Induction Motor (Asynchronous Motor)

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Induction MotorInduction Motor(Asynchronous Motor)(Asynchronous Motor)

ELECTRICAL MACHINESELECTRICAL MACHINES

Compiled byCompiled by

Prof Mitali RayProf Mitali Ray

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Learning Outcomes

• At the end of the lecture, student should to:– Understand the principle and the nature of 3 phase principle and the nature of 3 phase

induction machines.induction machines.

– Perform an analysis on induction machines which is the most rugged and the most widely used machine most widely used machine in industry.in industry.

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Contents

– Overview of Three-Phase Induction Motor– Construction– Principle of Operation– Equivalent Circuit

• Power Flow, Losses and Efficiency

• Torque-Speed Characteristics

– Speed Control – Overview of Single-Phase Induction Motor

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Overview of Three-Phase Induction Motor

• Induction motors are used worldwide in many residential, commercial, industrial, and utility applications.

• Induction Motors transform electrical energy into mechanical energy.

• It can be part of a pump or fan, or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer.

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Introduction

General aspectsGeneral aspects• A induction machine can be used as either a induction

generator or a induction motor. • Induction motors are popularly used in the industry• Focus on three-phase induction motor• Main features: cheap and low maintenance• Main disadvantages: speed control is not easy

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Construction

• The three basic parts of an AC motor are the rotor, stator, and enclosure.

• The stator and the rotor are electrical circuits that perform as electromagnets.

Squirrel Cage Rotor

MZS FKEE, UMP

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Construction (Stator construction)

• The stator is the stationary electrical part of the motor.stationary electrical part of the motor.• The stator core of a National Electrical Manufacturers Association

(NEMA) motor is made up of several hundred thin laminationsseveral hundred thin laminations.• Stator laminations are stacked togetherstacked together forming a hollow cylinderhollow cylinder.

Coils of insulated wireCoils of insulated wire are inserted into slots of the stator core. are inserted into slots of the stator core.• Electromagnetism is the principle behind motor operation. Each Each

grouping of coilsgrouping of coils, together with the steel core it surrounds, form an electromagnet. The stator windings are connected directly to the power source.

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Construction (Rotor construction)

• The rotor is the rotating part of the electromagnetic circuit.

• It can be found in two types:– Squirrel cage– Wound rotor

• However, the most common type of rotor is the “squirrel cage” rotor.

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• Induction motor types:

Squirrel cage type:Squirrel cage type:Rotor winding is composed of copper bars embedded in

the rotor slots and shorted at both end by end ringsSimple, low cost, robust, low maintenance

Wound rotor type:Wound rotor type:Rotor winding is wound by wires. The winding terminals

can be connected to external circuits through slip rings and brushes.

Easy to control speed, more expensive.

Construction (Rotor construction)

MZS FKEE, UMP

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Construction (Rotor construction)

Wound Rotor

Squirrel-Cage Rotor

/rotor winding/rotor winding

Short circuits allShort circuits allrotor bars.rotor bars.

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Construction (Enclosure)

• The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings). The stator is mounted inside the frame. The rotorThe rotor fits inside fits inside the statorthe stator with a with a slight slight air gapair gap separating it from the stator. There is separating it from the stator. There is NONO direct physical connection between the rotor and the direct physical connection between the rotor and the stator.stator.

Stator

Rotor

Air gap

• The enclosure also The enclosure also protectsprotects the electrical the electrical and operating parts of the motor and operating parts of the motor from from harmful effects of the environmentharmful effects of the environment in which in which the motor operates. the motor operates. Bearings, mounted on Bearings, mounted on the shaftthe shaft, support the rotor and allow it to , support the rotor and allow it to turnturn. . A fanA fan, also mounted on the shaft, is , also mounted on the shaft, is used on the motor shown below used on the motor shown below for coolingfor cooling..

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Construction (Enclosure)

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Nameplate

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Rotating Magnetic Field

• When a 3 phase stator winding is connectedconnected to a 3 phase voltage supply, 3 phase current will flow in the windingsflow in the windings, which also will inducedinduced 3 phase flux in the stator.

• These flux will rotate at a speed called a Synchronous Speed, Synchronous Speed, nnss. The flux is called as Rotating magnetic Field

• Synchronous speed: speed of rotating flux

• Where; p = is the number of poles, and f = the frequency of supply

p

fns

120

a Fc

-93 10 113 216-1.5

-1

-0.5

0

0.5

1

1.5

a’

c’ b’

b c

a

a’

c’ b’

b c

a

a’

c’ b’

b c

a

a’

c’ b’

b c

Fb

Fa F

FbFc

F

Fa

F

Fb

Fc Fc Fb

F

Space angle () in degrees

FFa Fc

Fb

t = t0= t4

t = t1t = t2 t = t3

t = t0= t4

RMF(Rotating Magnetic Field)

AC Machine Stator

Axis of phase a

a’a’

-90 -40 10 60 110 160 210 260-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Fa

Space angle (theta) in degrees

t0

t01

t12

t2

a

MMF Due to ‘a’ phase current

1 Cycle

Amp

timet0t1 t2 t3 t4

t01 t12

Currents in different phases of AC Machine

Slip Ring Rotor

•The rotor contains windings similar to stator.

