electrical machines-iifiles.teceee-in.webnode.in/200000077-ce6c2cf65e... · overview of...

UNIT‐3 Three Phase Induction Motor

ELECTRICAL MACHINES-II

Learning Outcomes

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

– Perform an analysis on induction machines which isthe most rugged and the most widely used machine inindustry.

2Dr. K.Srinivasan,Prof/EEE/Tagore Engg College

Contents

– Overview of Three‐Phase Induction Motor

– Construction & Principle of Operation

– Slip‐ Torque characteristics

– Equivalent Circuit

– Losses & Efficiency

– Tests on 3Ph IM

– Synchronous Induction Motor

– Double cage rotors

– Induction generator

Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 3

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 otherform of mechanical equipment such as a winder, conveyor,or mixer.


Introduction

General aspects• A induction machine can be used as either ainduction generator or a induction motor.

• Induction motors are popularly used in theindustry

• Main features: cheap and low maintenance

• 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.


Construction

Cutaway in a typical wound-rotor IM. Notice the brushes and the slip rings

Brushes

Slip rings


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.


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.



• 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.


Construction

Squirrel cage rotor

Wound rotor

Notice the slip rings




Wound Rotor

Squirrel-Cage Rotor

/rotor winding

Short circuits allrotor bars.

Construction (Enclosure)

• The enclosure consists of a frame (or yoke) and two endbrackets (or bearing housings). The stator is mounted inside theframe. The rotor fits inside the stator with a slight air gapseparating it from the stator. There is NO direct physicalconnection between the rotor and the stator.


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.

Construction (Enclosure)


Nameplate


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=


18

AC Machine Stator

Dr. K.Srinivasan,Prof/EEE/Tagore Engg College

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)

1 Cycle

Amp

timet0t1 t2 t3 t4

t01 t12Currents in different phases of AC Machine

Principle of Operation• Torque producing mechanism

When a 3 phase stator winding is connected to a 3 phasevoltage supply, 3 phase current will flow in the windings,hence the stator is energized.A rotating flux Φ is produced in the air gap. The flux Φ inducesa voltage Ea in the rotor winding (like a transformer).The induced voltage produces rotor current, if rotor circuit isclosed.The rotor current interacts with the flux Φ, producing torque.The rotor rotates in the direction of the rotating flux.


Slip


27```Dr. K.Srinivasan,Prof/EEE/Tagore Engg College

Direction of Rotor Rotates

• Q: How to change the direction of

• rotation?

• • A: Change the phase sequence of the

• power supply.


29

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)



Power flow in induction motor

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

Efficiency

WattxWhpxPIVP

otherwisePPPPPP

givenarePifPP

out

ssin

mo

lossesino

losses

in

out

746746cos3

,

,

%100

=×==

−=−=

×=

θ

η

μ


Equivalent Circuit of Induction Machines• 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

• 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.

34

f f



• 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.

36

f fr


Equivalent Circuit of Induction Machines• Step3 Eliminate f2

Keep the rotor current same:


39

LOAD TEST ON 3 PHASE INDUCTION MOTOR

1. Torque, T= (T1~T2) x R x 9.81 , Nmwhere R is the radius of the brake drum in metre

2. Output Power PO = 2пNT 60 (watt)3. Input Power Pi = W1+W2 (watt) 4. % Efficiency η = Po Pi x1005. % Slip = NS‐N x100 where NS = 120f P (rpm)

NS

6. Power factor cos ф = Pi VL IL √3


40

SEPERATION OF LOSSES IN 3PHASE IM

VO IOWO=

W1+W2Wcu WO– Wcu

(watts)


41

NO LOAD AND BLOCKED ROTOR TEST ON THREE PHASE INDUCTION MOTOR

NO LOAD TEST1.The output voltage is adjusted to its rated value using three phase variac.2.The readings of ammeter, voltmeter and wattmeter was noted.3.Reduce the output voltage of the variac to zero and switch OFF the supply.

BLOCKED ROTOR TEST1.The circuit in no load test is modified to conduct blocked rotor test replacings meters of appropriate ratings.2.Adjust the variac such that the ammeter reads the rated current.4.Note down the corresponding wattmeter and voltmeter readings.


42

EQUIVALENT CIRCUIT DETERMINATION:

ΦO = cos-1( WO )√3VLIL

ΦSC = cos-1( WSC )√3VSCISC

ISN = ISC ( VOC/VSC)PSN = PSC ( ISN/ISC)2

%SLIP = NS-N X100NS

IO = IW2 + Iμ2

IW = IOcosΦOIμ = IOsinΦORO = V1/IWXO = V1/ IμRO1 = R1 + R2’XO1 = X1 + X2’

FROM BLOCKED ROTOR TEST

ZO1 = VSC√3 ISC

WSC = 3 ISC2RO1

RO1 = WSC3ISC

2

XO1 = ZO12 – RO1

2


INDUCTION GENERATOR


SYNCHRONOUS INDUCTION MOTOR


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