GATE 2018 EE – Question 34

Electrical Machines · DC machines: separately excited, series and shunt, motoring and generating mode of operation and their characteristics, speed control of dc motors · 1 mark · Numerical answer

A separately excited dc motor has an armature resistance $R_a=0.05\ \Omega$. The field excitation is kept constant. At an armature voltage of 100 V, the motor produces a torque of 500 Nm at zero speed. Neglecting all mechanical losses, the no-load speed of the motor (in radian/s) for an armature voltage of 150 V is ________ (up to 2 decimal places).

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Correct answer: 600

Explanation

At zero speed the armature current is $\frac{100}{0.05}=2000$ A, so $K\phi=\frac{500}{2000}=0.25$ Nm/A. The no-load speed at 150 V is $\omega=\frac{150}{0.25}=600$ rad/s.