Question

What will happen if excitation of DC shunt motor is changed?

(a) Torque will remain constant

(b) Torque and power both will change

(c) Torque will change but power will remain constant

(d) Torque, power and speed, all will change

The question was asked during an internship interview.

I would like to ask this question from Speed Control Using Field Control of Shunt Motor topic in division DC Motors of DC Machines

Answer 1

Correct choice is (c) Torque will change but power will remain constant

To explain I would say: The motor will accelerate the mechanical load connected during this period but no increase in the mechanical load as Pload = T1W1 = T2W2 where W2 >W1. So, at the higher speed there is less electrical torque for the same mechanical load / power.