Motor Speed Control
One can control the speed of a DC shunt motor in two ways:
- By varying the current supplied to the rotor
- By varying the current supplied to the stator
As the voltage around the rotor and stator is the same, the speed of the motor can be controlled by controlling the current through the stator or rotor. The current through the stator and rotor branches can be controlled by varying their resistance or by using a silicon controlled rectifier (SCR). The resistance can be increased in the shunt winding and armature branch by placing a rheostat in series. As the armature handles much higher current than the field winding, so the rheostat for controlling current in the armature branch is quite large. This is why having the current controlling rheostat in the field winding is generally preferred.
Shunt field current can change the speed of the motor by 10-20%. As the current through the shunt winding increases, the speed of the rotor increases, thus yielding higher back EMF to maintain the equivalent decrease in current of armature. Conversely, by decreasing the current through the shunt winding, the speed of the motor can be decreased.
Speed of the shunt DC motor also decreases when the motor is operated at lower voltage than its rated voltage, but this makes it inefficient with a tendency to become overburdened and overheat. Generally, motors come with a specified rated speed in RPM and rated voltage. When a shunt DC motor functions below its full voltage, its torque is reduced. For these reasons, it is recommended not to operate the motor below its specified voltage rating.