Question

100 V, 2 A, 90 rpm separately excited dc motor with armature resistance (Ra) equal to 8 ohms.  Calculate back emf developed in the motor when it operates on 3^th/4 of the full load. (Assume rotational losses are neglected)

(a) 100 V

(b) 87 V

(c) 88 V

(d) 90 V

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This interesting question is from Methods of Starting Electric Motors topic in portion Starting of Electric Drives

Answer 1

Right choice is (c) 88 V

Explanation: Back emf developed in the motor can be calculated using the relation Eb = Vt-I×Ra. In question, it is asking for 3^th/4  load, but the data is given for full load so current becomes 3^th/4  of the full load current = 2÷1.33 = 1.5 A. 100 V is terminal voltage it is fixed so Eb = 100-1.5×8 = 88 V.