Page 117 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

where, f = frequency of the supply and P = number of stator poles.
Thus, synchronous speed changes with change in supply frequency, and thus running speed also changes.
However, this method is not widely used. This method is used where, only the induction motor is supplied by a
generator (so that frequency can be easily changed by changing the speed of prime mover).
c Changing the number of stator poles: From the above equation, it can be also seen that synchronous speed
(and hence, running speed) can be changed by changing the number of stator poles. This method is generally
used for squirrel cage induction motors, as squirrel cage rotor adapts itself for any number of stator poles.
Change in stator poles is achieved by two or more independent stator windings wound for different number of
poles in same slots.

For example, a stator is wound with two 3phase windings, one for 4 poles and other for 6 poles.
For supply frequency of 50 Hz
i Synchronous speed when 4 pole winding is connected, Ns = 120 x (50/4) = 1500 RPM
ii Synchronous speed when 6 pole winding is connected, Ns = 120 x (50/6) = 1000 RPM

2 Speed control from rotor side
a Rotor rheostat control: This method is similar to that of armature rheostat control of DC shunt motor. But
this method is only applicable to slip ring motors, as addition of external resistance in the rotor of squirrel cage
motors is not possible.
b Cascade operation: In this method of speed control, two motors are used. Both are mounted on a same
shaft so that both run at same speed. One motor is fed from a 3phase supply and other motor is fed from the
induced emf in first motor via slip-rings. The arrangement is as shown in Fig 1.

Motor A is called main motor and motor B is called auxiliary motor.
Let, N = frequency of motor A
s1
N = frequency of motor B
s2
P = number of poles stator of motor A
1
P = number of stator poles of motor B
2
N = speed of the set and same for both motors
f = frequency of the supply

Now, slip of motor A, S = (N - N) / N .
s1
1
s1
Frequency of the rotor induced emf in motor A, f1 = S1f. Now, auxiliary motor B is supplied with the rotor induce
emf therefore, N = (120f ) / P = (120S f) / P . Now putting the value of S = (N - N) / N s1
1
2
2
s2
s1
1
1
120f (N N)
N s1
s2 P N
2 s1
At no load, speed of the auxiliary rotor is almost same as its synchronous speed. i.e. N = Ns2. From the above
equations, it can be obtained that
N 120f
P P
1 2
104
CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75