EEEB443 Control & Drives

Induction Motor ReviewByDr. Ungku Anisa Ungku AmirulddinDepartment of Electrical Power EngineeringCollege of Engineering

Dr. Ungku Anisa, July 2008 1EEEB443 - Control & Drives

OutlineIntroductionConstructionConceptPer-Phase Equivalent CircuitPower FlowTorque EquationT- CharacteristicsStarting and BrakingReferences

Dr. Ungku Anisa, July 2008 EEEB443 - Control & Drives 2

IntroductionInduction motors (IM) most widely usedIM (particularly squirrel-cage type) compared to

DC motorsRuggedLower maintenanceMore reliableLower cost, weight, volumeHigher efficiencyAble to operate in dirty and explosive environments


IntroductionIM mainly used in applications requiring

constant speedConventional speed control of IM expensive or

highly inefficientIM drives replacing DC drives in a number of

variable speed applications due toImprovement in power devices capabilitiesReduction in cost of power devices


Induction Motor – ConstructionStator

balanced 3-phase windingdistributed winding – coils

distributed in several slotsproduces a rotating magnetic

fieldRotor

usually squirrel cageconductors shorted by end

ringsRotating magnetic field induces

voltages in the rotorInduced rotor voltages have

same number of phases and poles as in stator winding


a

b

b’

c’

c

a’

120o120o

120o

Induction Motor – ConceptStator supplied by balanced 3-phase AC source (frequency f Hz or

rads/sec ) field produced rotates at synchronous speed s rad/sec

(1)

where P = number of polesRotor rotates at speed m rad/sec (electrical speed r = (P/2) m)Slip speed, sl – relative speed (2)

between rotating field and rotorSlip, s – ratio between slip speed and synchronous speed (3)


fPPs 42

fP

ns

120

mssl

s

mss

Induction Motor – ConceptRelative speed between stator rotating field and rotor induces:

emf in stator winding (known as back emf), E1

emf in rotor winding, Er

Frequency of rotor voltages and currents: (4)Torque produced due to interaction between induced rotor currents

and stator field Stator voltage equation:Rotor voltage equation:


12 E ILπf j I R V slssss

sffr

rlrrr

r

rlrrrr

ILπfj IsRE

ILπf js I R sE

2

2

Induction Motor – ConceptE1 and Er related by turns ratio aeff

Rotor parameters can be referred to the stator side :


reffrreffr

eff

rrreff

LaLRaR

aIIEaE

2'2'

'1

Rr/s

+

Vs

–

RsLls Llr

+

E1

–

IsIr

Im

Lm

+

Er

–

Induction Motor – Per Phase Equivalent Circuit

Rs – stator winding resistance

Rr’ – referred rotor winding resistance

Lls – stator leakage inductance

Llr’ – referred rotor leakage inductanceLm – mutual inductanceIr’ – referred rotor current


Rr’/s+

Vs

–

RsLls Llr’

+

E1

–

Is Ir’

Im

Lm

Induction Motor – Power Flow


cos3 LT

in

IV

P

mLout TP

Stator Copper

Loss (SCL)

ssSCL RIP 23

Rotor Copper

Loss (RCL)

'2'3 rrRCL RIP

Airgap Power Pag

ConvertedPower Pconv

Rotational losses Prot

(Friction and windage, core and stray losses)

s

RIP r

rag

'2'3

s

sRIP rrconv

13 '2'

Electrical Power

Mechanical Power

agRCL sPP

Note:

agconv PsP 1

Induction Motor – Torque EquationMotor induced torque is related to converted power by:

(5)

Since and , hence

(6)

Substituting for Ir’ from the equivalent circuit:

(7)


agconv PsP 1 sr s 1

2

2'

2'3

lrlsr

s

s

s

re

XXs

RR

V

s

RT

m

conve

PT

s

rr

s

age s

RIPT

'2'3

Induction Motor – T- CharacteristicT-

characteristic of IM during generating, motoring and braking


Induction Motor – T- Characteristic

Maximum torque or pullout torque occurs when slip is:

