Induction Motor
Transportation Prime-mover
Induction Motor
6
Application Of Slip Ring Motor
Wound rotr
compressor
Central air conditoining
7
Inside View of An Induction Motor
8
Stator
Rotor
9
Construction
1- STATOR
A three-phase windings is put in slots cut on the
inner surface of the stationary part. The ends of
these windings can be connected in star or delta to
form a three phase connection. These windings
are fed from a three-phase ac supply.
10
Construction
2- Rotor
it can be either:
a- Squirrel-cage (brushless) (SCIM)
The squirrel-cage winding consists of bars
embedded in the rotor slots and shorted at both
ends by end rings.
The squirrel-cage rotor is the most common type
because it is more rugged, more economical, and
simpler.
Short circuits all
rotor bars.
/rotor winding
11
Construction
b- Slip ring (wound-rotor) (WRIM)
The wound-rotor winding has the same
form as the stator winding. The windings
are connected in star. The terminals of the
rotor windings are connected to three slip
rings. Using stationary brushes pressing
against the slip rings, the rotor terminals
can be connected to an external circuit.
12
Construction-Wound Rotor Induction Motor (WRIM)
Slip rings
Cutaway in a
typical woundrotor IM.
Notice the
brushes and the
slip rings
Brushes
13
Advantages of Slip Ring and Squirrel Cage Motor
SQUIRREL CAGE
SLIP RING
cheaper and more robust
slightly
the starting torque is much
higher and the starting current
much lower
higher efficiency and power
factor
the speed can be varied by means
of external rotor resistors
explosion proof, since the
absence of slip-rings and brushes
eliminates risk of sparking.
Three-Phase Induction Motor
Stator
ib
ia
Stator Windings
Rotor
Rotor Windings
ic
15
Principle of Operation
If the stator windings are connected to a threephase supply; a rotating field will be produced in the
air-gap. This field rotates at synchronous speed ns. This
rotating field induces voltages in the rotor windings.
Since the rotor circuit is closed, the induced voltages in
the rotor windings produce rotor currents that interact
with the air gap field to produce torque. The rotor will
eventually reach a steady-state speed nm that is less
than the synchronous speed ns.
The difference between the rotor speed and the
synchronous speed is called the slip, s ,
ns
120 f s
P
S
ns n
ns
S
s
s
fs is the supply frequency
P is the total number of poles.
2 n
60
n in rpm
ω in rad/s
16
Rotating Magnetic Field
Currents in different phases of AC Machine
t01 t12
Amp
t0
t1
t2
t3
t4
time
1 Cycle
17
MMF due to ac current in phase “a”
Axis of phase a
a
.
Fa
Axis of
a’ +
phase a
1
0.8
0.6
0.4
0.
a 2
0
F
-0.2
-0.4
-0.6
-0.8
-1
-90
t1
t12
a’
t0
t01
a’
a
t2
-40
10
60
110
160
210
260
Space angle ( ) in degrees
Pulsating mmf
18
MMF due to three-phase currents in 3-ph winding
1.5
Fc ’
a
b
c’
Fa
c
b
a’ Fb
Fb a
c’
b
0.5
F
a’
t = t1
Fb
-1
-1.5
-93
10
113
216
Space angle ( ) in degrees
Fb a
Fc ’
b
c
0
-0.5
t = t0
F
t = t0
Fa F Fc
1
F
’
c
Fa
b’
c
b
Fc a’
t = t2
c’
a
b’
c
b
Fc a’ Fb
F t = t3
MMF’s at various instant (Rotating mmf)
19
Rotating Magnetic Field
a
c
b
Time
t1
t2
t3
20
b
a
c
Rotating Magnetic Field
Time
t1
t2
t3
ia
c
ib
c
a
b
a
b
ic
21
Rotating Magnetic Field
3
max
2
b
a
3
max
2
c
3
2
3
2
Time
3
max
2
t1
ia
t2
t3
3
max
2
c
max
At t1
max
At t2
3
max
2
3
max
2
ib
At t3
a
b
ic
3
max
2
22
Principle of Operation: Definitions
nm = the rotor speed (the motor speed) w.r.t. stator
ns = the speed of stator field w.r.t. stator or the synch. speed
nr = the speed of rotor field w.r.t rotor
S = the slip
fs = the frequency of the induced voltage in the stator (stator
or supply frequency)
fr = the rotor circuit frequency or the slip frequency
nr
nS
fr
nm
snS
p
p
( nr )
( nS
120
120
Slip rpm
p
nm )
( S nS ) S f S
120
23
Induced EMF
The instantaneous value of the induced voltage
in N turns coil is given by:
e
N
Let
d
dt
m
e
phase voltage is 1/√3
of the normal voltage
N
sin( t )
m
cos( t )
N 2 f
m
sin( t
90 )
The r.m.s. value of the induced voltage per phase is
E rms
4 . 44 f N
ph
where
Nph is the number of turns in series per phase
f is the frequency
p is the flux per pole
Kw is the winding factor
p
K
w
phase voltage is equal
to the line voltage.
