THREE PHASE INDUCTION MOTOR, Rotating Magnetic Field (RMF), Slip, Construction, Torque-speed characteristics Speed control, Power flow, Equivalent Circuit, Maximum torque

THREE PHASE INDUCTION MOTOR
Advantages/D-Advantages/Applications
Rotating Magnetic Field (RMF)
Slip and slip frequency
Construction
Principal of Operation
Torque-speed characteristics
Speed control
Power flow
Equivalent Circuit
Maximum torque
Specifications

1. THREE PHASE INDUCTION MOTOR
•Advantages/D-Advantages/Applications
•Rotating Magnetic Field (RMF)
•Slip and slipfrequency
•Construction
•Principal of Operation
•Torque-speed characteristics
•Speed control
•Power flow
•Equivalent Circuit
•Maximum torque
•Specifications

2. Introduction
•The induction motors are basically ac motors.
•They can operate on either single phase or three
phase ac supply, however the single phase
induction motors are suitable only for few
applications.
•In almost 85% applications the three phase
induction motors are preferred.
•Depending on the type of rotor, the induction
motor are classified into two types, slip ring
induction motors and squirrel cage induction
motors.

3. Advantages of induction motors over
DC motors
1.Low maintenance requirement.
2.Ruggedness, smaller size and weight.
3.Low cost.
4.They can operate in dusty and explosive
environment.
5.They can operate at much higher speed.
6.It can produce sufficient torque.
7.Speed control by using thyristors can give a wide
range of speeds.

4. Disadvantages of induction motors:
1.The efficiency of induction motors varies
with speed.
2.They have low starting torque.
3.They have a lagging and low power factor.
4.Speed control by electrical methods is not
easy.

5. Applications of induction motors
1.Fans
2.Pumps
3.Extruders
4.Conveyors
5.In food and chemical industries
6.Paper and sugar industries etc.
7.Chemical, textile, mines and traction etc.

6. Rotating Magnetic Field (RMF)
•The induction motor operates on the principle of rotating
magnetic field(RMF) which is produced by the stator
winding of the induction motor in the air gap between the
stator and the rotor.
•The stator is a three phase stationary winding which can be
either star connected or delta connected.
•Let the flux produced by line current I
Rbe ø
R, that produced
by I
Ybe ø
Yand that produced by I
Bbe ø
B.
•Mathematically
ø
R= ø
msin ωt
ø
Y= ø
msin (ωt-120
0
)
ø
B= ø
msin (ωt-240
0
)

7. Production of RMF:
•The effective or total flux (ø
T) in the air gap
between the stator and rotor is equal to the
phasorsum of the three component fluxes ø
R, ø
Y
andø
B.
∴ø
T= ø
R+ø
Y+ø
B
•The magnitude of ø
Tat any value of θfrom 0 to
360 is constant.
•ø
Trotates in the clockwise direction in space.
One rotation of ø
Tcorresponding to one cycle.

8. Direction of RMF
•The direction of RMF depends on the
sequence of the ac supply being connected to
the stator winding.
•RMF rotates in the clockwise direction if the
phase sequence is R, Y, B.
•But if the sequence is changed, the direction of
RMF will reverse.

9. Construction of Induction Motor
Fig.(1): construction of three phase induction motor

10. Principle of Operation
•The three phase stator winding of induction motor is connected to
the three ac supply.
•Due to ac voltage applied, current starts flowing in the stator
conductors.
•Due to three phase stator current, a rotating magnetic field (RMF)
of constant amplitude and rotating at a constant speed is set up in
the air gap between stator and rotor.
•The rotor winding is not rotating. So rotating magnetic field cuts
the stationary rotor conductors and induced emfin the rotor
winding.
•The rotor induced voltage gives rise to currents.
•So rotor current will flow in such a direction that the rotor will
experienced a force that accelerates it in same direction as that of
RMF.

11. Synchronous Speed (N
s)
•The synchronous speed is the speed at which
the rotating magnetic field rotates.
•N
sis dependent only on the stator frequency f
1
and number of poles P.
N
s= 120 f
1/P

12. Slip and slipfrequency
s 
N
synN
rot


syn1 
rot
1
2
N
syn 
syn
Slip,
f
r sf
1f
1 f
2
N = N
s(1-s)
s = 1 when the rotor is atstandstill.
s = 0 when the motor runs at synchronous speed. Possible???
s0.025–0.07,normally.Theslipfrequency,sf
1,isthe
frequency of the voltage and current induced in therotor.
Rotor inducedvoltage/phase:
E
24.44N
rˆ
rf
r4.44N
rˆ
rsf
1
Rotor voltages and currents are at the slip frequencysf
1.
Slipfrequency,

13. Torque-speed characteristics
•slip = (N
s–N)/N
s
1.Forward motoring
2.Plugging
3.Regeneration

14. Forward motoring:
The forward motoring region corresponds to the values of slip
between 0 and 1.
In this region, the motor rotates in the same direction as that of
rotating magnetic field.
The torque increases as the slip increases while air gap flux remains
constant.
Once the torque reaches its maximum value at critical slip s
m., the
torque decreases with increase in slip due to reduction in air gap flux.
The region of (0 < s <s
m) is known as the stable region of operation
and the operating point of the motor should be in this region of the
characteristics.
This is stable region because in this region with increase in the torque
demand, the motor speed decreases.
The region of (s
m<s <1) is unstable region because in this region with
increase in torque the speed of the motor increases.

