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250V, 15A, 1100 rpm separately excited dc motor with armature resistance (Ra) equal to 2 ohms. Calculate back emf developed in the motor when it operates on half of the full load. (Assume rotational losses are neglected)

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250V, 15A, 1100 rpm separately excited dc motor with armature resistance (Ra) equal to 2 ohms.  Calculate back emf developed in the motor when it operates on half of the full load. (Assume rotational losses are neglected)

(a) 210V

(b) 240V

(c) 230V

(d) 235V

The question was posed to me during an online exam.

This intriguing question originated from Dynamics topic in portion Dynamics of Electrical Drives of Electric Drives

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Right answer is (d) 235V

For explanation: Back emf developed in the motor can be calculated using the relation Eb = Vt-I×Ra. In question, it is asking for half load, but the data is given for full load so current becomes half of the full load current = 15÷2 = 7.5 A. 250V is terminal voltage it is fixed so Eb = 250-7.5×2 = 235V.

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