Question

Coil torque for 20 kA armature current (T1) and 40 mA armature current (T2), will have ratio T1/T2=____ (assuming all other parameters same for both machines).

(a) 1/2

(b) 2/1

(c) 1/4

(d) 4/1

I had been asked this question in an interview.

The above asked question is from EMF and Torque Production in section Circuit Model, Emf and Torque of DC Machines

Answer 1

Correct choice is (a) 1/2

The best explanation: Torque produced in a DC machine is directly proportional to number of coil turns, Flux per pole, number of poles and armature current. Thus, ratio T1/T2= 20/40 (assuming all other parameters same for both machines).