6 Motor Basics Handout

8/6/2019 6 Motor Basics Handout

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Efficient Motor Use

Attention:

Contrary to popular belief, motors do not run on electricity!

Motors run on the pre-installed smoke from the factory.

The electricity only keeps the smoke in.

If the smoke gets out, the motor is no good!!!


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INTRODUCTION

Motors and Engines are energy conversion devices:

Electric motors convert electrical energy to mechanical energy.

Engines convert chemical energy (gasoline, diesel, natural gas, etc.) to mechanical energy.

A 1 horsepower electric motor can provide the work of approximately 8 people.

Electric motors are a significant and important portion of most utilities load.

Depending on the numbers used, anywhere from 50 to 65 percent of the electricity sold byelectric utilities is used to power electric motors.

Other utilities (natural gas) and fossil fuel suppliers would like to increase their market shareby promoting engine use over electric motor use. Competition for this market can be intense

depending on local fuel prices.

Efficiency of Electric Motors & Engines

Electric Motor - 50 to 95%Gasoline Engine - 25%Diesel Engine - 40%Natural Gas Engine - 37%

Advantages of Electric Motors

Low initial cost. ($/Hp)Relatively economical operation. ($/Hr)Long life with a minimum of service requirements. (Hours)Simple and efficient operation.Low noise and exhaust emissions.Compact size. (Hp/cubic inches)Capable of withstanding significant temporary overloads. (100%)Capable of being remotely started and controlled.Easy to start and stop.

Advantages of Engines

Portability from location to location.Simple speed control.No electric demand charge.No requirements for line extension in remote locations.


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Simple Electromagnet

Motors Operate on the Principle of Alternating Magnetic Poles

PRINCIPLES OF MOTOR OPERATION

All motors (AC or DC) are comprised of two important parts -- the stator or stationary partand the rotor or rotating part.

All motor operation is governed by the interaction between stator and rotor magnetic fields.

The fields can be produced by a permanent magnet and/or an electromagnet.

Electromagnets are based on the principle that whencurrent is passed through a wire, a magnetic field isproduced around the wire in turn magnetizing the ironnail

Electric motors operate on the principle that a currentcarrying conductor (rotor) placed inside a magneticfield becomes a magnet itself due to the interactionfrom the stators magnetic field.

The basic principle of torque and motor rotation for a motor in its simplest form is shown byusing a permanent magnet and two electromagnets.

The resultant force (and thus torque) produced by the opposing magnetic fields causes therotor to turn.

If the current direction in the electromagnet is changed every 180 degrees of revolution, thepermanent magnet will continue to rotate.


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Motor Enclosures (Examples)

ELECTRIC MOTOR PARTS

A. Motor Enclosures

The enclosure for the motor provides several important purposes;

Hold the motor parts togetherDissipate the heat produced when the motor operate.

The enclosure may also be designed to protect the motor from the expected operatingenvironment.

Electric motors are required to operate in many different environments ranging from cleanand dry to extremely dirty, wet, and corrosive or from normal to very high temperatures.

Manufacturers provide a variety of motor enclosures designed to protect against varioustypes of adverse conditions.


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Simple 2 Pole Motor Stator (Example)

Stator Core

B. Stator

The stator is the stationary part of theelectric motor generally made of pairs ofslotted cores made of thin sections of softiron.

The cores are wound with insulated copperwire to form one or more pairs of magneticpoles.

When the copper wire comprising the statoris connected to an electrical source, thestator windings form electromagnets andproduce magnetic fields.

The stator may have several sets of windings including running windings, separate starting windings, and separate windings foroperation with different voltages.


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Squirrel Cage Rotors (Examples)

C. Rotor

The rotor is the rotating part of the electric motor. Induction motors generally contain asquirrel cage rotor or a wound rotor.

Squirrel Cage Rotors

The squirrel cage rotor (derived from its appearance similar to an exercise cage for hamsters)is made of conductive copper, brass, or aluminum bars that are parallel to the shaft and shortcircuited by rings in which they are physically supported at each end. Bar size, shape andresistance significantly influence the operational characteristics of this type of motor.

The magnetic field from the stator induces an opposing magnetic field into the bars on thesquirrel cage causing the rotor to push away from the stator's magnetic field.


