disciples of AC and fought a prolonged, bitter war against proponents of DC (namely Thomas Edison) in what was known at the time as the “War of the Currents”. Remember, at this time the electrical infrastructure of the US and the rest of the world was in its infancy and there were vast fortunes to be made by those who produced the global standard. There are many other reasons why an AC system is superior to a DC system

Alternating current is the natural output of rotating a coil through a magnetic field; it makes sense that we would use electricity in the form that we generate it in.

Transmission

Distribution

Ease of voltage conversion and

Ability to produce a rotating magnetic field As we’ll see, nearly all household and commercial electricity is generated by the electric utility: using enormous generators with coiled conductors passing through powerful magnetic fields at high speed creating vast amounts of AC power.


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Generating AC in a conductor Like we discussed before in the principles of magnetism lesson if you pass a conductor through a magnetic field you will generate current flow in that conductor. This diagram illustrates a simple, two pole rotor generator with a single winding, placed between two magnetic poles.

The detailed mechanics of how this happens go something like this: 1. As the conductor moves downward through the magnetic field, cutting the magnetic lines of force (A),

a positive-going current is generated. 2. When the conductor is parallel to lines of magnetic force (B), the generated positive-going current is at

a maximum. 3. As the conductor moves downward cutting the lines of magnetic force (C), a negative-going current is

generated in which the current decreases from the maximum to zero. 4. The conductor then moves upward cutting the lines of magnetic force generating a negative-going

current until it reaches a negative maximum when the conductor is parallel to the lines of magnetic force (D).

5. Finally, as the conductor continues to move upward cutting lines of magnetic force, a positive-going electric current is generated (E).

Notes:


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Factors that determine the voltage that will be created in a conductor

No. of turns in the coil.

Strength of the magnetic field (no. of flux lines that

are cut through)

Speed of the relative motion between the conductor

and the magnetic field.

Proximity of the conductor to the magnetic field.

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Points to ponder on AC Generation

In the same way that winding a conductor into a coil increases the magnetic field that is created when a current is run through the conductor, winding a coil increases the amount of current produced when the conductor passes through a magnetic field. As a segment of each coil passes through the magnetic field, a little bit of current is created in each coil. The resulting electromagnetic pulse adds up so every winding of the coil adds to the created current.

The critical point is that the conductor has to cut across the lines of magnetic flux. If the conductor moves in parallel with the flux lines, not much happens. Like a waterwheel, to be most effective, you need the water hitting the blades head on.

There has to be relative motion involved. Remember that there is no flow in the lines of flux. Either the conductor has to move through the magnetic field, or the magnetic field has to move so its flux lines cut through the conductor.

Note: There’s a case when it’s not necessary for either of the components (conductor or magnet) to move. If you turn an electromagnet on and off repeatedly, the rise and fall of the resulting magnetic field does the “moving” for you.

The speed of an electrical impulse traveling through a conductor is instantaneous – a little bit of current created in different parts of the conductor is the same thing as a larger current being introduced at the end of the conductor.

Closer the conductor is to the magnetic field the larger the voltage generated in the conductor.

Direction of current flow The polarity of the magnet and the direction of the conductor’s motion through the flux lines determine the direction of induced current flow through the conductor. This is the same as for electromagnets – the direction of the current flowing through the coil determines which end will be the N pole and which will be the S pole. If you can put together a mechanical device where a coil and magnet pass one another repeatedly, you will have a Generator. All you need are the three components: a coil, a magnetic field, and relative motion in close proximity between the two.

Q: Why are we talking about the voltage that will be

created, and not the current?

A: Because the current is a function of the load of

the circuit. Voltage is the inherent quality of the

power-producing component.


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Sine wave form

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R S Voltage Peak Voltage

Frequency The waveform shown here is the output from any rotating electrical machine with construction similar to what we have discussed so far. Per this diagram, note that there are 360 electrical degrees in one cycle. In the US and in many other countries, the standard AC frequency is 60 cycles per second (or Hertz- abbreviated Hz). In other parts of the world, the standard frequency is 50Hz. Peak-to-peak Amplitude In the diagram above the maximum values of the waveform are +1 and -1. These maximum values are associated with the “peaks” of the waveform and thus are known as peak values. The real peak to peak value would be 2 volts for this waveform. RMS Value In the waveform above we know that the peak value is 1 (or -1) but that doesn’t mean that the value is continuous. On the contrary, the value is always changing over time. To keep things straight, we need a value that takes any point in time into account and gives us a constant. The mathematical functions of Root Mean Square (RMS) give us this meaningful value. In general, you could take the peak value and multiply it by .707 and you would get a good RMS approximation.

For example the voltage coming out of the socket in your home is around 120VAC RMS. If you wanted to obtain the peak value you could take the inverse of .707 (which is 1.414) and multiply it by 120. This would give you about 169.7 volts!

Notes:


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Three phase power

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Phase The waveform above is a single phase sine wave. That means that there is only one source of potential. For many applications this waveform is perfectly acceptable. The devices in your home work just fine when you plug them in. Engineers who design the appliances you use every day have taken some of the short comings of single phase power into account, and you are unaware of any issues with the AC power. If you look closely at the waveform, you will notice that at the 0 degree point and at the 180 degree point, that there is no potential. At these moments in time, there is in fact no energy at all and your appliance is essentially turned off. Granted, the moment is very brief- but it’s an outage just the same. In many industrial applications, this interruption is simply intolerable. The solution to the problem of interruptions and lack of rotating field capability of single phase power is to supply three phase power instead. The figure shows three distinct sine waves- three distinct sources of potential, with 120 electrical degrees of separation between each phase. It’s important to realize that these wave forms really are electrically isolated from each other and that there is a difference in potential from phase to phase. The nice part about three phase power is that at any point in time, the algebraic sum of all three phases is equal to the source voltage. There is no point in time where power is unavailable.

Ohm’s Law for AC Circuits

In general, resistive circuits behave the same with

AC as they do with DC and the Ohms law formulas

In the electrical world, any circuit can be broken

down into one of three types of circuit:

– Resistive circuit

– Inductive circuit

– Capacitive circuit

Inductive and capacitive circuits also show an

opposition to current flow, its referred to as

impedance.

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Ohm’s law for AC Circuits In general, resistive circuits behave the same with AC as they do with DC and the Ohms law formulas learned in earlier sections of this course apply to either form of current. There are special means of calculating the opposition to current flow for devices other than resistors. In the electrical world, any circuit can be broken down into one of three types of circuit:

Resistive circuit

Inductive circuit

Capacitive circuit or combinations thereof. As mentioned earlier,

Q: How much current does a generator’s output provide?

A: The sine wave graph we’ve seen depicts the voltage

output of an AC power source, but it can also illustrate the

amount of current drawn by a circuit or load using AC

power.

When connected to a constant load, the circuit will draw

current in an amount proportional to the AC voltage

output at any given moment.

With larger generators there is more copper and more

steel (and a larger prime mover) and thus the more current

it can produce. The size of the coil wires needs to be large

enough to carry the current that could be drawn from the

generator.


