Download EEEB283 Electrical Machines & Drives

Induction Motor Review
By
Mr.M.Kaliamoorthy
Department of Electrical & Electronics Engineering
PSNA College of Engineering and Technology
1
Outline
 Introduction
 Construction
 Concept
 Per-Phase Equivalent Circuit
 Power Flow
 Torque Equation
 T- Characteristics
 Starting and Braking
 References
2
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
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
Induction Motor – Construction
 Stator
 balanced 3-phase winding
 distributed winding – coils
distributed in several slots
 produces a rotating magnetic
field
a
120o
120o
c’
b’
 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
c
b
a’
120o
5
Induction Motor – Construction
6
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
2
4
(1)
120
s    f
ns 
f
P
P
P
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
sl  s  m
• Slip, s – ratio between slip speed
  m
and synchronous speed
(3)
s s
s
7
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:
f r  sf
(4)
• Torque produced due to interaction between induced rotor currents
and stator field
• Stator voltage equation: Vs  Rs I s  j2πf Lls I s  E1
• Rotor voltage equation:
sEr  Rr I r  js2πf Llr I r
Er   Rr
 s
 I  j 2πf L I
 r
lr r

8
Induction Motor – Concept
• E1 and Er related by turns ratio aeff
Rs
Lls
Llr
Is
+
+
Vs
Im
+
Lm E1
–
Ir
–
Er
Rr/s
–
• Rotor parameters can be referred to the stator side :
E1  aeff Er
2
Rr'  aeff
Rr
I r'  I r
aeff
2
L'r  aeff
Lr
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Induction Motor –
Per Phase Equivalent Circuit
Rs
Is
Lls
Llr’
Ir ’
+
+
Lm
Vs
Im
–
•
•
•
•
•
•
E1
Rr’/s
–
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
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Induction Motor – Power Flow
Airgap
Power
Pag
Converted
Power Pconv
Mechanical
Power
Rr' P  3I ' 2 R '  1  s 
Pag  3I r
conv
r
r
 s 
s
'2
Electrical
Power
Pout  TLm
Pin 
3VT I L cos
Stator
Copper
Loss (SCL)
Rotor
Copper
Loss (RCL)
'2
r
'
P

3
I
R
r
PSCL  3I Rs RCL
2
s
Rotational losses Prot
(Friction and windage, core and
stray losses)
Note:
Pconv  1  s Pag
PRCL  sPag
11
Induction Motor – Torque Equation
• Motor induced torque is related to converted power by:
Te 
Pconv
m
(5)
• Since Pconv  1  s Pag and r  1  s s , hence
'2
r
3I Rr'
Te 

s
ss
Pag
(6)
• Substituting for Ir’ from the equivalent circuit:
3Rr'
Te 
ss 
Rr'
 Rs 
s

Vs
2


2
   X ls  X lr  


2
(7)
12
Induction Motor –
T- Characteristic
• T-
characteristic
of IM during
generating,
motoring and
braking
13
Induction Motor –
T- Characteristic
 Maximum torque or pullout
torque occurs when slip is:
Te
Pull out
Torque
(Tmax)
smax  
Rr'
Rs   X ls  X lr 
 The pullout torque can be
calculated using:
Trated
2
(8)
2
r
0
s
1
smax
rated
s
0
Tmax 
3
Vs
2
2s  R  R 2   X  X 2 
s
ls
lr
 s
(9) 
14
Induction Motor –
T- Characteristic
Linear region of operation
(small s)
 Te  s
 High efficiency
Te
Pull out
Torque
(Tmax)
 Pout = Pconv – Prot
 Pconv = (1- s )Pag
Trated
 Stable motor operation
s
0
smax
rated
r
s
1
0
15
Induction Motor –
NEMA Classification of IM
s
 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
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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 motorload 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
Induction Motor – Starting
• Methods for starting:
– Stat-delta starter
– Autotransformer starter
– Reactor starter
– Soft Start
18
Induction Motor – Starting
• Star-delta starter
– Special switch used
– Starting: connect as ‘star’ (Y)
• Stator voltages and currents
reduced by 1/√3
• Te  VT2  Te reduced by 1/3
– When reach steady state speed
• Operate with ‘delta’ ( )
connection
– Switch controlled manually or
automatically
19
Induction Motor – Starting
• Autotransformer starter
– Controlled using time relays
– Autotransformer turns ratio aT
• Stator voltages and currents
reduced by aT
• Te  VT2  Te reduced by aT2
– Starting: contacts 1 & 2 closed
– After preset time (full speed
reached):
• Contact 2 opened
• Contact 3 closed
• Then open contact 1
20
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
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
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
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
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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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