motor

1. Induction Motor Review
By
Mr.M.Kaliamoorthy
Department of Electrical & Electronics Engineering
PSNA College of Engineering and Technology
1

2. Outline
 Introduction
 Construction
 Concept
 Per-Phase Equivalent Circuit
 Power Flow
 Torque Equation
 T- Characteristics
 Starting and Braking
 References
2

3. Introduction
 Induction motors (IM) most widely used
 IM (particularly squirrel-cage type) compared to
DC motors
Rugged
Lower maintenance
More reliable
Lower cost, weight, volume
Higher efficiency
Able to operate in dirty and explosive
environments
3

4. Introduction
• IM mainly used in applications requiring
constant speed
– Conventional speed control of IM expensive or
highly inefficient
• IM drives replacing DC drives in a number of
variable speed applications due to
– Improvement in power devices capabilities
– Reduction in cost of power devices
4

5. Induction Motor – Construction
 Stator
 balanced 3-phase winding
 distributed winding – coils
distributed in several slots
 produces a rotating magnetic
field
 Rotor
 usually squirrel cage
 conductors shorted by end rings
 Rotating magnetic field induces
voltages in the rotor
 Induced rotor voltages have
same number of phases and
poles as in stator winding
5
a
b
b’
c’
c
a’
120o
120o
120o

6. Induction Motor – Construction
6

7. Induction Motor – Concept
• Stator supplied by balanced 3-phase AC source (frequency f Hz or 
rads/sec )
– field produced rotates at synchronous speed s rad/sec
(1)
where P = number of poles
• Rotor rotates at speed m rad/sec (electrical speed r = (P/2) m)
• Slip speed, sl – relative speed
(2)
between rotating field and rotor
• Slip, s – ratio between slip speed
and synchronous speed (3)
7
f
P
P
s 


4
2

 f
P
ns
120

m
s
sl 

 

s
m
s
s


 


8. Induction Motor – Concept
• Relative speed between stator rotating field and rotor induces:
– emf in stator winding (known as back emf), E1
– emf in rotor winding, Er
• Frequency of rotor voltages and currents:
(4)
• Torque produced due to interaction between induced rotor currents
and stator field
• Stator voltage equation:
• Rotor voltage equation:
8
  1
2 E
I
L
πf
j
I
R
V s
ls
s
s
s 


sf
fr 
 
  r
lr
r
r
r
r
lr
r
r
r
I
L
πf
j
I
s
R
E
I
L
πf
js
I
R
sE
2
2











9. Induction Motor – Concept
• E1 and Er related by turns ratio aeff
• Rotor parameters can be referred to the stator side :
9
r
eff
r
r
eff
r
eff
r
r
r
eff
L
a
L
R
a
R
a
I
I
E
a
E
2
'
2
'
'
1




Rr/s
+
Vs
–
Rs
Lls Llr
+
E1
–
Is
Ir
Im
Lm
+
Er
–

10. Induction Motor –
Per Phase Equivalent Circuit
• Rs – stator winding resistance
• Rr’ – referred rotor winding resistance
• Lls – stator leakage inductance
• Llr’ – referred rotor leakage inductance
• Lm – mutual inductance
• Ir’ –referred rotor current
10
Rr’/s
+
Vs
–
Rs
Lls Llr’
+
E1
–
Is Ir’
Im
Lm

11. Induction Motor – Power Flow
11

cos
3 L
T
in
I
V
P 
m
L
out T
P 

Stator
Copper
Loss (SCL)
s
s
SCL R
I
P 2
3

Rotor
Copper
Loss (RCL)
'
2
'
3 r
r
RCL R
I
P 
Airgap
Power
Pag
Converted
Power Pconv
Rotational losses Prot
(Friction and windage, core and
stray losses)
s
R
I
P r
r
ag
'
2
'
3
 




 

s
s
R
I
P r
r
conv
1
3 '
2
'
Electrical
Power
Mechanical
Power
ag
RCL sP
P 
Note:
  ag
conv P
s
P 
 1

12. Induction Motor – Torque Equation
• Motor induced torque is related to converted power by:
(5)
• Since and , hence
(6)
• Substituting for Ir’ from the equivalent circuit:
(7)
12
  ag
conv P
s
P 
 1   s
r s 
 