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Wound Rotor Induction Motor

Wound Rotors

The wound rotor motor operates on the same principles as the squirrel cage motor but differsin the construction of the rotor.

Instead of shorted bars, the rotor is made up of windings which terminate at slip rings on theshaft.

Connection of external resistance to the rotor circuit, via the slip rings, permits variation ofmotor torque-speed characteristics.

Speed range variation of about 5:1 can be achieved by adding external resistance to the rotorcircuit. However, this is at the expense of electrical efficiency unless a slip energy recoverycircuit is used.

Prior to the advent of AC Adjustable Speed Drives, wound rotor motors were one of the fewoptions available for changing the speed of an AC motor. As AC Drives have become more

commonplace, wound rotor motors are not seen as often.

Advantages: Speed Control from an AC Motor, High Starting Torque, Low StartingCurrent

Disadvantages: Expensive, High Maintenance Requirement of Slip Rings & Brushes, LowEfficiency


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Sleeve Type Bearing

Ball or Roller Type Bearing

D. Bearings

There are two types of bearings commonly used in motors: Sleeve bearings and Ball orRoller bearings. Most manufacturers today supply sleeve bearings on their general purposemotors with the option of upgrading to Ball or Roller Bearings.

Sleeve Bearings

Sleeve bearings are made of a soft metalsuch as bronze or babbitt and are quieterthan antifriction bearings.

They cannot support thrust loads and are

designed to operate only with horizontalshafts.

Oil is used to lubricate this type of

bearing, and supports the moving surfaceswith a thin film while they are turning.Operation without sufficient lubrication

will cause immediate damage.

An oil wick, oil soaked yarn, or oil ring may be used to transport oil from a reservoir tolubricate the bearing and shaft. An oil ring is a large loose fitting ring with its top halfresting on the shaft and its bottom half in an oil reservoir. The presence of these devices canbe confirmed via a filler plug in the top of the bearing.

Ball or Roller Bearings

Ball or Roller bearings use rollingelements between the bearing housingand the rotating shaft.

These bearings generally use grease as alubricant.

Some ball and roller bearings used inmotors are sealed and need nomaintenance, but many are unsealed and

require periodic re-packing with greasefrom a grease gun.

E. Other Parts -Other motor parts withspecific functions include:

1. Conduit Boxes2. Eye Bolts


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Synchronous Speed 120 X FrequencyNumber of Poles

Synchronous = Theoretical No Load Speed

MOTOR SPEED/POWER/TORQUE

MOTOR SPEED

There are two common speed terms/ratings used in the motor industry;

Synchronous SpeedRated Speed.

Synchronous Speed

This is the speed at which a motorsmagnetic field rotates.

Synchronous speed is the motorstheoretical speed if there was no load on

the shaft and friction in the bearings.

The two factors affecting synchronous

speed are the frequency of the electricalsupply and the number of magnetic polesin the stator.

Synchronous Speed is calculated as:

Where: Frequency = Electrical frequency of the power supply in Hz.Number of poles = Number of electrical poles in the motor stator.

Since the frequency of the power supply is usually fixed (typically 60 Hz), the number ofmagnetic poles (or simply poles) is the principal design factor affecting motor speed.

Example:

A 4 pole motor is connected to a 60 hertz electrical supply. What is the synchronous speed of themotor?

120 X 60 hertzSynchronous Speed = ------------------- = 1800 rpm

4 poles


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SlipSynchronous Speed Running Speed

Synchronous Speed

x 100

Rated = Full Load Speed

Motor Slip

The rotor of an induction motor does not rotate at synchronous speed, but lags this speedslightly. This lag is expressed as a percentage of the synchronous speed called the "slip".

Because the rotor "slips" with respect to the rotating magnetic field of the stator, voltage andcurrent are induced in the rotor.

The larger the slip, the higher the current induced in the rotor which creates a strongermagnetic field allowing the motor to produce more torque.

As the motor load increases, slip and torque also increase.

Motors can be characterized as low, normal or high slip motors depending on their design.

Rated Speed

The speed listed on a motor nameplate is theactual speed at the motor's rated power output andnot the motor's synchronous speed.

As load on an induction motor increases, theactual operating speed of the motor decreasesslightly to allow the motor to produce more

torque.

The actual amount of the speed change is dictatedby the design of the motor and the amount of loadthe motor must drive.

