Answer MET Question 30

Question 136: Compare methods of obtaining speed regulation of three-phase induction motor generally used in tankers by means of: A. Rotor resistance; B. Cascade system; C. Pole-changing. Give examples where each system may be employed with advantage.

Answer: A. Rotor resistance control:By introducing external resistance in the rotor circuit, the speed of the motor can be reduced. The change in speed depends upon both rotor circuit resistance and load.This method is similar to armature rheostat control method of DC shunt motors.

Advantages :

a. Absence of in-rush starting current

b. Availability of full-rated torque at starting

c. High live power factor

d. Absence of line current harmonics

e. Smooth and wide range of speed control

Main drawbacks of this speed control:

a. Reduced efficiency because the slip energy is wasted in the rotor circuit resistance.

b. Speed changes very widely with load variation, also speed depends on rotor resistance.

c. Unbalance in voltage and current if rotor circuit resistance are not equal

d.This method is applicable to three-phase slip-ring induction motor only.

e. Due to power loss in the resistance ,this method is used where speed changes are required for short period only.

f. With increase in rotor resistance copper losses increase and operating efficiency decreases.

B. Speed control by Cascade Connection: This method needs two motor, one of them is wound motor. The two motors are mechanically coupled together to drive a common load. The starter of slip ring induction motor is connected to three-phase supply and its rotor is connected to stator of the other machine. This method is used for cranes There are four possible speeds can be obtained by this arrangement.

$\displaystyle \small N = \frac{120f}{P_1}$

$\displaystyle \small N = \frac{120f}{P_2}$

$\displaystyle \small N = \frac{120f}{P_1+P_2}$

$\displaystyle \small N = \frac{120f}{P_1-P_2}$ 

C. Speed Control By Pole Changing: The change of number of poles is done by having two or more entirely independent stator windings in the same slots. Each winding gives a different number of poles, so we will get different speeds. Due to cost and complex switching arrangements, it is not practical to provide more than two arrangements of poles (ie, two normal speeds). This method is applicable to squirrel cage induction motor only. It is not practically applicable with wound rotor.