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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.
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 4
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
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 5
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.
7Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
Construction
Cutaway in a typical wound-rotor IM. Notice the brushes and the slip rings
Brushes
Slip rings
8Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
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.
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 9
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.
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 10
Construction (Rotor construction)
• 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.
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 11
Construction
Squirrel cage rotor
Wound rotor
Notice the slip rings
12Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
Construction (Rotor construction)
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 13
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.
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 14
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)
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 15
Nameplate
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 16
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=
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 17
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.
21Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
Slip
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25Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
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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.
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 28
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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)
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 30
Power flow in induction motor
Power Flow Diagram• Ratio:
Pag Prcu Pm
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 31
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
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College 32
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
33Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
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
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
Equivalent Circuit of Induction Machines
35Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
Equivalent Circuit of Induction Machines
• 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
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
Equivalent Circuit of Induction Machines• Step3 Eliminate f2
Keep the rotor current same:
37Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
Equivalent Circuit of Induction Machines
38Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
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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
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
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SEPERATION OF LOSSES IN 3PHASE IM
VO IOWO=
W1+W2Wcu WO– Wcu
(watts)
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
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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.
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
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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
Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
43Dr. K.Srinivasan,Prof/EEE/Tagore Engg College
INDUCTION GENERATOR
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SYNCHRONOUS INDUCTION MOTOR
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