EEEB283 Electrical Machines & Drives
Download
Report
Transcript 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
P
f
P
ns
f
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: V s R s I s j 2 πf Lls I s E 1
• Rotor voltage equation:
sE r R r I r js 2 πf L lr I r
R
E r r I r j 2 πf L lr I r
s
8
Induction Motor – Concept
• E1 and Er related by turns ratio aeff
Rs
Lls
Llr
Is
+
+
Vs
+
Lm E1
Im
–
Ir
Er
–
Rr/s
–
• Rotor parameters can be referred to the stator side :
Ir
E 1 a eff E r
Ir
R r a eff R r
L r a eff L r
'
2
'
'
a eff
2
9
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
10
Induction Motor – Power Flow
Airgap
Power
Pag
Converted
Power Pconv
'
Pag 3 I
Electrical
Power
'2
r
Rr
Pconv
s
Mechanical
Power
1 s
3I R
s
'2
r
'
r
Pout T L m
Pin
3V T I L cos
Stator
Copper
Loss (SCL)
PSCL 3 I R s
2
s
Rotational losses Prot
Rotor
Copper
Loss (RCL)
'2
r
(Friction and windage, core and
stray losses)
PRCL 3 I R r
'
Note:
Pconv 1 s Pag
PRCL sP ag
11
Induction Motor – Torque Equation
• Motor induced torque is related to converted power by:
Te
• Since Pconv 1 s Pag
Pconv
(5)
m
and r 1 s s , hence
Te
Pag
s
'2
r
'
3I Rr
s s
(6)
• Substituting for Ir’ from the equivalent circuit:
'
Te
3Rr
Vs
2
2
'
s s
Rr
2
X ls X lr
R s
s
(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)
'
s max
Rr
R s X ls X lr
2
(8)
2
The pullout torque can be
calculated using:
Trated
r
0
s
1
smax
rated
s
0
T max
3
2 s R
s
Vs
2
2
2
R s X ls X lr
(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
16
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
24
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.
25