Unit-III-Machines.ppt

8/14/2019 Unit-III-Machines.ppt

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Unit III

AC machines Introduction


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AC Motors


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AC Motors

AC motors convert AC electrical energy to Mechanical energy.


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AC Motors

AC motors:1. the armature of rotor is a magnet (different to DC motors).

2. the stator is formed by electromagnets (like in DC motors).


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Effects of AC Supply on Magnetic Poles

Consider the rotor to be a permanent magnet. Current flowing through conductors energize the magnets and develop N and S

poles. The strength of electromagnets depends on current. First half cycle current flows in one direction. Second half cycle it flows in opposite direction.

As AC voltage changes, the poles alternate.


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Using AC Supply to Make an ElementaryMotor (1)

Consider the AC voltage at 0 degrees, then, no current will flow, and thereis no magnetism.


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As voltage increases, current starts to flow and electromagnets gainstrength and North and South poles appear.

(Use left hand rule to find poles).

The rotor magnet is pushed CW, and the rotor and motor starts to rotate.


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When voltage decreases, the current decreases also, the electromagnetloses the strength, and when V=0 there is no magnetism.


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Now, AC voltage builds up as part of the negative cycle.

Then, current flows in opposite direction, and the magnets reversepolarity.

Therefore, the CW rotation continues.


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This process is repeated over and over, as AC voltage goes through itscycles, and we have continuous rotation.


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AC Motor Rotation The whole picture


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Limitation of the Elementary Motor

The initial position of the rotor determines the direction of the motorrotation.

Indicate the rotation in the figures below :


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Practical AC Motor By adding another pair of electromagnets the limitation mentioned

before is removed. Two electromagnets = Vertical & Horizontal Two phases with phase difference = 90 deg.


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Effect of Two Pole-Pairs(Observe the pole rotation)


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Operation of the Practical AC Motor

Fig. of page 124 shows a CCW rotation

Can you see it?


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Magnetic Poles Revolve in AC Motors From the previous slide we can see that the poles rotate around the

circumference of the motor. The rotor, no matter how it is positioned at rest, will be locked-in with the

magnetic field and will turn in one direction only. (Same rotation as the poles).


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Reactor Start AC Motor(One phase + Inductor)

Two parallel branches connected to the power supply. First branch: Start winding through a centrifugal switch. Second branch: Run winding (through an inductor). The current in the second branch lags the current in the first branch (Remember

ELI). This phase difference makes motor work.


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Induction Motors


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Introduction

Three-phase induction motors are the mostcommon and frequently encountered machinesin industry

simple design, rugged, low-price, easy maintenance wide range of power ratings: fractional horsepower to

10 MW run essentially as constant speed from zero to full load

speed is power source frequency dependent not easy to have variable speed control requires a variable-frequency power-electronic drive for

optimal speed control


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Construction

An induction motor has two main parts a stationary stator

consisting of a steel frame that supports a hollow,cylindrical core

core, constructed from stacked laminations (why?),having a number of evenly spaced slots, providing thespace for the stator winding

Stator of IM


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Construction

a Revolving rotor composed of punched laminations, stacked to create a series of

rotor slots, providing space for the rotor winding one of two types of rotor windings conventional 3-phase windings made of insulated wire ( wound-

rotor ) similar to the winding on the stator aluminum bus bars shorted together at the ends by two

aluminum rings, forming a squirrel-cage shaped circuit ( squirrel-cage )

Two basic design types depending on the rotor design squirrel-cage wound-rotor


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Construction

Squirrel cage rotor

Wound rotor

Notice theslip rings


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Construction

Cutaway in atypical wound-

rotor IM.Notice thebrushes andthe slip rings

Brushes

Slip rings


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Rotating Magnetic Field

Balanced three phase windings, i.e.mechanically displaced 120 degrees formeach other, fed by balanced three phasesource

A rotating magnetic field with constantmagnitude is produced, rotating with a speed

Where f e is the supply frequency and P is theno. of poles and n sync is called thesynchronous speed in rpm (revolutions perminute)

120e

sync

f n rpm

P


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Rotating Magnetic Field


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Principle of operation

This rotating magnetic field cuts the rotor windings andproduces an induced voltage in the rotor windings Due to the fact that the rotor windings are short circuited, for

both squirrel cage and wound-rotor, and induced currentflows in the rotor windings

The rotor current produces another magnetic field A torque is produced as a result of the interaction of those

two magnetic fields

Where ind is the induced torque and BR and BS are the magneticflux densities of the rotor and the stator respectively

ind R skB B


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Induction motor speed At what speed will the IM run?

