Induction Motor

Abhisek Pal
Department of Electrical Engineering
Indian School of Mines
Dhanbad-826004, India
E-mail: abhisekpal.ee@gmail.com
Induction Motor
Theory, Operation & Control

Presentation Outline
 Introduction
 Working Principles
 Characteristics
 Methods of starting
 Methods of speed control
 Basic principles of Thyristor controlled variable speed
drives
 Industrial application & control of electrical motors – A
brief review
11-04-2016 A. Pal, EE Dept., ISM Dhanbad 2

What is motor ?
Motor is a device which converts electrical energy into
mechanical energy via the principle of electromagnetic
interaction.
Introduction

Introduction (contd…)
 Induction Motor (IM)
Induction motor is also called asynchronous motor as it runs at a speed
other than the synchronous speed.
 Why Induction motor (IM) is so popular :
rugged,
compact,
easier to maintain ,
cost-effective,
 highly efficient and reliable.
 Applications of Induction Motor (80% of the Motors)
Squirrel Cage Induction Motor (SCIM)
(90% of the IMs)
 Lathes
 Drilling machines
 Agricultural and industrial
pumps
 Industrial drives.
Slip Ring Induction Motor (SRIM)
 Lifts
 Cranes
 Conveyors

 Major parts of Induction Motor:
(Like any other electrical motor, induction motor have two main parts
namely stator and rotor.)
Stator: As its name indicate stator is a stationary part of
induction motor. A three phase supply is fed to the stator of
induction motor.
Rotor: The rotor is a rotating part of induction motor. The
rotor is connected to the mechanical load through the shaft.
LOAD
Rotor of SRIMSCIM

Depending upon the type of rotors, the three phase induction
motor are classified as:
 Squirrel Cage Induction Motor (SCIM), and
 Slip Ring Induction Motor (SRIM) or Wound Rotor Induction Motor
(WRIM).

Rotor of SCIM
Rotor of SRIM

Working of Induction Motor
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
Fb
FcFb
Fc Fc Fb
Space angle () in degrees
Fc
Fb
t = t0= t4
t = t1
t = t2 t = t3
t = t0= t4
RMF (Rotating Magnetic Field) [video link]

RMF (Rotating Magnetic Field) [video link]

tcoscosFF ma 
   
 120120  tcoscosFF mb
   
 240240  tcoscosFF mc
     tcosFFFFt,Fm.m.f.Resultant mcba
2
3
a’
c’
b’
b c
a

 
   
   
αsinF
2
3
90-αcosF
2
3
tα,F,90ωtAt
45-αcosF
2
3
tα,F,45ωtAt
αcosF
2
3
tα,F,0ωtAt
m
m
m
m







a''ph
ofAxis
b''ph
ofAxis
c''ph
ofAxis
asallymathematicexpressedbecanFandF,Fm.fsm.Three cba

 
 



tcosF
FFF
t,Fm.m.f.Resultant
m
cba
2
3

Working of Induction Motor (Contd…)

 IEEE standard equivalent circuit of IM
Per Phase Equivalent Circuit of Induction Motor
I1
1R1X
mX
'
X2
'
R2
s
s
R
1'
21V
slips
stator,torefresistancerotorR,resistancetatorsR
stator,torefreactancerotorXreactance,statorX
reactance,gmagnetizinX
A,currentstatorI
V,voltageappliedstatorV
,where
'
'
m






21
21
1
1

Electromagnetic Torque
Speed–Torque Characteristics
 2'
2
'2
2
'2
s
e
sXR
RsE
ω
3
T 22


 2'
21
2
'
1
'
2
1
s
e
XX
s
R
R
s
R
V
ω
3
T
2
2



















   
s
rs
s
rs
'
21
'
21
2
1
s
e
n
nn
or
ω
ωω
sslip,
Ωstator,torefresistancerotorRΩ,resistancestatorR
Ωstator,torefreactancerotorXΩreactance,statorX
Vemf,RotorE
Vvoltage,supplyV
rpsspeed,sSynchronouω
Nmtorque,neticElectromagT
where,








'
2
2
2
e_max
2X
KE
T 
 2'
2
'2
2
'2
e_st
XR
RE
KT 22


sω
3
Kwhere, 
Maximum Torque
Starting Torque

 Speed-Torque characteristics
Characteristics
Starting Torque
Maximum Torque

Power Flow Diagram
Pin (Motor)
Pin (Stator)
Pcore loss
(Pc)
Pair Gap
(Pag)
Pdeveloped
Pmechanical
Pconverted
(Pm)
Pout, Po
Pstator copper
loss, (Pscu)
Protor copper
loss (Prcu)
Pwindage, friction,
etc
(P - Given)
cosIV 113
s
'R
'I 22
23
2
0
3 







c
RMS
R
V 'R'I 2
2
23





 
s
s
'R'I
1
3 2
2
2
Whp 7461 
1
2
13 RI
Pin (Rotor)
%
PPP
P
,Efficiency
cufl
100
0
0



 lossesfrictionwindage,core,lossttanConsPfl 

Starting Methods of Induction Motor
Selection of starting methods are based on
 Design and size of the motor,
 Type of driven load, and
 Capacity of supply line
Methods of starting
 Full voltage or Direct On-Line (DOL) starting,
 Reduced voltage starting
 Stator reactor or resistor starting,
 Auto-transformer starting,
 Star-delta starting

Full voltage or Direct On-Line (DOL) starting
Features:
• Simplest & inexpensive,
• Starting current 6-10
times full-load current
at low pf,
• Suitable for < 5 HP
motors
*May cause
objectionable voltage
drop in supply mains.

