Question

By series armature resistance method, we can get _________________________

(a) Speed above rated speed

(b) Speed equal to rated speed

(c) Speed below rated speed

(d) All speeds are possible

This question was posed to me in an internship interview.

The query is from Speed Control Using Armature Control in DC Motor topic in portion DC Motors of DC Machines

Answer 1

Correct choice is (c) Speed below rated speed

To explain: The drawback of armature speed control method is we get only speeds which are below the rated speed. As the value of series resistance increases, back emf decreases which result in lowering of the speed.