Question

In variable speed motor ____________

(a) Stronger commutating field is needed at low speed than at high speed

(b) Weaker commutating field is needed at low speed than at high speed

(c) Same commutating field is needed at low speed and at high speed

(d) Cannot be determined

This question was posed to me in my homework.

The query is from Speed Control Using Field Control of Series Motor in section DC Motors of DC Machines

Answer 1

Correct answer is (b) Weaker commutating field is needed at low speed than at high speed

The explanation: According to commutating fields of machine, speed and overall performance of a machine is determined. For low speeds of motor in a variable speed motor we need weaker commutating field.