GATE 2018 EE – Question 62

Electrical Machines · DC machines: separately excited, series and shunt, motoring and generating mode of operation and their characteristics, speed control of dc motors · 2 marks · Numerical answer

A 200 V DC series motor, when operating from rated voltage while driving a certain load, draws 10 A current and runs at 1000 r.p.m. The total series resistance is $1\ \Omega$. The magnetic circuit is assumed to be linear. At the same supply voltage, the load torque is increased by 44%. The speed of the motor in r.p.m. (rounded to the nearest integer) is ________.

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Correct answer: 825

Explanation

For a series motor on a linear magnetic circuit, $T\propto I^2$, so $I=10\sqrt{1.44}=12$ A. The back emf is $E_1=200-10=190$ V and $E_2=200-12=188$ V. Since $E\propto\phi N\propto IN$, $N_2=1000\times\frac{188}{190}\times\frac{10}{12}=824.6\approx825$ rpm.