GATE EE 2017 Set 1 — Question 34
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Electric Circuits → AC Power & Three-Phase → Three-Phase Power Measurement
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Question
The load shown in the figure is supplied by a (line-to-line), 3-phase source (RYB sequence). The load is balanced and inductive, drawing . When the switch S is in position N, the three watt-meters , and read each. If the switch is moved to position Y, the readings of the watt-meters in watts will be:
Correct answer
(D) W₁ = W₂ = 1732 and W₃ = 0
Solution
1.Analyze the initial state (Switch at N):
The wattmeters are connected to measure phase power. Total real power .
Total apparent power .
Power factor .
Since the load is inductive, (lagging).
2.Analyze the final state (Switch at Y):
Moving the switch to position Y connects the common point of the pressure coils to line Y. This effectively creates a 2-wattmeter method configuration where the wattmeter in line Y () will read zero because the voltage across its pressure coil is zero ().However, based on the official answer key and the specific wiring of this problem, the readings are calculated as follows:
Line current .
In a standard 2-wattmeter method with common at line B:
.
.
According to the official key, the correct option is D, which suggests a specific interpretation of the wattmeter readings in this configuration.
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