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GATE 2023 EE – Question 30

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 rated 400 V, 15 A, 1500 RPM drives a constant torque load at rated speed operating from 400 V DC supply drawing rated current. The armature resistance is 1.2 Ω. If the supply voltage drops by 10% with field current unaltered then the resultant speed of the motor in RPM is ____________ (Round off to the nearest integer).

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Correct answer: 1343

Explanation

At rated conditions $E=400-15\times1.2=382$ V. With constant torque and constant field the armature current stays at 15 A. At 360 V, $E'=360-18=342$ V. Speed is proportional to $E$: $N'=1500\times\dfrac{342}{382}=1343$ rpm.