Question

780 V, 97 A, 1360 rpm separately excited dc motor with armature resistance (Ra) equal to 9 ohms.  Calculate back emf developed in the motor when it operates on one-fourth of the full load. (Assume rotational losses are neglected)

(a) 564.75 V

(b) 561.75 V

(c) 562.45 V

(d) 565.12 V

I got this question in a job interview.

The query is from Modified Speed Torque Characteristics of DC Series Motors in division Characteristics of DC & AC Motors of Electric Drives

Answer 1

The correct choice is (b) 561.75 V

To explain: Back emf developed in the motor can be calculated using the relation Eb = Vt-I×Ra. In question, it is asking for one-fourth load, but the data is given for full load so current becomes one-fourth of the full load current = 97÷4 = 24.25 A. 250 V is terminal voltage it is fixed so Eb = 780-24.25×9 = 561.75 V.