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Inductive Reactance (Xl) caused by the action of changing magnetic fields offers opposition to current flow or Impedance (Z). There is such a thing as Capacitive Reactance (Xc) and it too offers opposition to current flow. The opposition to current flow caused by Xc is also termed Impedance (Z).

Transformers only work with AC input

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Importance of AC Power in transformers AC power causes the magnetic field to continually reverse itself; this is why the transformer can only work with AC power. If a DC voltage was applied to the primary winding, a brief voltage spike would be induced in the secondary winding. Once the electromagnetic field stabilizes, the “motion” required to generate an induced current disappears. Therefore a consistent current cannot be induced in the secondary winding using a DC voltage input. But if AC voltage is applied to the primary windings of the transformer, the continual rising, falling, and reversing of the voltage corresponds to exactly the same motion of a coil through a magnetic field as we saw in our alternator example. As the input voltage rises to its peak, the induced magnetic field also rises to its peak, inducing a correspondingly rising and falling current in the secondary winding. (The flux in the core must be dynamic for the transformer to function.) When the input current reverses itself, the direction of the magnetic flux in the core is also reversed, and thus the output voltage “alternates” as well. The magnetic flux generated in the core rise, fall, and reverses itself following the same sine waveform of the AC voltage. An oscillator or a rapidly opening and closing switch provide an AC-like input current to the transformer.


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Look back at Transformers

Uses :Step - up Step - downIsolating voltage spikes and DC “line noise” from AC current.

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Turns Ratio The factor to consider when calculating the output voltage is not so much the number of turns in each winding, but the ratio of turns between the two. For example, if the ratio between the primary and the secondary windings is 2:1, the transformer output voltage will be half the input voltage. This is referred to as a step-down transformer. If the ratio is 1:3 (for each turn of the primary winding there are three turns of the secondary winding), then the output voltage will be three times the input voltage (this is known as a step-up transformer). The secondary winding of a transformer may have taps at different locations in the secondary winding, for the purpose of obtaining different output voltages. Generally, a transformer in a well designed circuit will follow the following equation x. Where “V” is voltage, “N” is number of turns in the winding, and “I” is the current; subscripts “p” stands for primary, and “s” stands for secondary.

Some transformers are equipped with adjustable position taps, allowing user-adjustable output voltage.

The bottom line for transformers Under no-load conditions, they draw minimal current. The more load place on the output, the higher the current draw from the power supply will be. This phenomenon provides a very elegant solution: the transformer draws more current when it is working harder and less current when it is working less hard. Of course, both windings need to have sufficient current carrying capacity for the maximum loads that will be experienced. It is certainly possible to connect the transformer output to a load that either exceeds the

Q: Does the amount of current a circuit draws from the secondary winding affect the primary winding’s current draw?

A: If there is no load connected to the secondary winding, only a small amount of current is drawn by the primary winding

– just enough to generate the magnetic field in the core. This is because the induced magnetic field generates a counter-

current in the primary winding, opposite to the applied voltage. The self-induced voltage is almost as large as the applied

voltage.

When current is drawn from the secondary windings, current flow in the secondary coil creates a magnetic field in the core,

which opposes the magnetic field generated by the primary windings. This causes a reduction in the total flux of the core,

which allows (requires) more current to flow in the primary winding.

The upshot is that if current draw from the secondary winding increases, current in the primary winding increases

proportionally, maintaining the proper output voltage as determined by the windings ratio.

In a step-down transformer, the current increase is proportionate to the ratio difference.

Q: How Much Current Can the Secondary Winding

Provide?

A: It might sound like transformers give you something for

free – taking an input voltage and increasing it to whatever

voltage you desire. But as we learned earlier, there’s no

free lunch with electricity or any type of energy. The cost

of stepping up an AC voltage through a transformer is that

the amount of current that can be drawn from the

secondary winding is proportionally less than the current

drawn by the primary winding. If you double the voltage,

the available current is cut in half. The general rule for

transformers is: “Power in Power out.”

In a step-down transformer, the current increase is

proportionate to the ratio difference


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current rating of the secondary winding, or causes the primary winding to draw more current than it is rated for. (Remember that the primary winding can only draw as much current as the power supply is able to provide.) In general, step-up transformers use a heavy gauge wire for the primary winding and a thinner lighter gauge wire for the secondary winding. Conversely, step down transformers have thinner wire in the primary winding and heavier wire in the secondary winding. Transformers are rated according to maximum input voltage and current, and by output voltage and maximum current draw. The voltage and current ratings are usually expressed as a VA (Volt-Amp) rating that indicates how much current is available at a specific voltage. Uses for Transformers

Stepping up voltage for transmission from power plan to point of use.

“Stepping down” very high transmission line voltages to lower commercial and residential voltages (220 and 120 VAC)

Isolating voltage spikes and DC “line noise” from AC current. Isolation Transformers Isolation transformers can greatly reduce the effect of voltage spikes contained in the source current. A significant voltage spike in the input voltage (transformer primary) may be unnoticeable in the output current (transformer secondary). For this reason isolation transformers are made with an equal number of turns in the primary and secondary windings.

Note: Actually, the lamp will probably appear to be lit all the time. If the AC input is 60 Hz, the lamp receives another half wave of current 60 times each second. This is too fast to see the bulb dimming and re-brightening. This kind of half-wave rectifier is kind of hard on the circuit components too – applying repeated loads to the diode and the lamp, rather than a continuous steady current.

Look back at Diodes

Half-wave rectification

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Half-wave Rectification With each cycle of the AC input, current flows through the diode half the time, lighting the lamp in an increasing and then declining brightness as the sine wave rises and falls. The diode only passes current in one direction. During the other half cycle of the AC input, output voltage is against the diode, preventing current from flowing through the circuit. Voltage still exists in the circuit, for example if you connected an AC voltmeter across the diode, but no current flows and the lamp will not light. The above circuit is called a half-wave rectifier (it only uses half of the AC waveform). You can connect any DC load, not just bulbs. The input can be any AC source: any voltage, frequency and waveform shape, as long as the specifications of the diode and wiring are not exceeded.


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Typical AC adapter for DC loads

9/29/201175

A typical AC adaptor for DC loads A typical AC adaptor power supply for a household appliance (e.g., a cell phone charger or laptop power supply) uses a step-down transformer to reduce the AC voltage, and then rectifies that lower voltage to the desired DC output voltage. As we noted earlier, the step-down transformer draws an amount of input current proportional to the load connected at the DC output terminals. When using a half-wave rectifier, there is no DC voltage production at all half of the time, and there are gaps between the DC peaks. A better DC output can be obtained by using a full-wave rectifier. As the AC power supply runs, the resulting DC output has an effective voltage of about 45% of the RMS value of the transformer’s output.

