Question

40 V, 7 A, 70 rpm DC separately excited motor having a resistance of 0.3 ohms excited by an external dc voltage source of 4 V. Calculate the torque developed by the motor on half load.

(a) 18.10 N-m

(b) 4.24 N-m

(c) 40.45 N-m

(d) 52.64 N-m

This question was posed to me in an interview for internship.

My query is from Induction Motors in chapter Speed Control of Direct Current Motors & Induction Motors of Electric Drives

Answer 1

Correct choice is (a) 18.10 N-m

For explanation I would say: Back emf developed in the motor during the full load can be calculated using equation Eb = Vt-I×Ra = 37.9 V and machine constant Km = Eb÷Wm which is equal to 5.17. Torque can be calculated by using the relation T = Km× I = 5.17×3.5 = 18.10 N-m.