Question

What will happen if field current of motor in Hopkinson’s test is decreased?

(a) Current through motor armature will increase

(b) Current through motor armature will decrease

(c) Current through motor armature will remain constant

(d) Motor armature current can’t be determined

I got this question in an international level competition.

This is a very interesting question from Hopkinson’s Test topic in portion Efficiency and Testing of DC Machines

Answer 1

The correct option is (a) Current through motor armature will increase

Explanation: If field current through motor is decreased, speed of the motor will increase due to inverse proportionality, back emf of generator will increase thus it’ll become greater than back emf of a motor, so to compensate this effect armature current in generator will increase thus, motor armature current will also increase.