A bridge rectifier circuit

Bridge rectifier input and output

Full-wave (Bridge) Rectifier A full-wave rectifier uses the entire AC wave to create DC output – hence the name, “full wave.” The full-wave rectifier contains four diodes which direct the two halves of the AC input to the DC output terminals. The DC output voltage from a full-wave rectifier is shown on the left. With both the positive and the negative parts of the AC waveform being converted to DC, there are no gaps where no voltage is being produced. Also, the DC peaks are much closer together than in the half-wave rectifier, so the DC voltage is smoother. Because the AC input passes through two diodes instead of one, the voltage drop across the full-wave rectifier is about 1.4 volts, instead of the 0.7 we saw in the half-wave rectifier. Nonetheless, because twice as many DC waves are created for each AC cycle, the effective voltage output is about 90% of the AC wave’s R S value. Of course diodes have voltage and current ratings. If subjected to excessive voltage or current draw, the diode can burn out. If excessive voltage is applied backwards to the diode it can be short circuited. Again, it is the engineer’s job to select diodes with the proper ratings and safety margin for a given application. The 0.7 voltage drop across each diode can generate a lot of head when used in a power supply. This is why any power supply is


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Effective voltage from full wave rectification

likely to feel warm to the touch when running.

Filtering The DC ripple that we saw produced by the half-wave and full-wave rectifiers may be acceptable for some circuits (like our little lamp), but most DC applications require a higher quality voltage – one that doesn’t bounce around so much. There are two ways to improve the quality of the DC output from a rectifier: Add a capacitor Add an induction coil

Filtering

Adding a Capacitor to a half wave rectifier

9/29/201178

Filtering by adding a capacitor to a half wave rectifier A very common use for large capacitors is filtering the “ripple” voltage out of the DC output of a power supply. A polarized capacitor must be used and installed correctly in parallel with the power supply’s output terminals, after the rectifier, as shown: When the rectified DC half-wave output begins rising from zero, the capacitor starts absorbing a charge. After the DC voltage peaks and begins declining to zero, the capacitor starts releasing its charge into the DC part of the circuit. The effect of this repeated charging + discharging of the capacitor is to add DC voltage to the circuit during those parts of the cycle when the DC voltage from the rectifier is lower than the effective voltage output. The voltage output (shown by the red line) illustrates that DC voltage supplied to the load even when the output from the rectifier is zero. The downward slant of the red line indicates that voltage output from the capacitor declines between cycles.


64 | Cummins Inc

Filtering

Adding a Capacitor to a full-wave rectified circuit

9/29/201179

Filtering by adding a capacitor to a full wave rectifier Notice that the ripple is less pronounced – the DC output is “cleaner” – in the full-wave rectified output than in the half-wave rectified output. Capacitor Sizing To perform properly in a rectifier/power supply, the capacitor has to have an appropriate voltage rating, and it has to have a large enough capacity so that: It collects the required amount of charge during

the time the AC sine wave is rising. It discharges enough, over a long enough period

of time, while the sine wave is declining, to be useful in adding DC voltage to the output when needed.

Filtering

Adding an inductor

9/29/201180

Filtering using an Inductor An induction coil – a wound coil with an iron or ferrite core – may be placed in the rectifier circuit, either before or after the capacitor to provide additional filtering of the DC output. An induction coil opposes any sudden change of voltage As the DC output from the rectifier rises and falls, the induction created in the coil smoothes out the change, tending to keep the voltage from changing as much as it otherwise would. The amount of induction created in the inductor depends on the number of turns in the coil and the permeability of the core material.

Notes:


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Lesson 7 : Alternators

Objectives

Understand the construction and function of an Alternator

Understand the difference between a rotating field vs. rotating armature design

Understand Brushed vs. Brushless Alternators

Understand Self-excited vs. Separately-Excited Alternators


Basic Alternator Components and operation The alternator is the component in a genset that converts the rotational mechanical energy into electrical energy using the ideas of current induction that we discussed in the principles of magnetism. There are many components in an alternator but the two main features are the: Rotor: the rotating part of an alternator, it has copper coil windings. Stator: the stationary part of an alternator, it also has copper coil windings. Operation Depending on the type of generator (rotating field, or rotating armature), one of these components (rotor or stator) will be magnetized. We will discuss the difference between rotating field, and rotating armature shortly. Since the rotor is connected to the engine, there is a relative rotary motion between the rotor and the stator. This generates AC electricity since we have all three items necessary for a voltage to be induced in the conductor, namely, a magnetic field, relative motion (cutting the magnetic flux lines), and a conductor.

Examples of some Alternators from Cummins Generator Technologies


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Typical Construction

Rotor: The rotating portion of any piece of electrical equipment.

Stator: The stationary portion of any piece of electrical equipment.

Field: The part that generates a magnetic field.

Armature: The part that gets current induced in it.

Winding: Is a continuous piece of wire (usually copper) that is

wound around a steel assembly- there are stator and rotor windings.

Excitation: The process of getting electrons to flow in a conductor.

Exciter: The component that produces excitation (can be a rotor or

stator).

9/29/201183

Typical Generator Construction In the examples that we’ve seen previously, a conductor was moving in the presence of a magnetic field causing current to flow. The premise of relative motion between a conductor and a magnetic field causing current flow should be established in your mind by this time. If it isn’t, go back and review the section on Inducing Current in a Conductor. (Lesson 4)

Notes:

As we learn more about the construction of generators (and electric motors) it’s important to remember a few terms:

Rotor: The rotating portion of any piece of electrical equipment.

Stator: The stationary portion of any piece of electrical equipment.

Field: The part that generates a magnetic field.

Armature: The part that gets current induced in it.

Excitation: The process of getting electrons to flow in a conductor.

Exciter: The component that produces excitation (can be a rotor or stator).

Winding: Is a continuous piece of wire (usually copper) that is wound around a steel assembly- there are stator and rotor windings.

As we study the basics of generator design, you should refer to the terms above until you have them thoroughly understood in your head.


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Rotating Armature vs. Rotating Field

vs.

9/29/201184

Types of alternators In the world of alternators, there are two prevalent designs that are commonly used:

Rotating Armature

Rotating Field. Beyond that, there are two excitation systems that are typically employed:

Brushed

Brushless We will discuss the rotating field- brushless design in greater detail than the others, simply because it is by far the most widely used design. Not only do Cummins and its competitors use this design, the public utilities (at their generating stations) use it too.

Notes:


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Rotating Armature Brushed Design Let’s start with the simplest design, a rotating armature design.

DC is fed into the stator of the generator producing a magnetic field

The rotating armature windings get current (AC) induced in them.

To deliver this current to a usable point, the rotor must be equipped with “Slip Rings”. The slip rings are rings of copper that are connected to the end of the windings.

Since the slip rings are constantly in motion, brushes are required to transmit the current to a stationary point where it can be connected to a load. The brushes are usually made of carbon and are held tightly against the slip rings by springs.

Design pros and cons: The design of this type of generator is simple but suffers from the fact that the slip rings and brushes are wear items that need to be frequently maintained. Additionally, in applications where larger currents are involved, the slip rings and brushes need to be quite large and are susceptible to failure due to arching. This design was common in the early days of electrical power production but has decreased in popularity due to more reliable designs like Rotating Field machines.

Notes:


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Rotating Field-Brushed design In a rotating field design, the stator windings get current induced in them by a rotating magnetic field.

The rotor windings are fed a DC (through brushes and slip rings) producing an electromagnet with north and south poles.

The current flow into the windings of the rotor is known as Excitation Current. In most cases the excitation current is relatively small compared to the amount of current that the output of the stator can produce and thus the wear on the slip rings due to arcing is greatly reduced.

