module3 induction motor

7/30/2019 Module3 Induction Motor

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Induction Motor

(Asynchronous Motor)

ELECTRICAL MACHINES

Compiled by

Prof 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 is

the most rugged and the most widely used machine

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 intomechanical 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 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 aselectromagnets.


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Squirrel Cage Rotor


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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 ofseveral hundred thin laminations.

Stator laminations are stacked togetherforming 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 anelectromagnet. The stator windings are connected directly to thepower 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 terminalscan be connected to external circuits through slip ringsand brushes.

Easy to control speed, more expensive.



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Wound Rotor

Squirrel-Cage Rotor

/rotor winding

Short circuits all

rotor 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 from

harmful 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 statorwinding is connected to a 3 phasevoltage supply, 3 phase current will flow in the windings,which also will induced3 phase flux in the stator.

These flux will rotate at a speed called a SynchronousSpeed, n

s

. 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

RMF


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a Fc

-93

10

113

216

-1.5

-1

-0.5

0

0.5

1

1.5

a

cb

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)


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AC Machine Stator


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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


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1 Cycle

Amp

timet0

t1 t2 t3 t4

t01 t12Currents in different phases of AC Machine


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Slip Ring Rotor

The rotor contains windings similar to stator.

The connections from rotor are brought out using slip rings that

are rotating with the rotor and carbon brushes that are static.


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Slip and Rotor Speed

1. Slip s

The rotor speed of an Induction machine is different from thespeed 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( snnOR

n

nns sr

s

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= slip

f = supply frequency

sf

pnnf rsr

)(

fsfiii

iipnn

f

nnRotorAt

ipn

f

nstatorAt

Note

r

rsr

p

f

rs

s

p

f

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 is produced in the air gap. The flux induces a voltage Ea in the rotor winding (like atransformer).

The induced voltage produces rotor current, if rotor

circuit is closed.The rotor current interacts with the flux , producing

torque. 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.

E i l t Ci it f I d ti


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Conventional equivalent circuit

Note:

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

phase equivalent circuit. The equivalent circuitalways 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 ofE2 is the same as that ofE1 since the rotor is at

standstill. At standstill s=1.


Machines

f f



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Machines



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Machines

Step2 Rotor winding is shorted

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

Note:

the frequency ofE2 isfr=sfbecause rotor is rotating.

f fr



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Step3 Eliminatef2

Keep the rotor current same:


Machines



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Step 4 Referred to the stator side

Note: X2 and R2 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.

Rcrepresents core loss, which is the core loss of stator side.


Machines



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IEEE recommended equivalent circuit

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

rotational loss.


Machines



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IEEE recommended equivalent circuit

Note: can be separated into 2 PARTS

Purpose :

to obtain the developed mechanical


Machines

I1 1R1X

mX

'

2X'2R

s

sR

1'21V

s

R2

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

ZZZZ

jXRZ

neglectedRjXZ

neglectedRjXRZ

jXsRZ

//

;

;

;//

;''

ZRZm

Zs

Vs1

Is1 Im1 IR1

T

s

sZ

V

I

1

1


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Analysis of Induction Machines

m

RM

m

R

RM

R

s

T

mRRM

Z

VI

Z

VIHence

VZ

ZZV

RulesDiv idingVoltage

1

1

1

1

11

,

//

,

ZRZm

Zs

Vs1

Is1 Im1 IR1

11

11

,

s

Rm

mR

s

Rm

Rm

I

ZZ

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)

PdevelopedPmechanical

Pconverted

(Pm)

Pout, Po

Pstator copper

loss, (Pscu)

Protor copper

loss (Prcu)Pwindage, friction,

etc

(P - Given)

cos3 ssIV

s

RI RR

''32

2

3

c

RM

R

V ''32

RR RI

s

sRI RR

1''3

2

Whp 7461

ss RI2

3

Pin (Rotor)


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Power Flow Diagram

Ratio:

Pag Prcu Pm

s

RI RR

''3 2 ''3 2 RR RI

s

sRI RR

1''3 2

s1 11

s1

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

WattxWhpxPIVP

otherwise

PPPPPP

givenarePif

PP

out

ssin

mo

lossesino

losses

in

out

746746cos3

,

,

%100


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Torque-Equation

Torque, can be derived from power equation in term ofmechanical power or electrical power.

nPTHence

sradn

whereTPPower

260,

)/(60

2,,

r

oo

r

m

m

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

parameters. This is determined by using approximationmethod

.. .

.. .

.. .

)1('

'3

)1('

'3

2

2

r

RR

r

mm

mrmR

Rm

s

s

RI

PT

TPandss

RIP

22

2

)'()'(

'

2

)(3

RR

R

s

RM

msXR

sR

n

VT

Hence, Plot Tmvs 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

s

stXXRR

R

n

VT

sTorqueStarting

22

2

max

22max

)'()(

1

6022

)(3

)'()('

Rssss

s

Rs

R

XXRRnVT

XRRs


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Speed Control

There are 3 types of speed control of 3 phaseinduction machines

i. Varying rotor resistance

ii. Varying supply voltage

iii. Varying supply voltage and supply frequency


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Varying rotor resistance

Forwound rotor only Speed is decreasing

Constant maximum

torque

The speedat which maxtorque occurs changes

Disadvantages:

large speed regulation

Power loss in Rext reduce the efficiency

T

ns~nNL

T

nr1nr2nr3 n

nr1< nr2< nr3R1R2R3

R1


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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 methoduses power electronics

circuit 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

Varying supply voltage and supply


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The best method since

supply voltage and supply

frequency is varied to keepV

/f constant Maintain speed regulation

uses power electronics

circuit for frequency andvoltage controller

Constant maximum torque

Varying supply voltage and supply

frequency

T

nNL1

T

nr1nr2nr3 n

f

decreasing

nNL2nNL3

module3 induction motor

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