(8)

The pullout torque can be calculated using:

(9)


22

2

max 2

3

lrlsss

s

s XXRR

VT

22

'

max

lrlss

r

XXR

Rs

r

s

Trated

Pull out Torque(Tmax)

Te

0 ratedsmax s

1 0

Induction Motor – T- Characteristic

Linear region of operation (small s)Te sHigh efficiency

Pout = Pconv – Prot

Pconv = (1- s )Pag

Stable motor operation


r

s

Trated

Pull out Torque(Tmax)

Te

0 ratedsmax

s

1 0

Induction Motor – NEMA Classification of IM

NEMA = National Electrical Manufacturers Association Classification based on T- characteristicsClass A & B – general purposeClass C – higher Tstart (eg: driving compressor pumps)Class D – provide high Tstart and wide stable speed range but low efficiency


s

Induction Motor – StartingSmall motors can be started ‘direct-on-line’Large motors require assisted startingStarting arrangement chosen based on:

Load requirementsNature of supply (weak or stiff)

Some features of starting mechanism:Motor Tstart must overcome friction, load torque and inertia of motor-

load system within a prescribed time limit Istart magnitude ( 5-7 times I rated) must not cause

machine overheating Dip in source voltage beyond permissible value


Induction Motor – StartingMethods for starting:

Stat-delta starterAutotransformer starterReactor starterSoft Start


Induction Motor – StartingStar-delta starter

Special switch used Starting: connect as ‘star’ (Y)

Stator voltages and currents reduced by 1/√3

Te VT2 Te reduced by 1/3

When reach steady state speed Operate with ‘delta’ ( )

connectionSwitch controlled manually or

automatically


Induction Motor – StartingAutotransformer starter

Controlled using time relaysAutotransformer turns ratio aT

Stator voltages and currents reduced by aT

Te VT2 Te reduced by aT

2

Starting: contacts 1 & 2 closedAfter preset time (full speed

reached): Contact 2 opened Contact 3 closed Then open contact 1


Induction Motor – StartingReactor starter

Series impedance (reactor) added between power line and motor

Limits starting currentWhen full speed reached,

reactors shorted out in stages


Induction Motor – StartingSoft Start

For applications which require stepless control of Tstart

Semiconductor power switches (e.g. thyristor voltage controller scheme) employed Part of voltage waveform

applied Distorted voltage and current

waveforms (creates harmonics)When full speed reached, motor

connected directly to line


Induction Motor – BrakingRegenerative Braking:

Motor supplies power back to line Provided enough loads connected to line to absorb power

Normal IM equations can be used, except s is negativeOnly possible for > s when fed from fixed frequency source

Plugging:Occurs when phase sequence of supply voltage reversed

by interchanging any two supply leadsMagnetic field rotation reverses s > 1Developed torque tries to rotate motor in opposite direction If only stopping is required, disconnect motor from line when = 0Can cause thermal damage to motor (large power dissipation in rotor)


Induction Motor – BrakingDynamic Braking:

Step-down transformer and rectifier provides dc supply

Normal: contacts 1 closed, 2 & 3 opened

During braking: Contacts 1 opened, contacts 2 & 3 closed

Two motor phases connected to dc supply - produces stationary field

Rotor voltages inducedEnergy dissipated in rotor

resistance – dynamic braking


ReferencesChapman, S. J., Electric Machinery Fundamentals, McGraw

Hill, New York, 2005.Rashid, M.H, Power Electronics: Circuit, Devices and

Applictions, 3rd ed., Pearson, New-Jersey, 2004.Trzynadlowski, Andrzej M. , Control of Induction Motors,

Academic Press, 2001.Nik Idris, N. R., Short Course Notes on Electrical Drives,

UNITEN/UTM, 2008.Ahmad Azli, N., Short Course Notes on Electrical Drives,

UNITEN/UTM, 2008.

Dr. Ungku Anisa, July 2008 25EEEB443 - Control & Drives

EEEB443 Control & Drives

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