Question

340 V, 45 A, 1400 rpm DC separately excited motor having a resistance of .7 ohm excited by an external dc voltage source of 90 V. Calculate the torque developed by the motor on full load.

(a) 94.73 N-m

(b) 94.52 N-m

(c) 93.37 N-m

(d) 94.42 N-m

I had been asked this question at a job interview.

This key question is from Modified Speed Torque Characteristics of DC Series Motors in portion Characteristics of DC & AC Motors of Electric Drives

Answer 1

The correct answer is (a) 94.73 N-m

Easy explanation: Back emf developed in the motor during the full load can be calculated using equation Eb = Vt-I×Ra = 308.5 V and machine constant Km = Eb÷Wm which is equal to 2.1053. Torque can be calculated by using the relation T = Km × I = 2.1053×45 = 94.73 N-m.