The of N-S-N… poles of magnetism in the rotor, rotating in the presence of the stator windings produces an AC output in the stator windings.

The stator windings connect directly to the load- eliminating any losses and maintenance issues that would have been present with a rotating armature design.

Design pros and cons: The slip rings and brushes still remain and they are still a maintenance consideration. But they will wear out a lot less since the current transmitted through it is much less than in a rotating armature design.

Notes:


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Rotating Field-Brushless design (self-excited) The design shown above is a Rotating Field-Brushless design. In most cases this design is known simply as a brushless system (the rotating field being implied). This example includes a PMG which we will discuss shortly. Let’s focus on the components

In this design, there are two stators- the Exciter Stator and a Main Stator.

There are two rotating components as well- the Exciter Rotor and the Main Rotor.

The rotors are mounted on a common shaft. The shaft is supported by a bearing on one end (near the exciter rotor) and by the flywheel on the other end. There are designs that utilize two bearings as well.

The drive disks fit into a recess machined into the flywheel where they are bolted in place.

This arrangement is known as a Single Bearing alternator. Realize that the flywheel, the bearing next to it and the crankshaft are all part of the engine (or prime mover) and not part of the alternator. Not shown are the housing of the alternator or the full construction of the stators (which would encompass the rotors).

Each section of the genset will be introduced as we talk about the sequence of operation. Note: This is the culmination of everything we have learnt so far, feel free to ask the instructor if anything is confusing or not clear. At rest, when the rotor is not spinning, there is no current flow anywhere in the alternator. A small

amount of residual magnetism remains in the exciter stator. This means that even though the windings of the exciter stator have no current flowing through them, there is still some magnetism present in the steel.


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Technical Training

Self-excited alternator start-up

12/1/201189

Voltage

Time

~70V

t1

Operating

Voltage

t2

Self-excited

At t1AVR starts supply voltage

back to the exciter stator

Voltage generated gets bigger

until it reaches the operating

voltage, at t2 at which point

the AVR starts to regulate

Self-excited alternator start-up

At t1AVR starts supply voltage back to the exciter stator

Voltage generated gets bigger until it reaches the operating voltage, at t2 at which point the AVR starts to regulate

Excitation system : Exciter stator

N S N S

From A.V.R Output terminals

Exciter Stator Operation

The exciter stator iron core has some retentivity and it stores a fraction of magnetism from prior operations. This is known as the ‘residual magnetism’. This residual magnetic is used during initial startup.

Exciter stator purpose The exciter stator is the source of the magnetic field in the exciter system.

Excitation system : Exciter stator

Wound Exciter Stator – P7 type

Exciter Stator Typical Construction

High-Remanence(stores Residual Magnetism) steel core

Copper windings

10 - 14 pole magnet field (varies)


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SHAFT

W U

W

U

V

V

•

Excitation system : Exciter rotor

Exciter Rotor Operation When in operation, the rotor (driven by the

prime mover) turns. The exciter rotor windings pass through the small residual magnetic field in the exciter stator causing a current to be induced.

The current induced in the exciter rotor windings is AC (since the poles of the rotor are moving past a fixed magnetic field).

Excitation system : Exciter rotor

Exciter Rotor – UC type

Similar in principal to the rotating field design we discussed previously, the objective of this design is to create a consistent N-S-N… relationship in the poles of the main rotor. If we were to feed the current from the exciter rotor (AC) into the main rotor directly, we would have a constantly changing polarity. This situation is undesirable.

The exciter rotor generates AC, to create a stable DC power supply to the main rotor; this AC needs to be rectified.

Rotating rectifiers are used to achieve this.

SHAFT

Varistor

(Surge Suppressor)

3 Phase A.C Input

from Exciter Rotor.

(Insulated Terminals)

Aluminium

Heat-sinks

Split Two-Piece

Rectifier Hub

Rectified output

to Main Rotor

Excitation system : Rotating Rectifier

Rotating Rectifiers What we need is a direct current (DC) to be fed into the main rotor windings. Lucky for us, there is a means of converting AC to DC. By using a device called a Rotating Rectifier (consisting of Diodes) we can provide direct current to the main rotor from the exciter stator. Rotating Rectifiers are mounted to the exciter rotor. There is a set of rectifiers that provides positive potential and a set of rectifies that provides negative potential.


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3 Phase full wave rectification produces a D.C output 1.35 X A.C input voltage

3 phase A.C Input to Rectifier - (175Hz to 210Hz for 14 pole Exciter)

D.C output to Main Rotor

+

-

Excitation system : 3 phase rectification

3 phase rectification The rotating rectifier comprises of diodes that rectifies the 3 phases of AC voltage into a smooth DC voltage. Generally, 3 phase full wave rectification produces a DC output of 1.35 x AC input into the rectifier. As you can see from the last plot the rectified voltage is much smoother.

SHAFTD.C

Main Rotor

Main Rectifier

Main Rectifier Main Rotor connections

Main Rectifier Main Rotor Connections • The Rectifier Output is a smooth D.C Supply

across the Aluminium Heat Sinks • This is fed to the Main Rotor windings


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Excitation system : Main Rotor

Main

Rotor

Notes:

Main

Rotor

Excitation system : Main Rotor

Notes:

SHAFT

S

S

NN SHAFT

S

N

4 Pole2 Pole

SHAFT

N

6 Pole

Main Rotor Poles & Frequency

Generator Frequency (HZ) = Speed (N) X Number of poles (P)

120

120

PNf

Main Rotor : Number of Poles If you want to obtain AC voltage at a specific frequency from a generator, the required shaft speed (rpm) is determined solely by the number of poles in the generator. Using the equation below, one can easily determine the number of poles in a machine if the rotational speed is known. Conversely, if the number of poles is known, the rotational speed can be determined.

Where f is the Frequency, P is the number of poles and N is the rotational speed. Typically, the engine’s output shaft is connected directly to the generator’s input shaft (1:1 speed ratio). The drive could also be geared up or down


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Main Rotor Poles & Frequency

f frequency

N rotations per min

P # of poles in the rotor

120

PNf

FrequencyNumber of Poles

Speed

Hz # RPM50 2 3000

50 4 1500

50 6 1000

60 2 3600

60 4 1800

60 6 1200

for a specific application (just like your car’s transmission.) Effect of Changing Speed The most pronounced (and immediate effect) is

a change in frequency. As the rate of change in lines of flux being cut goes down the (by the change in prime mover speed) the frequency drops.

Generally the rotation speed is kept at a constant in generators, to maintain a constant frequency.

Laminated Steel core

Main Stator

Output Leads

Copper Windings

Main Stator The direct current in the main rotor provides us

with a rotating field that in turn causes an AC current to be induced in the windings of the main stator.

As the prime mover approaches normal operating speed, the windings of the main stator cut the lines of flux on the main stator with greater speed, resulting in higher a voltage being produce in the stator.

Main Stator Construction

Main Stator Typical Construction • Core built from high grade Electrical Steel, to

reduce Iron losses (heat). • Each lamination is electrically insulated to

minimise Eddy Currents in the core. • 12 Ends out Re-connectable, 6 Ends out Star /

Delta, or Dedicated Windings. • Class ‘H’ Insulation as standard, 125 º C

Temperature rise in 40ºC Ambient.


