Question

The speed of a DC motor can be varied by changing ______________

(a) Field current

(b) Applied voltage

(c) Resistance in series with armature

(d) Field current, applied voltage or resistance in series with armature any method will work

I had been asked this question in an interview for job.

This interesting question is from Speed Control Using Armature Control in DC Motor in chapter DC Motors of DC Machines

Answer 1

Correct option is (d) Field current, applied voltage or resistance in series with armature any method will work

The explanation is: In field control method we change the flux produced by machine with the help of field current. If applied voltage is changed, definitely speed will change. By adding resistance in series with armature voltage of the armature can be varied to change the speed.