•The connections from rotor are brought out using slip rings thatare rotating with the rotor and carbon brushes that are static.

MZS FKEE, UMP

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Slip and Rotor Speed1.1. Slip Slip ss

– The rotor speed of an Induction machine is different from the speed of Rotating magnetic field. The % difference of the speed is called slip.

– Where; ns = synchronous speed (rpm)

nr = mechanical speed of rotor (rpm)

– under normal operating conditions, s= 0.01 ~ 0.05, which is very small and the actual speed is very close to synchronous speed.

– Note that : s is not negligibles is not negligible

)1( snnORn

nns sr

s

rs

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Slip and Rotor Speed

• Rotor SpeedRotor Speed – When the rotor move at rotor speed, nr (rps), the stator flux will

circulate the rotor conductor at a speed of (ns-nr) per second. Hence, the frequency of the rotor is written as:

• Where; s = slip

f = supply frequency

sf

pnnf rsr

)(

fsfiii

iipnn

f

nnRotorAt

ipn

f

nstatorAt

Note

r

rsr

pf

rs

s

pf

s

.:)()(

).....(120

)(

:

).....(120

:

:

120

120

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Principle of Operation

• Torque producing mechanismTorque producing mechanismWhen a 3 phase stator winding is connectedconnected to a 3

phase voltage supply, 3 phase current will flow in the flow in the windingswindings, hence the stator is energized.

A rotating flux Φ is produced in the air gap. The flux Φ induces a voltage Ea in the rotor winding (like a transformer).

The induced voltage produces rotor current, if rotor circuit is closed.

The rotor current interacts with the flux Φ, producing torque. The rotor rotates in the direction of the rotating flux.

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Direction of Rotor Rotates

• Q: How to change the direction of

• rotation?

• • A: Change the phase sequence of the

• power supply.

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• Conventional equivalent circuit Note:

● Never use three-phase equivalent circuit. Always use per- phase equivalent circuit.

● The equivalent circuit always bases on the Y connection always bases on the Y connection regardless of the actual connection of the motorregardless of the actual connection of the motor.

● Induction machine equivalent circuit is very similar to the single-phase equivalent circuit of transformer. It is composed of stator circuit and rotor circuit

Equivalent Circuit of Induction Machines

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• Step1 Rotor winding is openStep1 Rotor winding is open (The rotor will not rotate)

• Note: – the frequency of E2 is the same as that of E1 since the rotor is at

standstill. At standstill s=1.

Equivalent Circuit of Induction Machines

f f

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Equivalent Circuit of Induction Machines

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Equivalent Circuit of Induction Machines

• Step2 Rotor winding is shortedStep2 Rotor winding is shorted(Under normal operating conditions, the rotor winding is shorted. The slip is s)

• Note:

– the frequency of E2 is fr=sf because rotor is rotatingrotor is rotating.

f fr

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• Step3 EliminateStep3 Eliminate ff22

Keep the rotor current same:

Equivalent Circuit of Induction Machines

MZS FKEE, UMP

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• Step 4 Referred to the stator sideStep 4 Referred to the stator side

• Note:– X’2 and R’2 will be given or measured. In practice, we do

not have to calculate them from above equations.– Always refer the rotor side parameters to stator side.– Rc represents core loss, which is the core loss of stator side.

Equivalent Circuit of Induction Machines

MZS FKEE, UMP

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• IEEE recommended equivalent circuitIEEE recommended equivalent circuit

• Note:– Rc is omitted. The core loss is lumped with the

rotational loss.