24
Equivalent Circuit Per Phase
• At standstill (nm= 0 , S = 1)
The equivalent circuit of an induction motor at standstill is the
same as that of a transformer with secondary short circuited.
R1
R2
I
Ir I2
Ic
I1
V
X2
X1
Rc
Xc
E2
E1
N2
N1
E1
N1
I2
I
N2
E2
N1
N2
I2
I1
25
Equivalent Circuit Per Phase (cont.)
At any slip S
When the rotor rotates with speed nm the rotor circuit
frequency will be:
fr
S fS
Therefore induced voltage in the rotor at any slip S will
be E2S = S E2 , similarly X2S = SX2
and the rotor equivalent circuit per-phase will be:
sX2
I2
I2
sE2
Where
X2
X2
R2
I2
E2
SE 2
R2 jSX 2
I2
R2/S
R2
E2
E2
( R2 / s ) jX 2
R2(1-S)/S
26
Combined Equivalent Circuit
At any slip S
X1
R1
I
Rc
R2 /s
N1 : N2
Ic
I1
V
X2
I2
Xc
E1
E2
27
Combined Equivalent Circuit
R1
X 2’
X1
V
Xc
Rc
R'2
N1
R2
N2
I2 ’
Ic
I1
2
'
I2
R 2’ /s
E 1 E 2’
N2
I2
N1
'
X2
N1
X2
N2
2
28
R1
X 2’
X1
V
E 1 E 2’
Xc
Rc
X 2’
X1
E1
Xc
Rc
R '2
s
R 2’
I2 ’
Ic
I1
V
I2 ’
Ic
I1
R1
R 2’ /s
R '2
R'2
(1 s)
s
E 2’
R '2
(1
s
s)
29
Combined Equivalent Circuit
Airgap &
Magnetic Circuit
Stator Circuit
I1
V1
R1
Io
X1
E1
Rc
Rotor Circuit
X
Xm
Load +
Rotational losses
'
2
I’2
'
2
R
(1 S ) R2'
S
30
R1
Rc
R 2’
I2 ’
Ic
I1
V
X 2’
X1
Xc
E1
E 2’
R'2
(1 s)
s
Equivalent to transformer’s
secondary windings
Equivalent to transformer’s
primary windings
Equivalent to transformer’s
load
31
R1
X 2’
X1
V
I2 ’
Ic
I1
E1
Xc
Rc
R1
X1
X 2’
R 2’
I2 ’
Ic
Rc
R'2
(1 s)
s
E 2’
I1
V
R 2’
Xc
E 1 E2 ’
R'2
(1 s)
s
32
Approximate Equivalent Circuit
Xeq
I1
Req
I2 ’
Ic
V
Rc
R'2
(1 s )
s
Xc
Req R1
X eq X 1
'
R2
'
X2
33
Determination of the Equivalent Circuit Parameters
1. Resistance Determination
The winding’s resistance can be approximated by applying a DC voltage to a
stationary machine’s winding and measuring the current. However, AC resistance
is slightly larger than DC resistance (skin effect).
For Y-Connection
For -Connection
Vdc
I dc
Vdc
I dc
2
Rdc
3
2 Rdc
Rdc
Rdc
3 Vdc
2 I dc
1 Vdc
2 I dc
Determination of the Equivalent Circuit Parameters
2. No-Load Test ( S = 0 )
A
W1
I
V
I.M.
