1.

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 VI 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»

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.



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