Question

State whether true or false:
Consider a separately excited DC motor. If field current is increased while armature terminal voltage is held constant then speed of motor will also increase.
(A) True
(B) False

Answer 1

Hint: In a separately excited DC motors, the armature windings and the field will get the supply separately. Thus, the armature current has no influence in field windings.

Complete solution:
Generally, in DC motors, the electrical energy is converted to mechanical energy. The motors will have both stator and rotor. The stator will have field windings and the rotor will have armature windings or coils and they can rotate. We know that in separately excited DC motors; the armature current does not flow through the field windings. This is because the field windings are separately excited using an external supply.
Thus, the torque which causes the rotation of the motor is depending on the varying field flux. And it will be independent of the armature current. When we are considering a separately excited dc motor, the magnetic flux will be proportional to the current produced. And when the flux of the field is interacting with flux of armature, then the motor rotates. The torque is proportional to the square of the current.
The statement, if field current is increased while armature terminal voltage is held constant then speed of motor will also increase is true.

The answer is option A.

Note: In separately excited DC motors, both field coil and armature coil are separately given the DC supply. Thus, the field current influences the speed of the motor. The current input to the DC motor and the speed of the DC motor is directly proportional, so current increases, speed also increases.