GATE EE 2015 Set 1 — Question 58
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Electrical Machines → DC Machines → Generator & Motor Characteristics
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Question
A separately excited DC motor runs at on no load when its armature terminals are connected to a DC source and the rated voltage is applied to the field winding. The armature resistance of this motor is . The no-load armature current is negligible. With the motor developing its full load torque, the armature voltage is set so that the rotor speed is . When the load torque is reduced to of the full load value under the same armature voltage conditions, the speed rises to . Neglecting the rotational losses, the full load armature current (in Ampere) is _______.
Correct answer
8 to 8
Solution
1.At no load: , , . Back EMF .
2.Since field voltage is constant, flux is constant. . The constant .
3.At full load torque : speed , armature voltage . Back EMF .
4.The armature voltage equation is .
5.Torque . When load torque is reduced to , .
6.At load: speed , same armature voltage . Back EMF .
7.The armature voltage equation is .
8.Equating the two expressions for :
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