Question

440 V, 77 A, 700 rpm DC separately excited motor having a resistance of 0.11 ohms excited by an external dc voltage source of 24 V. Calculate the torque developed by the motor on full load.

(a) 453.51 N-m

(b) 451.24 N-m

(c) 440.45 N-m

(d) 452.64 N-m

I have been asked this question in a job interview.

My question is based upon Energy Relations During Starting topic in portion Starting of Electric Drives

Answer 1

The correct choice is (a) 453.51 N-m

To explain: Back emf developed in the motor during the full load can be calculated using equation Eb = Vt-I×Ra = 431.53  V and machine constant Km = Eb÷Wm which is equal to 5.88. Torque can be calculated by using the relation T = Km × I = 5.88×77 = 453.51 N-m.