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Strengthening of the excitation magnetic field Some of the voltage generated from the main stator is tapped and fed into the AVR (Automatic

Voltage Regulator). The AVR takes AC voltage from the main stator, rectifies it and sends DC current to the exciter stator.

This current enhances the magnetic field (which was originally only a small residual amount) providing more lines of flux that are cut by the exciter rotor. The result is more current being delivered to the windings of the main rotor which in turn creates more lines of flux that are cut by the main stator.

This “build up” continues up to the point where the AVR starts to regulate. It does this by sensing the input (from the main stator) and cutting back (or increasing) on the output

(to the exciter stator) until a normal operating threshold is achieved. The regulation process continues as long and the alternator is producing power.

Notes:

Notes:


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Pros and Cons of Design This type of system has no slip rings or brushes. Consequently, there are none of the maintenance issues that would be present with a brush type system. In addition to being brushless, this system is known as a “Self Excited” design. It gets its name from the fact that the excitation current is derived from the output of the main stator, and not from an external source. The excitation system is dependent on residual magnetism, thus the time it takes for the generator to attain normal operating conditions may be longer than usual. This doesn’t occur in a P G excitation system.

Notes:

Rotating Field-Brushless design (Separately-excited)

Q: Why wouldn’t you just crank the magnets up all

the way, all the time, to get the most power

possible out of the generator? Isn’t the extra

output essentially free?

A: Because the load only requires as much current

as it will use. There is nothing free about the

generation. You’d be working the equipment

harder (and using more fuel) for no gain.

Q: Why Would You Need to Adjust the Generator’s

Output?

A: To react to load changes. As more or less

power is drawn by the attached loads, the

generator needs to supply more or less power to

meet the demand (within operational limits).


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A separately-excited system (or PMG excited) has an additional permanent magnet (PMG) rotor attached to the shaft. When in operation, the PMG rotor (driven by the prime mover) turns, this induces an AC voltage on the PMG stator. This acts as a source of power to the AVR. In a PMG excited genset the PMG is the main source of power to the AVR, not the main stator. However the main stator has AC Sensing lines that are connected to the AVR. These sensing lines are just there to help the AVR regulate voltage output from the alternator.

PMG excited gensets are very useful: When a load is applied to any generating system, there is a tendency for the output voltage to drop in proportion with the size of the load. That is, the greater the load the greater the voltage drop. The AVR we mentioned earlier does a nice job of detecting these drops and it increases the excitation current in an effort to compensate for the falling voltage. In many situations the self excited system is perfectly adequate and no real problems exist when the system is heavily loaded. Why do we need PMG excitation? There are certain loads that do present difficulties to self excited systems. One of the most difficult types of load for a genset to “pick up” is a motor load. Large motors present a nearly instantaneous demand for current when they are connected. In cases where the motor size is large enough, the output voltage of the alternator will drop to the point where the AVR cannot compensate. Remember, the internal electronics of the AVR are powered by the output voltage from the alternator. This voltage drop is so severe that the AVR shuts off (because the input power to it isn’t high enough to keep it operating, generally around 70V). Without the AVR there is no excitation current and the output from the alternator falls to zero. This condition is known as a “collapsed field”. Since the voltage drop was the reason the AVR shut off, wouldn’t it be nice if the AVR was powered by an external source (and was still able to sense the output from the alternator)? That way, the AVR would keep working no matter what the voltage output from the alternator was. Well, sure enough, that’s exactly what the PMG (Permanent Magnet Generator) does. It’s a small, totally independent generator whose sole function is to provide power to the AVR and the excitation system. The AVR senses the voltage


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from the main stator while the PMG provides power to the internal electronics and provides the actual excitation current. In this application, the AVR senses and adjusts excitation (it regulates). Cost and availability PMG excitation systems are standard on Cummins commercial gensets (around 500kW and up) and are optional on smaller commercial units. Because the PMG presents an extra cost, they are usually not found on consumer equipment. The same can be said for brushless excitation systems; they are commonly found on commercial generators, but not normally found on consumer equipment.

Technical Training

Separately-excited alternator start-up

12/1/2011123

Voltage

Time

Operating

Voltage

t2

Self-excited

Separately-excited (PMG)

t3

The PMG powers the

AVR directly thus, it

magnetizes the exciter

stator immediately.

Operating voltage is

reached much faster

than with a self-excited

genset

t3< t2

Separately-excited alternator start-up The PMG powers the AVR directly thus, it magnetizes the exciter stator immediately. Operating voltage is reached much faster than with a self-excited genset t3< t2


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Ceramic

Permanent

Magnets

Permanent Magnet Generator Assembly (PMG)

Notes:

Rare Earth

Permanent

Magnets

Permanent Magnet Generator Assembly (PMG)

Notes:

Typical 12 Wire Re-connectable Terminals – UC type

Main Stator output terminal

Main Stator output terminal

12 wire re-connectable enables Cummins/customers to change voltages or 3 phase to single phase.

Voltage ranges are strictly controlled within the specified limits of the stator winding.


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Types of three phase power Three-phase power is required for almost all industrial and high power commercial equipment. There are many economic benefits that make three phase power attractive for this type of equipment. Transformers, filters, power supplies, and conductors can all be smaller and less costly with a three phase system than with a single phase system of equivalent power. In the United States, there are two main types, or topologies, of three phase power. These are called Delta connected power, and Wye connected power. There are some similarities between Delta connected power and Wye connected power, and many differences. It is important to understand these two varieties of three phase power in order to properly specify power for your critical loads.

Delta Connected Power Delta connected power is developed from three, independent transformer or generator windings that are connected head to toe. There is no single point common to all phases. Delta power is named after the schematic resemblance of the windings to the Greek letter Delta (Δ).

Wye Connected Power Wye connected power is developed from three, independent transformer or generator windings that are connected at a common point, called a neutral or star point. Wye power is named after the schematic resemblance of the windings to the Greek letter Wye (Υ).

Delta vs. Wye Power Wye connected power has two different voltages available. The Phase to Phase voltage is the main system voltage (typically 208 VAC or 480 VAC in the United States). The Phase to Neutral voltage is also available, and is typically used for small single phase loads (120 VAC or 277 VAC). Delta connected power only has a single voltage level available: the Phase to Phase voltages. Other voltages can be obtained only by using step-up or step-down transformers.


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Generator Output ratings

Generator output ratings are given in Watts (or kilowatts-

kW).

Remember that Watts are a measure of power – more

useful than Volts or Amps alone because for a given

Wattage there are specific combinations of voltage and

current.

For example, if you have a 5000 Watt residential

generator and you know that the voltage is 240, then the

formula A= W/V would give you 20.8 Amps.

9/29/201194

Generator output ratings Generator output ratings are given in Watts (or kilowatts-kW). Remember that Watts are a measure of power – more useful than Volts or Amps alone because for a given Wattage there are specific combinations of voltage and current. For example, if you have a 5000 Watt home generator and you know that the voltage is 240, then the formula A= W/V would give you 20.8 Amps. One of the key principles to know about generators is that the Prime Mover produces kW and the Alternator produces kVA. This means that the prime mover needs to produce mechanical power for the alternator to produce volts and amps. As a rule of thumb, for any genset the engine is the limiting factor not the alternator. Under extreme loads, the engine will usually stall before the alternator stops producing electricity.

