GATE EE 2022 Set 1 — Question 62
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Electrical Machines → DC Machines → Speed Control & Starting
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Question
A 280 V, separately excited DC motor with armature resistance of 1 and constant field excitation drives a load. The load torque is proportional to the speed. The motor draws a current of 30 A when running at a speed of 1000 rpm. Neglect frictional losses in the motor. The speed, in rpm, at which the motor will run, if an additional resistance of value 10 is connected in series with the armature, is __________. (round off to nearest integer)
Correct answer
480 to 485
Solution
Given:
, , , .
Field excitation is constant, so flux is constant.
Load torque .For a DC motor, . Since is constant, .
Therefore, .Back EMF .
Case 1: .
Case 2: . .Also, . Since is constant, .
.Equating the two expressions for :
.Rounding to the nearest integer, the speed is 483 rpm.
, , , .
Field excitation is constant, so flux is constant.
Load torque .For a DC motor, . Since is constant, .
Therefore, .Back EMF .
Case 1: .
Case 2: . .Also, . Since is constant, .
.Equating the two expressions for :
.Rounding to the nearest integer, the speed is 483 rpm.
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