The amount of back e.m.f. of a shunt motor will increase when
Right Answer is:
When the Field is Weakened
The speed of DC shunt can be controlled by various Methods such as
- Flux Control Method
- Armature voltage control method
Flux Control Method
The speed of DC Shunt Motor is inversely proportional to the flux. The flux is dependent on the current through the shunt ﬁeld winding. Thus flux can be controlled by adding a rheostat (variable resistance) in series with the shunt ﬁeld winding. At Starting the rheostat R is kept at the minimum. The supply voltage is at its rated value. So the current through shunt field winding is also at its rated value. Hence the speed is also rated speed i.e at normal speed. When the resistance R is increased due to which shunt ﬁeld current Ish decreases, hence decreasing the ﬂux produced. As N =(I/Φ), the speed of the motor increases beyond its rated value. Now as we know that the speed of DC motor is directly proportional to the flux hence when the speed is increased the back EMF also increased. Thus by this method, the speed control above rated value is possible.