Bearing

Cartridge

N.D.E

Exciter RotorMain Rotor

Rectifier

Fan

Key

Main Stator

P.M.G

Rotor

Exciter Stator

Shaft

P.M.G

Stator

Bearing

Cartridge

D.E

Rotating Field (Brushless)

9/29/201193

The construction of a rotating field single bearing genset


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Power Factor The inductances and capacitances in AC circuits cause the point at which the voltage wave passes through zero to differ from the point at which the current wave passes through zero. When the current wave precedes the voltage wave, a leading power factor results, as in the case of

capacitive loads or overexcited synchronous motors. When the voltage wave precedes the current wave, a lagging power factor results. This is generally the

case with inductive loads. The power factor expresses the extent to which the voltage zero differs from the current zero. Considering one full cycle to be 360 degrees, the difference between the zero points can then be expressed as an angle. Power factor is calculated as the cosine of the angle between zero points and is expressed as a decimal fraction (.8) or as a percentage (80%). It is the ratio of kW and kVA. In other words kW = kVA x PF. Calculating Power

Single Phase (kW) = Volts Amperes PF

1000

Three Phase (kW) = Volts Amperes PF 1.73

1000

To calculate kVA, use the formulas above without the power factor.

Fabricated

Frame

Main Rotor

Main Stator

P.M.G

Rotor

P.M.G

Stator

Drive end

BracketNon drive

end Bracket

& Exciter

Stator assy

Main

Terminals

A.V.R.

Bearing Cap

BRUSHLESS GENERATOR

COMPLETE ASSEMBLY

9/29/201195

Typical construction of a Genset enclosure

Notes:


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Flashing the Field If the generator is disassembled or dropped, the residual magnetism in the main rotor can be reduced enough so the regulator cannot get itself started with the residual voltage out of the main stator windings. Cummins recommends that you use the resistors listed in the following chart, or a light bulb, as a current limiting device when flashing the field of a generator set. •6 Volts 10 Ohms •12 Volts 20 Ohms •24 Volts 40 Ohms DO NOT Flash the field for more than five (5) seconds, or you may damage the regulator or the exciter stator windings. Make sure you have a diode in the field flash apparatus you use to prevent the regulator from overcharging the battery. Batteries can explode when overcharged. The field flash apparatus (shown in heavy lines should be touched to either the exciter terminals at the regulator, or at the exciter stator. Make sure you observe proper polarity when connecting the field flash circuit. Remember, the exciter field circuit is not referenced to ground, so you have to touch both of the exciter terminals to create a current flow in the exciter stator winding.

Notes:


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Lesson 8: Transfer Switches

Objectives

Understand the operation of a typical Automatic Transfer Switch (ATS)

Know the different types of transfer switches

Familiarization with typical TS components

Understand transfer switch operation

Understand the importance of time delays


Transfer Switches

9/29/201198

There are many transfer Switch Types Automatic or manual Enclosed or Non-enclosed. Open Transition or Closed Transition

Technical Training

Basics of a Transfer Switch

The purpose of a transfer switch is to connect the load to any of two

power sources.

10/24/2011133

Basic Operation Remember, standby generator systems (Gensets) are used to provide power when electricity from the power utility is not available or has failed. The automatic transfer switch is the component that connects the loads of the system to utility or to genset. It should be noted that transfer switches can also be used to switch between two gensets, where one or the other supplies the power to the load. The transfer switch monitors utility and emergency standby generator set power. When utility power fails or is unsatisfactory, the control starts the generator set. Most transfer switches have the ability to sense the quality of the power of the generator as well as the utility. If the generator power is satisfactory, the ATS will transfer to the generator. An automatic transfer switch immediately senses when utility power has been interrupted, transferring the load to the generator.


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The transfer panel will sense when utility power is restored. It automatically times for user adjustable period to insure that the utility power is stable. If the power is stable the ATS will retransfer back to utility power. After the load has been retransferred, the ATS will keep the genset running for a cool down period. After the cool down period has expired, the genset shuts down.

Typical Transfer Switch Exterior It should be noted that there are many types of transfer switches available in the industry and they might not all look or function in a similar manner. The OTPC transfer switch has been used extensively in the Cummins family. Now let’s look at some visuals of the OTPC switch.

Typical Transfer Switch Interior Main Contacts: Long-life, high-pressure silver alloy contacts withstand thousands of switching cycles without burning, pitting or welding and provide 100% continuous current ratings Linear motor: For switching mechanism Power Module: with transformers for sensing voltage and supplying power to the digital board, also has some relays. Digital Board: the microprocessor which controls the switch is here Relays: These relays control and operate the linear motor. (also the start signal,load shedding)


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Time Delays Transfer switches have adjustable time delays, to optimize the process of transfer and retransfer between prime power and secondary power. These time delays are in place so that the equipment are not unnecessarily overworked and to make sure that stable power is supplied to the load. TDES = Time Delay Engine Start TDNE = Time Delay Transfer (Normal to Emergency) TDPT = Time Delay Programmed Transition TDEN = Time Delay Retransfer (Emergency to Normal) TDEC = Time Delay Engine Cooldown TDEL = Time Delay Elevator Notes:


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Notes: Open Transition (Break-Before-Make Transfer Switch) A Break-Before-Make transfer switch breaks contact with one source of power before it makes contact with another. It prevents back feeding from an emergency generator back into the utility line. During the time of the power transfer the flow of electricity is interrupted. The OTPC device is such a transfer switch. The linear motor in the OTPC has three switchable positions:

Source 1

Neutral

Source 2 Where the middle setting is neutral, therefore every time the linear motor is switching between the two sources it needs to switch from Source 1 Neutral Source 2, or Source 2 Neutral Source 1. In the neutral stage the load is not connected to either of the sources thus making this a break-before-make type switch.


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Sequence of Operation

Normal Utility Power Mode

Power Outage Occurs

– Normal Emergency

Utility Power Returns

– Emergency Normal

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Normal Utility Power Mode Under normal circumstances when utility power is available, utility power runs through the transfer panel contactors, and connects the power to the distribution panel and then to all the electrical loads in a home. The existing electrical system is not compromised in any way by integrating an automatic standby power system in a home. A battery charger installed in the transfer panel enclosure is powered by the utility to keep the battery fully charged. Power Outage Occurs When utility power voltage falls to less than 85% of nominal, or fails entirely, the generator system will automatically go through a start sequence and connect to a home. The transfer panel control constantly monitors the power quality from both the utility source and the generator set. When the transfer panel control senses unacceptable utility power, the control waits for a few seconds (adjustable TDES) and then sends a signal to start the generator set engine. If the utility power returns before this time period has passed, the generator set engine will not be signaled to start. When the start signal is received, the engine starts and reaches the proper operating speed and AC power is available at the generator set. The transfer panel control senses this, waits another set amount of time (TDNE) and will then transfer generator set power to the home through the transfer panel. This sequence of operations will usually occur in less than 10 seconds from the time the power outage occurred to the time when generator set power is connected. Utility Power Returns When utility power comes back on and returns to your home, the transfer panel control senses this and will watch for acceptable voltage. After checking for acceptable utility voltage for a certain time (TDEN), the transfer panel control will signal the transfer panel linear motor to re-transfer the load back to the utility source and disconnect the generator set source. At this point, the generator set is "off-line" and will operate for a set amount of time (TDEC) to properly cool down. After the cool


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down cycle, the generator set will be turned automatically off and reset to standby mode.

