Question

20 V, 10 A, 10 rpm separately excited dc motor with armature resistance (Ra) equal to .8 ohms.  Calculate back emf developed in the motor when it operates on the full load. (Assume rotational losses are neglected)

(a) 12 V

(b) 14 V

(c) 13 V

(d) 11 V

I had been asked this question by my college director while I was bunking the class.

This intriguing question originated from Dynamic Braking of DC Shunt Motors topic in chapter Braking of DC Motors & Induction Motors of Electric Drives

Answer 1

Right choice is (a) 12 V

Best explanation: Back emf developed in the motor can be calculated using the relation Eb = Vt – I×Ra. In question, it is asking for a full load. 20 V is terminal Voltage it is fixed so Eb = 20-10×.8 = 12 V.