 1
 




















2
2
'
2
'
3
lr
ls
r
s
s
s
r
e
X
X
s
R
R
V
s
R
T

m
conv
e
P
T


s
r
r
s
ag
e
s
R
I
P
T


'
2
'
3



13. Induction Motor –
T- Characteristic
• T-
characteristic
of IM during
generating,
motoring and
braking
13

14. Induction Motor –
T- Characteristic
 Maximum torque or pullout
torque occurs when slip is:
(8)
 The pullout torque can be
calculated using:
(9)
14
  




 



2
2
2
max
2
3
lr
ls
s
s
s
s X
X
R
R
V
T

 2
2
'
max
lr
ls
s
r
X
X
R
R
s




r
s
Trated
Pull out
Torque
(Tmax)
Te
0 rated
smax
s
1 0

15. Induction Motor –
T- Characteristic
Linear region of operation
(small s)
 Te  s
 High efficiency
 Pout = Pconv – Prot
 Pconv = (1- s )Pag
 Stable motor operation
15
r
s
Trated
Pull out
Torque
(Tmax)
Te
0 rated
smax
s
1 0

16. Induction Motor –
NEMA Classification of IM
 NEMA = National Electrical
Manufacturers Association
 Classification based on T-
 characteristics
 Class A & B – general
purpose
 Class C – higher Tstart (eg:
driving compressor
pumps)
 Class D – provide high Tstart
and wide stable speed
range but low efficiency
16
s

17. Induction Motor – Starting
• Small motors can be started ‘direct-on-line’
• Large motors require assisted starting
• Starting arrangement chosen based on:
– Load requirements
– Nature of supply (weak or stiff)
• Some features of starting mechanism:
– Motor Tstart must overcome friction, load torque and inertia of motor-
load system within a prescribed time limit
– Istart magnitude ( 5-7 times I rated) must not cause
• machine overheating
• Dip in source voltage beyond permissible value
17

18. Induction Motor – Starting
• Methods for starting:
– Stat-delta starter
– Autotransformer starter
– Reactor starter
– Soft Start
18

19. Induction Motor – Starting
• Star-delta starter
– Special switch used
– Starting: connect as ‘star’ (Y)
• Stator voltages and currents
reduced by 1/√3
• Te  VT
2  Te reduced by 1/3
– When reach steady state speed
• Operate with ‘delta’ ( )
connection
– Switch controlled manually or
automatically
19

20. Induction Motor – Starting
• Autotransformer starter
– Controlled using time relays
– Autotransformer turns ratio aT
• Stator voltages and currents
reduced by aT
• Te  VT
2  Te reduced by aT
2
– Starting: contacts 1 & 2 closed
– After preset time (full speed
reached):
• Contact 2 opened
• Contact 3 closed
• Then open contact 1
20

21. Induction Motor – Starting
• Reactor starter
– Series impedance (reactor)
added between power line
and motor
– Limits starting current
– When full speed reached,
reactors shorted out in stages
21

22. Induction Motor – Starting
• Soft Start
– For applications which require
stepless control of Tstart
– Semiconductor power switches
(e.g. thyristor voltage controller
scheme) employed
• Part of voltage waveform
applied
• Distorted voltage and current
waveforms (creates
harmonics)
– When full speed reached, motor
connected directly to line
22

23. Induction Motor – Braking
• Regenerative Braking:
– Motor supplies power back to line
• Provided enough loads connected to line to absorb power
– Normal IM equations can be used, except s is negative
– Only possible for  > s when fed from fixed frequency source
• Plugging:
– Occurs when phase sequence of supply voltage reversed
• by interchanging any two supply leads
– Magnetic field rotation reverses  s > 1
– Developed torque tries to rotate motor in opposite direction
– If only stopping is required, disconnect motor from line when  = 0
– Can cause thermal damage to motor (large power dissipation in rotor)
23

24. Induction Motor – Braking
• Dynamic Braking:
– Step-down transformer and
rectifier provides dc supply
– Normal: contacts 1 closed, 2 & 3
opened
– During braking: Contacts 1
opened, contacts 2 & 3 closed
– Two motor phases connected to
dc supply - produces stationary
field
– Rotor voltages induced
– Energy dissipated in rotor
resistance – dynamic braking
24

25. References
• Chapman, S. J., Electric Machinery Fundamentals, McGraw
Hill, New York, 2005.
• Rashid, M.H, Power Electronics: Circuit, Devices and
Applictions, 3rd ed., Pearson, New-Jersey, 2004.
• Trzynadlowski, Andrzej M. , Control of Induction Motors,
Academic Press, 2001.
• Nik Idris, N. R., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
• Ahmad Azli, N., Short Course Notes on Electrical Drives,
UNITEN/UTM, 2008.
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