System Testing The entire system can be tested manually by simulating a power failure. This can be done by manually opening the main circuit breaker ahead of the transfer panel. The standby power system will then go through the start sequence and pickup the home electrical loads. Manually re-closing the main utility breaker will then signal the standby power system to go through the retransfer sequence and return to standby mode. Manual Override Switch Some Cummins transfer switches have a built-in override switch that the operator can use to open or close the contactors manually, without opening up the transfer switch box. Battery Charger Some Cummins transfer switches have a built-in battery charger that automatically keeps the gensets battery charged up so that it will be able to provide DC voltage when needed.

Automatic Exercise Cycle The generator set should be exercised regularly for system readiness. The time of day can be set for the system to operate and it will automatically start and run for a preset period of time. During this exercise period, the power available from the generator set WILL NOT be connected to the home, and utility power will not be interrupted. Manual Exercise Operation The generator set can be operated at anytime from the generator set control panel behind the service access door. Simply move the "RUN/OFF/AUTO" switch to 'RUN", and the generator set will start and run. Power available from the generator set WILL NOT be connected to the home, and utility power will not be interrupted. The switch must be returned to the "AUTO" position for fully automatic operation.

Other Uses for Transfer Switches The power utility companies use transfer switches too. Part of the purpose in having power distributed through “grids” is to provide flexibility in the system. If demand in one part of the grid increases or if supply from one source decreases due to a problem, power from other parts of the grid can be re-routed to help meet the demand.

What Does a Transfer Switch Check for in the Source Voltage? The transfer switch continuously samples the utility company’s power and determines when conditions are okay or not okay. On the basis of these samples, the transfer switch decides when to drop the utility company’s power and use the genset instead. When the Genset is running, the transfer switch continues to sample the utility’s power and determines when it is okay to return to their power and shut the genset down. Some transfer switches are more or less sophisticated than others. Here are the criteria that a transfer switch might use when sampling the source current: Under- or over-voltage Under or over-frequency Phase difference (in degrees) Loss of phase Phase rotation


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TDPT

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Time delay program transition principles video

Transfer without TDPT

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Transfer without TDPT video

Transfer with TDPT

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Transfer with TDPT video


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Switches

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Actuation of switches video

Contacts

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Actuation of contacts video

Typical ATS mounting The following figure shows how a transfer switch is connected to the genset and its arrangement in a room. A couple of things to keep in mind when installing a transfer switch is to :

Have flexible unions when connecting to the genset (to account for vibrations)

Control wiring and Source 2 power wires are to be kept separate, so that there’s no interference to control wiring.


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Lesson 9: Genset Controls

Objectives

Introduction to types of Genset controls

Get a basic understanding of control functionalities


Genset Controls

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Genset controls Integrated microprocessor-based (digital) controls allow the genset and transfer switch to offer “smart” functionality — accessing critical performance data and communicating that data to each other as well as to other building management systems. Digital controls use the power of computers to allow the genset components to interact with each other. They provide more than the combination of a good engine, alternator, controls and transfer switches.

Benefits of Genset controls

Reliability

Flexibility

Physical Size

Data logging

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Benefits of Digital Controls Reliability – Using microprocessor based controls renders fewer points of failure than traditional discreet component and point to point wiring designs. Flexibility – Controls can be adapted to a variety of modes of operation with just a few changes in the menu. For more sweeping changes in operation, a new program may be loaded. This is in stark contrast to hardwired systems that take days or weeks to reconfigure. Physical Size (footprint) – The consolidation of controls allows the system to occupy much less physical space than conventional control arrangements. Data Logging – Integrated digital controls maintain records of control operations, warning conditions, and other events. Records are time stamped. Digital control systems facilitate state-of-the-art servicing of the entire paralleling control system by allowing interrogation, monitoring and adjustment


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of system parameters with a laptop computer. Interfacing with building management systems or SCADA systems is much easier. Genset functions such as speed governing and voltage regulation are integrated, offering superior performance compared to non integrated designs.

Typical controls you may find on Cummins gensets The power command series of genset controls are the most widely used in Cummins generators. The above figures show some of the controls that are used in the field currently. The functionalities and the cost of these controls vary depending on application and end use.

Functions/capabilitiesGeneralAVR (Automatic voltage regulation)Digital 3-phase sensing voltage regulatorGlow plug control Cycle cranking Full authority engine control Networking (LonWorks) Networking (PCCNet) Networking (Modbus) Fault history

Operator interfaceManual start/stop Auto/remote start Exercise function Emergency stop (local and remote) Alphanumeric screen Remote start input active led Fault reset

Paralleling capabilityFirst start sensor system kW and kVAR load sharing Base loading (utility bus) Power transfer control Peak shaving

Shutdown protection/indication — engineLow fuel level High fuel level Low oil pressure High engine coolant temperature Failure to crank shutdown Over crank (failure to start) High/low battery voltage/weak battery Overspeed

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Genset Functions/Capabilities General AVR (Automatic voltage regulation) Digital 3-phase sensing voltage regulator Glow plug control Cycle cranking Full authority engine control Networking (LonWorks) Networking (PCCNet) Networking (Modbus) Fault history Operator interface Manual start/stop Auto/remote start Exercise function Emergency stop (local and remote)


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Alphanumeric screen Remote start input active led Fault reset Paralleling capability First start sensor system kW and kVAR load sharing Base loading (utility bus) Power transfer control Peak shaving Shutdown protection/indication — engine Low fuel level High fuel level Low oil pressure High engine coolant temperature Failure to crank shutdown Over crank (failure to start) High/low battery voltage/weak battery Overspeed Shutdown protection/indication — alternator Under and over voltage Under and over frequency Overcurrent and short circuit Ground fault (earth leakage) Reverse power Reverse Var Measurement and instrumentation/alternator 3-phase L-L & L-N voltage, frequency 3-phase current kWh Total kVa Total kWe and kVAr PF Per phase kVAr, kWe Per phase kVa Measurement/instrumentation — engine Oil pressure Oil temperature Coolant temperature Engine speed Engine running hours Number of starts Battery voltage Exhaust temperature


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Lesson 10: Paralleling of Gensets

Objectives

Know the conditions required for parallel operation

Understand synchronisation

Understand the load sharing process

Know the types of paralleling

Understand Peak shaving vs. Base loading


Paralleling example : McMinnville Township, Tennessee The McMinnville Electric System, a Tennessee Valley Authority member utility, relies on a 20 MW diesel power plant to help the TVA meet its peak demand and provide emergency backup power for up to 40 percent of c innville’s load.