When a motors operating speed is lower thanits rated speed, it is an indication that themotor is overloaded or receiving low voltage.

High slip motors will have a rated speed

significantly lower than that of a low slipmotor.

Some applications like oil pump jacks andlarge impact loads require high slip motors toprotect the drivetrain components.


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1 Horse ower 746 Watts 0.746 K owatts

HorsepowerSpeed (in RPM) x Torque (in pound feet)

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MOTOR POWER

The motor industry rates their equipment differently thanmanufacturers of other types of electric equipment.

Nameplate Power = Output

The rated mechanical power of a motor is given on themanufacturers nameplate and quantifies the rate of work amotor is capable of performing at rated operating speed(the amount of load it can turn) without reducing its life.

Motor manufacturers rate the output power of theirmotors in units of horsepower (Hp), the measurement of power in the English system ofunits.

Motor output power can only be measured accurately with a dynamometer or prony brake.

Input Power

The input power to a motor is the amount of electric power it consumes to operate and drive the loadit is connected to.

Motor input power is commonly measured at the electrical supply to the motor using theMetric system term for power of kilowatts (kW).

The electrical power input to a motor can be measured with a watt-meter or a voltmeter,ammeter, and power factor meter.

Determining Motor Output Power (Horsepower)

Factors that affect mechanical power output of a motor are torque and operating speed.

Where: Speed = Motor speed in revolutions per minute (RPM)

Torque = Amount of torque produced (pound-ft)

Slower motors must produce more torque to deliver the same mechanical power output.

To withstand the greater torque, slow motors need stronger components than those ofhigher speed motors of the same power rating.

S Slower motors are generally larger, heavier and more expensive than faster motors ofthe equivalent power rating.


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Typical Motor Torque-Speed Curve

TORQUE-SPEED CHARACTERISTICS OF MOTORS

The amount of torque produced by a motor generally varies with speed.

This Torque-Speed characteristic depends on the type and design of a motor, and is oftenshown on a Torque-Speed Curve/Graph.

Some important factors indicated by a Torque-Speed graph include:

(a) Starting torque - the torque produced at zero speed;

(b) Pull-up torque - the minimum torque produced during acceleration from standstill tooperating speed;

(c) Breakdown torque - the maximum torque that the motor can produce before stalling.


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MOTOR POWER CALCULATIONS

Ohms Law: Volts = Amps x Ohms; or E = I x R

Horsepower-Kilowatt Relationship: 1 Horsepower = 746 Watts = 0.746 Kilowatts

Watt's Law:

Single Phase: W = E x I x p.f.

Three Phase: W = Eav x Iav x p.f.av x 1.732

Resistance Loads (heating elements, incandescent lights), power factor (p.f.) = 1.0 (100%).

Inductive Loads (motors, fluorescent lights, etc.), power factor < 1.0 (100%).

Actual Power Watts

Power Factor: p.f. = ------------------------ = ---------------------------Apparent Power Volts x Amps

Power Out

Efficiency: Efficiency = --------------------

Power In

Revolutions

Input Power (Electric Meter): kW = ------------------ x Kh x 3.6Seconds

E x I x p.f. x eff.

Single Phase Motor Horsepower (output):h.p. = ---------------------------

746

Eav x Iav x p.f.av x 1.732 x eff.

Three Phase Motor Horsepower (output): h.p. = ------------------------------------------746


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MOTOR POWER CALCULATION PROBLEMS

1. The voltage measured to a single phase motor is 123 volts. The current measured is 9 amps. Thepower factor was measured as 0.78. What is the power requirement of the motor in kilowatts and inhorsepower?

2. The voltages measured to a three phase motor are 453, 458, and 461. The current measurementswere 14.1, 13.9, and 13.8 amps respectively. The power factor was measured as 0.82. What is the

power requirement in kilowatts and in horsepower?

3. By timing the utility meter, the input power to a motor is found to be 3 kilowatts. Voltagemeasured on the power supply was 125 volts. The current measurement was 27 amps. What powerfactor is the motor operating at?

4. The power in to an electric motor is measured at the utility meter and found to be 5 kilowatts.Measurements on the motor shaft indicate the motor is producing 3 horsepower. What is theefficiency of the motor?