Stator reactor starting method
Features:
• Incurs small power loss,
• Reactor is more effective in
reducing stator voltage.

Auto transformer staring method
Features:
• Stator voltage is reduced by transformation,
• Most efficient.

Star-delta starting method
Features:
• Applicable for delta connected
stator winding under normal
running,
• Cheap, effective, used extensively.
*Not used for voltage > 3.3 kV
as delta winding becomes very
much expensive.

Methods of Speed Control
The speed controls from rotor side (for SRIM) :
 Adding external resistance,
 Injecting slip frequency emf into rotor side.
p
f*
ns
120

 
 s
p
f*120
snn sr


1
1
Hzfrequency,supplyf
polesofno.p
where,


Synchronous speed,
Rotor speed,
The speed control from stator side (for both SCIM/SRIM):
 Controlling supply voltage.
 V / f control or frequency control.
 Changing the number of stator poles.

Varying supply voltage
Methods of speed control (contd…)
T (Nm)
ns ~ nNL
TL
nr1nr2
nr3
n (rpm)
nr1 > nr2 > nr3
V1
V2
V3
V1> V2 > V3
V decreasing
Tmax

Varying supply frequency
Methods of Speed Control (contd…)
ns ns1 ns3
ns < ns1 < ns2 < ns3
Tmax
T (Nm)
n (rpm)
ns3
Constant Power
Region
Constant
Torque
Region

V / f control or Varying supply voltage and supply frequency
ns1nr1nr2
nr3
n (rpm)
f decreasing
ns2ns3
Tmax
TL
T (Nm)
fluxmachine
f
V


V / f control or Varying supply voltage and supply frequency

Varying rotor resistance

Varying rotor resistance
ns~nNL
TL
nr1nr2nr3
nr1 < nr2 < nr3 < nr2
R1
R2
R3
R1< R2< R3< R4
Tmax
T (Nm)
n (rpm)
nr3
R4

Variable Speed Drive (General Block Diagram)

Thyristor controlled variable speed drives

Thyristor Controlled Variable Speed Drive (Contd…)
 sTime


Thyristor Controlled Variable Speed Drive (Contd…)

Open Loop Speed Control of Induction Motor Drive
Induction
Motor
Power Processor
Unit
(Power Electronics
Converter)
Power
Source
Control
Unit
Open loop speed control
of IM drive
Command
signals

Closed Loop Speed Control of Induction Motor Drive

Speed Sensorless Control of Induction Motor Drive
Induction
Motor
Power Processor
Unit
(Power Electronics
Converter)
Power
Source
Control
Unit
v, i
feedback
Speed sensorless
control of IM drive
Command
signal

Reference
[1] M. G. Say, Alternating current machines, Pitman, 4th Ed., 1976.
[2] D. P. Kothari and I. J. Nagrath, Electric machines, Tata McGraw Hill, 2010.
[3] P. S. Bimbhra, Electrical machinery, Kaanna Publishers, 2010.
[4] B. K. Bose, Modern power electronics and ac drives, Prentice Hall, 2002.
[5] Electrical4u.com, ‘Electrical Engineering and Technology’, 2016. [Online].
Available: http://www.electrical4u.com/working-principle-of-three-phase-
induction-motor. [Accessed: 15- March- 2016].

Gratitude for your attention
Thank you

Discussion

~ END ~

Sinusoidally Distributed Field

Torque –Speed Characteristic for Stator Freq. Control

Torque –Slip Characteristic Adding External Rotor Resistances

Variable Speed Drives

Three Phase Inverter Switching State

Volt/Hz (or V/f) Control of Induction Motor
Block Diagram Schematic of V/f control of VSI fed 3-phase Induction Motor
drive:

PWM Waveform

Inverter Output Voltage Waveform

Speed – Torque Characteristic

Squirrel Cage Induction Motor

Rotor Internal structure: Squirrel Cage Induction Motor

Wound Rotor (or Slip Ring) Induction Motor

Wound Rotor Induction Motor
The 3-phase stator is
represented by 3 concentrated
coils displaced 120o from each
other. The wound rotor is also
represented by 3 concentrated
coils and it is rotating at the
speed n=0.8 pu. With balanced
3-phase sinusoidal currents in
the stator coils, a rotating
magnetic field at 1.00 pu speed
is established (represented by
the yellow space vector) which
overtakes the rotor at slip
speed (1-n), thereby inducing
in the rotor coils slip-
frequency voltages and
corresponding currents

 Speed control of DC motor
Ward –Leonard Method of Speed Control

 Advantages :
 It is a very smooth speed control system over a very wide range (from zero to
normal speed of the motor).
 The speed can be controlled in both the direction of rotation of the motor
easily.
 The motor can run with a uniform acceleration.
 Speed regulation of DC motor in this ward Leonard system is very good.
 Disadvantages :
˟ The system is very costly because two extra machines (motor-generator set)
are required.
˟ Overall efficiency of the system is not sufficient especially it is lightly loaded.
 Applications:
This Ward Leonard method of speed control system is used where a very wide
and very sensitive speed control is of a DC motor in both the direction of
rotation is required. This speed control system is mainly used in colliery
winders, cranes, electric excavators, mine hoists, elevators, steel rolling mills
and paper machines etc.
Ward –Leonard Method (contd…)

~ END ~

Induction Motor

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