Parallel Operation of Gensets

Parallel operation is the operation of two or more AC

power sources, that are in sync, whose output leads

are connected to a common load.

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McMinnville Township,

Tennessee

The McMinnville Electric

System, a Tennessee

Valley Authority member

utility, relies on a 20 MW

diesel power plant to help

the TVA meet its peak

demand and provide

emergency backup power

for up to 40 percent of

McMinnville’s load.

Parallel Operation of Gensets Parallel operation is the operation of two or more AC power sources, that are in sync, whose output leads are connected to a common load. Parallel operation is used: To increase the capacity of available power

without loss of supply to the customers distribution system.

To allow the Generator to be connected to a live system e.g. the Mains, (Grid, Utility), Multiple Generator systems, C.H.P

To allow shutdown of individual Generators for Maintenance or repair purposes.

To economise operating costs by running multiple generators according to load demand.

To provide an emergency back-up to critical supplies without losing power e.g. ; Hospitals, Ships, Computer data systems

Notes:


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Synchronization

In a paralleling application, synchronization is obtained when an

incoming generator set is matched with and in step to the same

frequency, voltage, and phase sequence as the operating power

source.

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GEN 1

GEN 2

50 HZ (1500 RPM)

50.1 HZ(1503 RPM)

IN PHASE

Synchronization In a paralleling application, synchronization is obtained when an incoming generator set is matched with and in step to the same frequency, voltage, and phase sequence as the operating power source.

Synchronising is carried out in order to parallel a

Generator to a live bus bar, either in Island mode (with multiple gensets as the only supply), or to the Utility.

Synchronising can be achieved manually, semi-automatically with check sync, or with fully automated PLC systems.

There are two main types of Generator paralleling systems, Isolated Bus and Infinite Bus.

Synchronizing two gensets • GEN 2 is supplying load and is running at 50HZ

(1500 RPM). • GEN 1 is INCOMING, and the engine speed has

been adjusted to slightly above 1500 RPM, (for example 1503 RPM, or 50.1 HZ).

• As there is a small relative difference between speeds, the Synchronising equipment should be indicating that the Generators are moving IN and OUT of phase SLOWLY enough to ALLOW TIME to close the breaker.


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• When the synchronising equipment indicates that the incoming generator (gen 1) is in phase with the bus-bar frequency, the circuit breaker can be safely closed.

• The incoming generator should always be slightly faster than loaded generators. This ensures that the incoming generator takes a small proportion of kwatt load when the breaker is closed. This will prevent reverse power protection tripping.

• The generators are now in parallel.

Notes:


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Isolated Bus As the name implies, the paralleling system is isolated from the utility grid. Generally, if a system has open transition transfer switches, it’s an isolated bus system.

Notes:


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Infinite Bus Generators may be paralleled to the utility grid (which is considered to be an infinite generating and distribution bus).

Notes:


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Peak Shaving & Base loading Electricity rates are generally tiered such that the more electricity you consume the higher rate that you will get charged for it. Paralleling systems are useful at times to offset this rising cost of electricity due to high usage. Sometimes the fuel and other costs associated with generating you own electricity on site may be cheaper than actually using the utility at higher consumption levels. Peak shaving and base loading are methods used to save money as well as take load off the grid (which is useful in highly congested utility grids). Let’s look at an electricity usage plot for a typical industrial facility.

Peak Shaving When peak shaving is in effect, the genset will kick-in and provide the electricity when the consumption rate is above a set amount. Generally this consumption rate is just under the rate when higher electricity rates factor in. The generator continually supplies a varying amount of power (depending on how much the load is) this is attained by varying the fuel rate.

Base Loading Base loading is essentially the opposite of peak loading. The genset provides a base amount power, while the utility provides a varying amount. In this scenario, the utility has a lot less burden.


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Lesson 11: Transmission and Distribution

Objectives

Get a basic understanding how power is transmitted and distributed through a power grid

Understand different types of power requirements depending on industrial, commercial, or residential applications.


Notes:


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Conclusion

Questions

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Last chance to ask questions!


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Knowledge Check

Q1: Complete the boxes with the correct component names.

Air Filter, Transfer Switch, Engine, Alternator, Turbo-Charger, Control System

Q2: Complete the boxes with the correct component names.

Camshaft, valves, engine block, crankshaft, piston, connecting rod, cylinder


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Q3: Reorganize the following four strokes in a gasoline engine, and describe what each step does.

Compression, Intake, Power, Exhaust

Q4: Name some main differences between a gasoline engine and a diesel engine?

Q5: Identify each of the different theories of current flow, in the box.

Q6: Which one of these theories(of current flow) is applied in wiring diagrams?

Stroke What happens


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Q7: Circle whether the following statements are true or false.

Two identical batteries connected in series will supply a voltage that is 2x (two times as much) the

rated voltage of an individual battery. TRUE / FALSE

Two identical batteries connected in parallel will supply a voltage that is 2x (two times as much) the

rated voltage of an individual battery. TRUE / FALSE

If the load attached to each of the circuits mentioned in the above two scenarios are the same. The

current in the circuit would be identical. TRUE / FALSE

Lesson 4: Principles of Magnetism

Q8: Fill in the blanks with the correct words provided below.

Reluctance, Saturation point, Retentivity, Mutual Induction, Permeability

_____________ is a measure of how easily a material can become magnetized – that is, how easily a

magnetic field can flow through a material, magnetizing it in the process.

A changing (collapsing/expanding) magnetic field from one coil induces a magnetic field in the

second; a voltage will be generated in the second coil (in the other circuit) as a result. This process is

called ______ ________.

The ability for a material to maintain some level of magnetism when the magnetic field is removed

from it is an intrinsic property of the material. This property is called its ________.

Materials that show high _____________ resist having their atoms lined up and becoming magnetic.

A conductor cannot be magnetized any further when it has reached its magnetic _______ _______.


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Q9: Match the component symbol to its name

Fuse

Resistor

Diode

Capacitor

Transformer

Q10: Circle whether the following statements are true or false.

Transformers require AC input to its primary coil induce a continuous voltage in the secondary

coil. TRUE/FALSE

A transformer has a DC voltage input to its primary windings, now this DC power source is

turned off. At that instant there’s a voltage induced in the secondary winding. TRUE/FALSE

Q11: Identify the components in the blank boxes below.


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Q12: Describe the sequence of operation of the self-excited brushless generator. Use the figure below

as a guide. The first two steps are provided to get you started.

Step Component used/affected

Description

1 Prime Mover Prime mover startsup and rotates the main shaft.

2 Exciter Stator/Rotor

Residual Magnetism in the exciter stator induces a AC Voltage in the Exciter Rotor

3

4

5

6

7


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Q13: The diagram below shows the transfer process in a typical ATS. Initially the load is connected to the

grid, and the utility power goes out. Please write in the step by step process that occurs afterwards.

Include all the time delays (TDES, TDNE, TDPT) and their definitions.

Step Name Description

1

2

3

4

5

6

7


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Q14: Identify which family each of these powercommands belong to (1.x , 2.x, 3.x) . Circle the control

family that has paralleling capabilities.

Documents

Fundamentals of Power Generation PG V1.2.pdf