5. A single phase motor's power supply measures 238 volts. The current to the motor measured 54amps. The motor operates with a power factor of 0.8 and the manufacturers listed efficiency is 90%.What is the output power of the motor in horsepower and in kilowatts?

6. A three phase motor's power supply measures 200, 205, and 207 volts. The currentmeasurements in each phase are 24.2, 24.1, and 24.0 amps. The motor operates with a power factorof 0.82 and the manufacturer's listed efficiency is 88%. What is the power output of the motor inhorsepower and kilowatts?


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DCPermanent MagnetSeries WoundShunt WoundCompound Wound

Split PhaseCapacitor RunCapacitor StartCapacitor Start/Run

Shaded Pole

Universal Squirrel Cage

Induction

RepulsionRepulsion StartWound

RotorSingle

HysteresisReluctanceSynchronous

AC

Wound Rotor

Induction

Design ADesign B

Design CDesign DDesign E

Squirrel Cage

Polyphase

Synchronous

CLASSIFICATION OF MOTORS

There are several major classifications of motors in common use, each with specificcharacteristics that suit it to particular applications.


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DIRECT CURRENT (DC) MOTORS

DC motors are used in small power requirement applications where precise speed control isrequired. The power requirements are generally not large since these motors are battery operated.

Historically, prior to the advent of reliable AC Adjustable Speed Drives, DC speed controlwas simpler, less costly and spanned a greater speed range than AC speed control systems.

ALTERNATING CURRENT (AC) MOTORS

Synchronous Motors

Synchronous Motors are constant speed motors most commonly used in very large industrialapplications or where exact speed, even with changing loading is required.

Universal Motors

Although most universal motors are operated on AC power, they can operate on either AC or DC.Tools and appliances are among the most frequent applications.

Induction Motors

Induction motors are very robust and reliable, and are the most common type of motor in use.Unfortunately, power factors tend to be poor for these motors when operated at less than 100 percentof their rated load. They come in three phase and single phase designs.

Three Phase Induction Motors

Three phase induction motors are the most widely used motors in industrial and commercialapplications. They fall into two subclassifications

Squirrel Cage Motors

Wound Rotor Motors

Single Phase Induction Motors

Single phase induction motors are used:S Where three phase power is not available (generally up to 10 horsepower).S For smaller sized motors (less than 1 horsepower) where three phase power is

available.

There are several sub-classifications which describe their starting and running modes.

1. Split Phase

2. Capacitor Run (Permanent Split Capacitor or PSC)3. Capacitor Start4. Capacitor Start - Capacitor Run5. Shaded Pole

Single phase motors do not generally produce enough torque at starting to turn themselvesand the connected load so they usually employ special starting windings to produceadditional torque during the starting period.


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THREE PHASE INDUCTION MOTORS

To facilitate the selection of three phase motors with different Torque-Speed characteristics,NEMA (National Electrical Manufacturers Association) has assigned designations A, B, C,D and E to describe standard characteristics of induction motors up to 200 horsepower.

Motors larger than 200 Horsepower are considered special purpose motors.

Design A motors used to be the industry standard prior to the advent of soft-start Design Bmotors.

Today, Design B motors are the most common and suit the majority of motor applicationsexcept where hard starting loads are encountered.

Design C motors are commonly used on hard starting loads like reciprocating pumps andcompressors.

Design D motors are commonly referred to as high slip motors and work well onapplications where the load fluctuates during operation.

The Design E category is relatively new and contains many of the newest ultra highefficiency motors manufacturers are producing with very low slip.

Design

Type

Starting

Torque

Starting

Current

X FLA

Breakdown

Torque

Full

Load

Slip

Typical Applications

A

B

C

D

E

highto 180%

normalto 150%

highto 200%

very high

to 275%

highto 190%

high5-7

normal4-6

normal4-6

normal

4-6

very high8-10

highto 275%

normalto 210%

lowto 210%

high

to 275%

highto 200%

normal0.5 - 5%

normal0.5 - 5%

normal1 - 5%

high

5 - 8%

low0.5 - 3%

fans, centrifugal pumps andcompressors, mediumefficiency

Same as A, high efficiency

Compressors, crushers,conveyors, medium efficiency

punch presses, shears, high

inertia loads, mediumefficiency

Same as A & B, very highefficiency


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Torque-Speed Curves of NEMA Design A, B, C and D Motors

Documents

6 Motor Basics Handout