V0
Ic
Rc
Im
Xm
No Load
W2
Measured values VoL = V1L , I L , and Pot = W1 ± W2
Calculate the per phase values V0 , I and P0= Pot /3
Po
cos( o )
V0 I
Ic
Rc
I cos(
V0
Ic
o
)
Xm
Im
V0
Im
I sin(
o
)
35
Determination of the Equivalent Circuit Parameters
3. Blocked rotor Test (S =1)
A
IbR1+R 2
W1
’
V
I.M.
Vb
X1+X’2
Zb
Blocked
W2
Measured values VbL < V1L , IbL , and Pbt= W1 ± W2
Calculate the per phase values Vb , Ib and Pb = Pot /3
Rb
R2
R1
Rb
R2
R1
Pb
,
2
Ib
Zb
&
Vb
,
Ib
Xb
X1
X1
X2
X2
Xb
2
Z b2 Rb2
36
Power Flow In Induction Motors
dev
= 3V1I1cos
37
Power Flow In Induction Motors
PAG
PRCL
Pdev
R2
S
3( I 2 ) 2 R2
3( I 2 ) 2
R (1 S )
3( I 2 ) 2
S
I1
2
PGA : PRCL : Pdev
X1
R1
1 : S : (1 S )
V1
X2’
I2’
R2’
I
Ic
Rc
Im
I2’
R2’(1-S)
S
Xm
PAG
38
Power Flow In Induction Motor
Input Power (Pin)
Stator Losses:
Copper losses (Pcu 1)
Core losses (Piron)
Airgap Power (Pg)
Rotor Copper Losses (Pcu 2)
Developed Power (Pd)
Rotational Losses (Protational)
Output Power (Pout)
39
Power Flow In Induction Motor
R1
Pin
Pcu 1
3
Piron
V2
3
Rm
Pcu 2
3I
' 2
2
Pg
R2' s Pg
V
Td
s
Protational
3I
' 2
2
’
Xc
s
E1 E2
’
Pg : Pcu 2 : Pd
R2'
(1 s) Pg (1 s)
s
Pout
R2
I2
Rc
'
' 2 R2
3 ( I2 )
Pd
X2
Ic
I1
3 V I1 cos 1
I12 R1
X1
T
’
’
'
R2
(1 s )
s
1 : s : (1 s )
Td
40
Torque Characteristics
Xeq
I1
I2’
Ic
V
I
R'2
(1 s )
s
Xc
V
'
2
R1
Td
Rc
Req
Pd
'
2
R
s
2
X eq2
3
'
R
( I 2' ) 2 2 (1 s )
s
3 V 2 R2' (1 s )
s
R1
'
2
R
s
2
X
2
eq
41
Torque Characteristics
s
0
Td
n
3 V 2 R2'
Pd
s
'
2
R
s
R1
s
2
X eq2
n
s
smax
s
1
Tst
T
max
ns
n
ns
s
s
Torque
42
Torque Characteristics
s
Small Slip
Maximum Torque
0
s max
Large Slip
1
T
st
T
max
Torque
43
Torque Characteristics
44
Maximum Torque
Td
Set
Td
s
smax
2
Pd
3V R
s
s
R1
0
R'2
2
2
R1 X eq
Tmax
2
'
2
R
s
3V
2 s R1
'
2
X
2
eq
2
2
R1
2
X eq
45
Starting of Induction Motor
Xeq
I1
I2’
Ic
V
Req
R'2
(1 s )
s
Xc
Rc
V
'
I 2st
R1
' 2
R2
2
X eq
46
Starting by Reducing Voltage
• Starting current is reduced (good)
• Starting torque is reduced (cannot start heavy loads)
• Maximum torque is reduced (Motor acceleration is
low)
• Speed at maximum torque is unchanged
47
Starting by Reducing Voltage
V
I '2st
n
R1
V2 < V1
ns
V1
Tmax
s max
3V
2 s R1
s
Tst 1
Tmax
2
X eq
2
2
R1
2
X eq
3 V 2 R2'
Tst
Tst 2
' 2
R2
' 2
2
R1 R
Torque
smax
X eq2
R'2
R12
2
X eq
48
Starting by Adding Rotor Resistance
n
Radd3 > Radd2 > Radd1
Radd3
ns
' 2
2
R1 R2
X eq
R'2
smax
2
R12 X eq
Radd2
Radd1
s max
Tmax
Tst1
Tst2
Tst3=Tmax
V
I '2st
3V2
2 s R1
R12
2
X eq
Torque
3 V 2 R2'
Tst
s
' 2
2
R1 R
X eq249
Starting by Adding Rotor Resistance
Effect of rotor resistance on torque-speed characteristic
Torque/Speed Curve for varying R2
Tmax
ST max
•
•
Vth2
3
2
S
( X th
T
X 2' )
R2'
( X th
R1< R2< R3
X 2' )
The
maximum
torque
is
independent
of
the
rotor
resistance. However, the value of
the rotor resistance determines
the slip at which the maximum
torque will occur. The torque-slip
characteristics for various values
of are shown.