15. Generating region:
For the generating region, the slip needs to be negative and in
between 0 and -1.
The torque produced is in opposite direction to that of the
motoring mode so it is shown to be negative.

16. Plugging or counter current braking:
The motor operates in the plugging or counter
current braking mode for values s > 1.
To get values of s > 1, N must be negative i.e.
Ns and N must have opposite directions i.e. the
RMF and rotor should rotate in opposite
directions.
This is achieved by interchanging any two
phases of the stator .

17. Speed control of Induction motor
•Speed control of induction motor by using variable frequency
drive (VFD):
We know that the actual speed N is given by,
N = N
s(1 –s)
and N
s= 120 f
1/ P
So we can change the actual speed by changing the synchronous
speed. But synchronous speed is changed by changing the stator
supply frequency f
1.
So theoretically we can control the speed by changing only f
1.
But only change in f
1, keeping V
1constant has an adverse effect on
the air gap flux because air gap flux is given by,
ø
ag∝(V
1/ f
1)
If f
1is reduced by keeping V
1constant then there is a possibility of
core saturation.

18. variable frequency drive (VFD).
Hence the ratio (V
1/f
1) is kept constant by
changing both the stator voltage V
1and
frequency f
1simultaneously. This is necessary
to keep the air gap flux constant.
Hence this method is called as constant (v/f)
control. It is also known as variable frequency
drive (VFD).
The block diagram of constant (V/f) control is
as shown in fig.(1).

19. variable frequency drive (VFD).
Fig.(1): constant (V/f) control for induction motor

20. •Operation:
The ac input of constant voltage and constant frequency
is applied to an AC to DC converter which is a rectifier.
At the output of AC to DC converter we get a DC
voltage. A capacitor bank is used to reduced the ripple
content in the DC voltage.
This DC voltage is applied at the input of an inverter.
This inverter converts the DC voltage into a 3 phase
variable voltage variable frequency AC voltage.
This voltage is applied to the stator winding of the
motor. Thus we get the constant V/f control.

21. Power flow in induction motor

22. Power relations3 cos 3 cos
in L L ph ph
P V I V I 2
11
3
SCL
P I R ()
AG in SCL core
P P P P   2
22
3
RCL
P I R conv AG RCL
P P P ()
out conv f w stray
P P P P

  

23. Equivalent Circuit
•We can rearrange the equivalent circuit as
follows
Actual rotor
resistance
Resistance
equivalent to
mechanical load

24. Torque, power and Thevenin’s Theorem
Then the power converted to mechanical (P
conv)11
2
2
22
2
()
eq eq
T
eq eq
VV
I
Z
R
R X X
s


  

 22
2
(1 )
conv
Rs
PI
s


And the internal mechanical torque (T
conv)conv
conv
m
P
T

 (1 )
conv
s
P
s

 22
2
s
R
I
s



25. Torque, power and Thevenin’s Theorem2
1 2
2
22
2
1
()
eq
conv
s
eq eq
V R
T
s
R
R X X
s



 




  

 2 2
1
2
22
2
1
()
eq
conv
s
eq eq
R
V
s
T
R
R X X
s






  



26. Torque-speed characteristics
Typical torque-speed characteristics of induction motor

27. Maximum torque
•Maximum torque occurs when the power
transferred to R
2/sis maximum.
•This condition occurs when R
2/sequals the
magnitude of the impedance R
eq+ j(X
eq+ X
2)max
222
2
()
eq eq
T
R
R X X
s
   max
2
22
2
()
T
eq eq
R
s
R X X



28. Maximum torque
•The corresponding maximum torque of an
induction motor equals
•The slip at maximum torque is directly
proportional to the rotor resistance R
2
•The maximum torque is independent of R
22
max
22
2
1
2 ()
eq
s
eq eq eq
V
T
R R X X


  


29. Maximum torque
Effect of rotor resistance on torque-speed characteristic

30. Specifications of induction motor
•Important specifications of a 3 phase induction motor are as
follows:
1.Number of phases
2.Stator connection
3.Rotor type
4.frequency
5.Rated stator voltage
6.Base speed RPM
7.Power output kW/HP
8.Insulation
9.Power factor
10.duty