GATE EE 2023 Set 1 — Question 30
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Electrical Machines → DC Machines → Generator & Motor Characteristics
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Question
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 . 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).
Correct answer
1340 to 1345
Solution
Given parameters:
.Step 1: Calculate initial back EMF ():
.Step 2: Determine conditions for the second case:
Supply voltage drops by 10%: .
Constant torque load and constant field current ( is constant) implies armature current remains constant: .Step 3: Calculate final back EMF ():
.Step 4: Calculate resultant speed ():
Since and is constant, .
.Rounding to the nearest integer, we get 1343 RPM.
.Step 1: Calculate initial back EMF ():
.Step 2: Determine conditions for the second case:
Supply voltage drops by 10%: .
Constant torque load and constant field current ( is constant) implies armature current remains constant: .Step 3: Calculate final back EMF ():
.Step 4: Calculate resultant speed ():
Since and is constant, .
.Rounding to the nearest integer, we get 1343 RPM.
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