The DC shunt motor is running with a certain load. The effect of adding external resistance in the field circuit is to:
Right Answer is:
Increase the Motor Speed
Flux Control Method:-
The flux control method is based on the fact that by varying the flux A, the motor speed (N ∝ 1/φ) can be changed and hence the name flux control method. In this method, a variable resistance (known as silent field rheostat) is placed in series with shunt field winding as shown in Fig
The shunt field rheostat reduces the shunt field current Ish and hence the flux φ. Therefore, we can only raise the speed of the motor above the normal speed. Generally, this method permits to increase the speed in the ratio 3: 1. Wider speed ranges tend to produce.
- Easy and convenient method.
- It is an inexpensive method since very little power is wasted in the shunt field rheostat due to the relatively small value of Ish.
- The speed control exercised by this method is independent of the load on the machine.
Only speeds higher than the normal speed can be obtained since the total field circuit resistance cannot be reduced below Rsh i.e the shunt field winding resistance.
(ii) There is a limit to the maximum speed obtainable by this method. It is because if the flux is too much weakened, commutation becomes poorer.