GATE EE 2020 Set 1 — Question 38
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Electrical Machines → DC Machines → Armature Reaction & Commutation
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Question
A dc shunt motor has an armature resistance of and a field resistance of . When the motor is operated on no-load at rated voltage, it draws an armature current of and runs at . When a load is coupled to the motor, it draws total line current of at rated voltage, with a reduction in the air-gap flux due to armature reaction. Voltage drop across the brushes can be taken as per brush under all operating conditions. The speed of the motor, in rpm, under this loaded condition, is closest to:
Correct answer
(C) 1220
Solution
1.Calculate shunt field current:
2.No-load condition:
Armature current .Total brush voltage drop .
Back emf .
Speed .
3.Loaded condition:
Total line current .Armature current .
Back emf .
4.Flux reduction:
Due to armature reaction, the flux is reduced by , so .5.Calculate loaded speed ():
Using the relation :6.Conclusion:
The speed is closest to . Thus, option (C) is correct.Continue learning with Success Tracker
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