GATE EE 2024 Set 1 — Question 55
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Electrical Machines → DC Machines → Losses & Efficiency
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Question
A 5 kW, 220 V DC shunt motor has 0.5 armature resistance including brushes. The motor draws a no-load current of 3 A. The field current is constant at 1 A. Assuming that the core and rotational losses are constant and independent of the load, the current (in amperes) drawn by the motor while delivering the rated load, for the best possible efficiency, is ________ (rounded off to 2 decimal places).
Correct answer
27.19 to 27.39
Solution
Given:
Rated Power W, V, .
No-load current A, Field current A.
No-load armature current A.
No-load power input to armature = W.
Armature copper loss at no-load = W.
Constant rotational losses W.
Field loss W.
Total constant losses W.
Efficiency is maximum when variable losses equal constant losses:
A.
Total current drawn A.
Rounding to 2 decimal places, we get 37.28 A.
Rated Power W, V, .
No-load current A, Field current A.
No-load armature current A.
No-load power input to armature = W.
Armature copper loss at no-load = W.
Constant rotational losses W.
Field loss W.
Total constant losses W.
Efficiency is maximum when variable losses equal constant losses:
A.
Total current drawn A.
Rounding to 2 decimal places, we get 37.28 A.
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