To get maximum torque at
starting::
ST max
1
i.e. R2
( X th
R3
R2
R1
T
n
nr3 nr2 nr1 n ~n
s
NL
nr1< nr2< nr3
X2)
51
Starting by Adding Rotor Resistance
• Starting current is reduced (good)
• Starting torque is increased (good)
• Maximum torque is unchanged (Motor
acceleration is high)
• Speed at maximum torque is reduced
52
Speed Control of IM by Changing Frequency
ns
Changing
ns
120 f
p
f
120
p
Change f
Continuous variation
Change p
Step variation
53
Speed Control of IM by Changing
Frequency
T
max
Speed
ns1
ns2
ns3
f1
3V2
R12
2 s R1
2
X eq
f1 > f2 > f3
f2
3 V 2 R2'
Td
f3
s
s
R1
'
2
R
s
2
'
2
' 2
2
X eq2
2
3V R
Tst
s
Torque
R1 R
X eq2
54
Classes of squirrel-cage motors
According to the National Electrical Manufacturing
Association (NEMA) criteria, squirrel-cage motors are
classified into class A, B, C or D. The torque-speed curves
and the design characteristics for these classes are :
Class
Starting
Current
Starting
Torque
Rated
Load Slip
A
Normal
Normal
< 5%
B
Low
Normal
< 5%
C
Low
High
< 5%
D
Low
Very High
8-13 %
D
A
C
B
55
Classes of squirrel-cage motors
1. Motors with standard locked-rotor torque (NEMA B)
• Good for fans, centrifugal pumps, machine tools…
2. High-starting torque motors (NEMA C)
• Good for starting under load – hydraulic pumps and piston-type
compressors
3. High-slip motors (NEMA D)
• Good for starting high-inertia loads
56
Double Squirrel-Cage Rotor Construction
Following double squirrel-cage arrangements can also be used to obtained a high
value of effective resistance at starting and a low value of the resistance at fullload operation.
It consists of two layers of bars, both short-circuited by end rings.
The upper bars are small in cross-section and have a high resistance.
They are placed near the rotor surface so that the leakage flux sees a path of high
reluctance; consequently, they have a low leakage inductance.
The lower bars have a large cross-section, a lower resistance and a high leakage
inductance.
Double squirrel-cage rotor bars
57
Double Squirrel-Cage Rotor Construction
At starting, rotor frequency is high and very little current flows
through the lower bars; the effective resistance of the rotor is
then the high resistance upper bars.
At normal low slip operation, leakage reactances are negligible,
and the rotor current flows largely through the low resistance
lower bars; the effective rotor resistance is equal to that of the
two sets of bars in parallel..
Double squirrel-cage rotor bars
58
Deep-Bar Rotor Construction
The use of deep, narrow rotor bars produces torque-slip characteristics
similar to those of a double-cage rotor.
Leakage inductance of the top cross-section of the rotor bar is
relatively low; the lower sections have progressively higher leakage
inductance.
At starting, due to the high rotor frequency, the current is concentrated
towards the top layers of the rotor bar.
At full-load operation, the current distribution becomes uniform and
the effective resistance is low.
Deep-bar rotor construction
59