Induction Motor (Asynchronous Motor) · 2014. 3. 13. · • Induction motor types: Squirrel cage type: Rotor winding is composed of copper bars embedded in the rotor slots and shorted

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Induction Motor(Asynchronous Motor)

ELECTRICAL MACHINES

Compiled byProf 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

induction machines.

– Perform an analysis on induction machines which isthe most rugged and the most widely used machinein 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 intomechanical energy.

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

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Introduction

General 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

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.

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Squirrel Cage Rotor

MZS FKEE, UMP

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Construction (Stator construction)• The stator is the stationary electrical part of the motor.• The stator core of a National Electrical Manufacturers Association

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

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

grouping 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: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: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.


MZS FKEE, UMP

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Wound Rotor

Squirrel-Cage Rotor

/rotor winding

Short circuits allrotor bars.

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Construction (Enclosure)

• The enclosure consists of a frame (or yoke) and two endbrackets (or bearing housings). The stator is mountedinside the frame. The rotor fits inside the stator with aslight air gap separating it from the stator. There is NOdirect physical connection between the rotor and thestator.

Stator

Rotor

Air gap

• The enclosure also protects the electricaland operating parts of the motor fromharmful effects of the environment in whichthe motor operates. Bearings, mounted onthe shaft, support the rotor and allow it toturn. A fan, also mounted on the shaft, isused on the motor shown below for 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 connected to a 3 phase voltage supply, 3 phase current will flow in the windings, which also will induced 3 phase flux in the stator.

• These flux will rotate at a speed called a Synchronous Speed, ns. 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

pfns

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 (θ) 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 t12Currents 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. Slip s

– 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 negligible

)1( snnORn

nns srs

rs −=−

=

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Slip and Rotor Speed• Rotor 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 = slipf = supply frequency

sfpnnf rsr

=−= )(

fsfiii

iipnnf

nnRotorAt

ipnf

nstatorAtNote

r

rsr

pf

rs

s

pf

s

.:)()(

).....(120

)(

:

).....(120

::

120

120

=÷

−=∴

=−

=∴

=

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Principle of Operation

• Torque producing mechanismWhen a 3 phase stator winding is connected to a 3

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

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

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

The rotor current interacts with the flux Φ, producingtorque. The rotor rotates in the direction of the rotatingflux.

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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 regardless 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 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.


f f

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• Step2 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 rotating.

f fr

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• Step3 Eliminate f2

Keep the rotor current same:


MZS FKEE, UMP

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• Step 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.


MZS FKEE, UMP

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• IEEE recommended equivalent circuit

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

rotational loss.


MZS FKEE, UMP

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• IEEE recommended equivalent circuit

Note: can be separated into 2 PARTS

• Purpose : – to obtain the developed mechanical


I1 1R1X

mX

'2X '

2R

ssR −1'

21V

sR2

ssRR

sR )1(2

22 −

+=

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Analysis of Induction Machines

• For simplicity, let assume

Is=I1 , IR=I2

(s=stator, R=rotor)

[ ]RmsTotal

sss

cmm

cmcm

RR

R

ZZZZjXRZ

neglectedRjXZneglectedRjXRZ

jXs

RZ

//;

;;//

;''

+=+=

==≠=

+=

ZRZm

Zs

Vs1φ

Is1φ Im1φ IR1φ

T

ss Z

VI φ

φ1

1 =

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Analysis of Induction Machines

=

=

=

m

RMm

R

RMR

sT

mRRM

ZV

I

ZV

IHence

VZ

ZZV

RulesDividingVoltage

φφ

φφ

φφ

11

11

11

,

//

,

ZRZm

Zs

Vs1φ

Is1φ Im1φ IR1φ

φφ

φφ

11

11

,

sRm

mR

sRm

Rm

IZZ

ZI

IZZ

ZI

RulesDividingCurrent

+

=

+

=

OR

Note : 1hp =746Watt

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Power Flow Diagram

Pin (Motor)

Pin (Stator)

Pcore loss(Pc)

Pair Gap(Pag)

PdevelopedPmechanicalPconverted

(Pm)

Pout, Po

Pstator copper

loss, (Pscu)Protor copper

loss (Prcu)Pwindage, friction,

etc(Pµ - Given)

θcos3 ss IV

sRI R

R''3 2

2

3

c

RM

RV ''3 2

RR RI

−

ssRI RR

1''3 2

Whp 7461 =

ss RI 23

Pin (Rotor)

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Power Flow Diagram• Ratio:

Pag Prcu Pm

sRI R

R''3 2 ''3 2

RR RI

−

ssRI RR

1''3 2

s1 11

−s

1

1 s s−1

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

ss

PP

m

rcu

−=

1

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Efficiency

WattxWhpxPIVP

otherwisePPPPPP

givenarePifPP

out

ssin

mo

lossesino

losses

in

out

746746cos3

,

,

%100

=×==

−=−=

×=

θ

η

µ

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Torque-Equation

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

nPTHence

sradnwhereTPPower

π

πωω

260,

)/(60

2,,

=

==

r

oo

r

mm

nPTTorqueOutput

nPTTorqueMechanical

Thus

π

π

260,

260,

,

=

=

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Torque-Equation• Note that, Mechanical torque can written in terms of circuit

parameters. This is determined by using approximation method

...

...

...

)1(''3

)1(''3

2

2

−==∴

=−=

r

RR

r

mm

mrmR

Rm

ss

RIPT

TPandss

RIP

ωω

ω

+

=∴ 22

2

)'()'('

2)(3

RR

R

s

RMm sXR

sRn

VT

πφ

Hence, Plot Tm vs s

Tm

ns

smax is the slip for Tmax 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

π

φ

+++

=

+±=

22

2

max

22max

)'()(1

6022

)(3

)'()('

Rssss

s

Rs

R

XXRRnV

T

XRRs

π

φ

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Speed Control

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

i. Varying rotor resistanceii. Varying supply voltageiii. Varying 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 electronicscircuit for voltage controller

• Suitable for fan type load• Disadvantages :

– Large speed regulation since~ ns

T

ns~nNL

T

nr1nr2nr3n

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 V/f 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

fdecreasing

nNL2nNL3

Documents

Induction Motor (Asynchronous Motor) · 2014. 3. 13. · • Induction motor types: Squirrel cage type: Rotor winding is composed of copper bars embedded in the rotor slots and shorted