Equivalent Circuit of Induction Machines

MZS FKEE, UMP

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• IEEE recommended equivalent circuitIEEE recommended equivalent circuit

Note: can be separated into 2 PARTSNote: can be separated into 2 PARTS

• Purpose : Purpose : – to obtain the developed mechanicalto obtain the developed mechanical

Equivalent Circuit of Induction Machines

I1 1R1X

mX

'2X

'2R

s

sR

1'21V

s

R2

s

sRR

s

R )1(22

2

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Analysis of Induction Machines

• For simplicity, let assume

IIss=I=I1 1 , I, IRR=I=I22

(s=stator, R=rotor)

RmsTotal

sss

cmm

cmcm

RR

R

ZZZZ

jXRZ

neglectedRjXZ

neglectedRjXRZ

jXs

RZ

//

;

;

;//

;''

ZZRRZZmm

ZZss

VVs1s1

IIs1s1 IIm1m1 IIR1R1

T

s

s Z

VI

1

1

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Analysis of Induction Machines

m

RMm

R

RMR

sT

mRRM

Z

VI

Z

VIHence

VZ

ZZV

RulesDividingVoltage

11

11

11

,

//

,

ZZRRZZmm

ZZss

VVs1s1

IIs1s1 IIm1m1 IIR1R1

11

11

,

sRm

mR

sRm

Rm

IZZ

ZI

IZZ

ZI

RulesDividingCurrent

OR

Note : 1hp =746WattNote : 1hp =746Watt

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Power Flow Diagram

PPinin (Motor) (Motor)

PPinin (Stator) (Stator)

PPcore losscore loss

(P(Pcc))

PPair Gapair Gap

(P(Pagag))

PPdevelopeddeveloped

PPmechanicalmechanical

PPconvertedconverted

(P(Pmm))

PPout, out, PPoo

PPstator copper stator copper

loss, loss, (P(Pscuscu))PProtor copper rotor copper

lossloss (P (Prcurcu))PPwindage, friction, etcwindage, friction, etc

(P (P - -

Given)Given)

cos3 ss IV

s

RI RR

''3 2

2

3

c

RM

R

V ''3 2RR RI

s

sRI RR

1''3 2

Whp 7461

ss RI23

PPinin (Rotor) (Rotor)

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Power Flow Diagram• Ratio:Ratio:

Pag Prcu Pm

s

RI RR

''3 2 ''3 2

RR RI

s

sRI RR

1''3 2

s

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1s

1

1 s s1

Ratio makes the analysis simpler to find the value of the particular power if we haveanother particular power. For example:

s

s

P

P

m

rcu

1

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Efficiency

WattxWhpxP

IVP

otherwise

PPP

PPP

givenarePif

P

P

out

ssin

mo

lossesino

losses

in

out

746746

cos3

,

,

%100

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Torque-Equation

• Torque, can be derived from power equation in term of mechanical power or electrical power.

n

PTHence

sradn

whereTPPower

2

60,

)/(60

2,,

r

oo

r

mm

n

PTTorqueOutput

n

PTTorqueMechanical

Thus

2

60,

2

60,

,

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Torque-Equation

• Note that, Mechanical torque can written in terms of circuit Note that, Mechanical torque can written in terms of circuit parameters. This is determined by using parameters. This is determined by using approximation approximation methodmethod

...

...

...

)1('

'3

)1('

'3

2

2

r

RR

r

mm

mrmR

Rm

ssR

IPT

TPandss

RIP

22

2

)'()'(

'

2

)(3

RR

R

s

RMm sXR

sR

n

VT

Hence, Plot Tm vs s

Tm

ns

ssmaxmax is the slip for T is the slip for Tmaxmax to occur to occur

s=1

Tst

Tmax

smax

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Torque-Equation

22

2

)'()'(

'

602

)(3

1,

RsRs

R

s

sst XXRR

Rn

VT

sTorqueStarting

22

2

max

22max

)'()(

1

6022

)(3

)'()(

'

Rssss

s

Rs

R

XXRRn

VT

XR

Rs

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Speed Control

• There are 3 types of speed control of 3 phase induction machines

i.i. Varying rotor resistanceVarying rotor resistance

ii.ii. Varying supply voltageVarying supply voltage

iii.iii. Varying supply voltage and supply frequencyVarying supply voltage and supply frequency

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Varying rotor resistance

• For wound rotor only• Speed is decreasing• Constant maximum

torque• The speed at which max

torque occurs changes• Disadvantages:

– large speed regulation

– Power loss in Rext – reduce the efficiency

T

ns~nNL

T

nr1nr2nr3 n

nr1< nr2< nr3R1R2R3

R1< R2< R3

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Varying supply voltage

• Maximum torque changes• The speed which at max

torque occurs is constant (at max torque, XR=RR/s

• Relatively simple method – uses power electronics circuit for voltage controller

• Suitable for fan type load• Disadvantages :

– Large speed regulation since ~ ns

T

ns~nNL

T

nr1nr2nr3 n

nr1> nr2 > nr3

V1

V2

V3

V1> V2 > V3

V decreasing

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• The best method since supply voltage and supply frequency is varied to keep VV//ff constant

• Maintain speed regulation

• uses power electronics circuit for frequency and voltage controller

• Constant maximum torque

Varying supply voltage and supply frequency

T

nNL1

T

nr1nr2nr3 n

f decreasing

nNL2nNL3