Can the IM run at the synchronous speed, why? If rotor runs at the synchronous speed, which is the

same speed of the rotating magnetic field, then therotor will appear stationary to the rotating magneticfield and the rotating magnetic field will not cut therotor. So, no induced current will flow in the rotor andno rotor magnetic flux will be produced so no torqueis generated and the rotor speed will fall below the

synchronous speed When the speed falls, the rotating magnetic field will

cut the rotor windings and a torque is produced


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Induction motor speed

So, the IM will always run at a speed lower thanthe synchronous speed

The difference between the motor speed and the

synchronous speed is called the Slip

Where n slip= slip speedn sync = speed of the magnetic fieldnm = mechanical shaft speed of the motor

slip sync mn n n


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The Slip

sync m

sync

n n s

nWhere s is the slip

Notice that : if the rotor runs at synchronous speed

s = 0

if the rotor is stationary

s = 1

Slip may be expressed as a percentage by multiplying the aboveeq. by 100, notice that the slip is a ratio and doesnt have units


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Capacitor Start AC Motor(One phase + Capacitor)

Here the capacitor provides the phase difference. The difference is that the current in the star winding leads the current in

the run winding (ICE). Similar effect as with the inductor, but it creates a motor with higher

starting power.

Refrigerators, compressors, air conditioners

8


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Three Types of Capacitor Start Motors

1. Capacitor Start (disconnects capacitor after motor speedpicks up)

2. Capacitor Run (Keeps the capacitor connected during theoperation of the motor, in order to keep the electric powerconsumption low)

3. Capacitor Start-Run (uses two capacitors, one for startingand one for running. This further improves PowerConsumption)


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Synchronous Speed AC motors always rotate with the speed of their revolving magnetic field. The speed of the revolving poles is the maximum possible speed of

rotation of the motor. It is called Synchronous Speed.


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Motor ConstructionThe Stator

The stator forms a hollow cylinder with coils of insulated wire inserted intoslots of the stator core. The coils, plus the steel core form the electromagnets.


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Motor ConstructionThe Rotor

There are two types of motor rotors: The wound rotor The squirrel cage The wound rotor has coils of wire wound in the slots of the rotor (Similar

to generator coils).

The Squirrel cage consists of bars of copper or aluminum electricallyconnected at each end with conducting rings. As the rotor rotates inside a magnetic field, it receives electromagnetic

induction, then current flows and form the rotor electromagnet .

0


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Types of Motor Enclosures

1. ODP Open Drip Proof

2. TENV Totally Enclosed Non-Ventilating

3. TEFC Totally enclosed Fan Cooled

4. XP Explosion Proof


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Types of Motor Enclosures ODP Open Drip Proof

Air flows through motor (fan blades help flow) Used in environments free from contaminants


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Types of Motor Enclosures TENV Totally Enclosed Non-Ventilating

Protect motor from corrosive and harmful elements Frame fins help to dissipate heat


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Types of Motor Enclosures TEFC Totally enclosed Fan Cooled

Similar to TENV except has external fan for cooling


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Types of Motor Enclosures

XP Explosion Proof Similar to TEFC but enclosures are cast iron


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Slip Slip is associated with synchronous speed. If the motor turned at the same RPM as the magnetic field, there would

be no relative motion between the rotor and the field. Therefore, no current would be induced into the rotor, and no magnetic

field would exist.

Rotor speed < synchronous speed

Slip = synchronous speed rotor speed

% slip = ( Ns Nr / Ns ) 100


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Three Phase AC Motor

It has three pairs of electromagnets, connected to one of the three phasesof the power supply. It provides a lot higher power that what single phase motor can deliver.


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AC Motor Data Plate Each motor has a plate mounted on its frame, with electrical and

mechanical information .


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Unit-III-Machines.ppt