Network Theory Questions and Answers – Power Measurement in Three-Phase Circuits

This set of Network Theory Multiple Choice Questions & Answers focuses on “Power Measurement in Three-Phase Circuits”.

1. The wattmeter method is used to measure power in a three-phase load. The wattmeter readings are 400W and -35W. Calculate the total active power.
a) 360
b) 365
c) 370
d) 375
View Answer

Answer: b
Explanation: Wattmeters are generally used to measure power in the circuits. Total active power = W1 + W2 = 400 + (-35) =365W.

2. The wattmeter method is used to measure power in a three-phase load. The wattmeter readings are 400W and -35W. Find the power factor.
a) 0.43
b) 0.53
c) 0.63
d) 0.73
View Answer

Answer: a
Explanation: We know tanØ = √3((WR – WY)/(WR + WY)) => tanØ = √3 (400-(-35))/(400+(-35))=2.064 => Ø = 64.15⁰. Power factor = 0.43.

3. The wattmeter method is used to measure power in a three-phase load. The wattmeter readings are 400W and -35W. Find the reactive power.
a) 751.44
b) 752.44
c) 753.44
d) 754.44
View Answer

Answer: c
Explanation: Reactive power = √3VLILsinØ. We know that WR – WY = 400-(-35)) = 435 = VLILsinØ. Reactive power = √3 x 435 = 753.44 VAR.
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4. The input power to a three-phase load is 10kW at 0.8 Pf. Two watt meters are connected to measure the power. Find the reading of higher reading wattmeter.
a) 7.165
b) 6.165
c) 6.165
d) 4.165
View Answer

Answer: a
Explanation: WR + WY = 10kW. Ø = cos-10.8=36.8o => tanØ = 0.75 = √3 (WR-WY)/(WR+WY)=(WR-WY)/10. WR-WY=4.33kW. WR+WY=10kW. WR=7.165kW.

5. The input power to a three-phase load is 10kW at 0.8 Pf. Two watt meters are connected to measure the power. Find the reading of lower reading wattmeter.
a) 1.835
b) 2.835
c) 3.835
d) 4.835
View Answer

Answer: b
Explanation: WR + WY = 10kW. Ø = cos-10.8=36.8o => tanØ = 0.75 = √3 (WR-WY)/(WR+WY)=(WR-WY)/10. WR-WY=4.33kW. WR+WY=10kW. WY=2.835kW.
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6. The readings of the two watt meters used to measure power in a capacitive load are -3000W and 8000W respectively. Calculate the input power. Assume RYB sequence.
a) 5
b) 50
c) 500
d) 5000
View Answer

Answer: d
Explanation: Toatal power is the sum of the power in R and power in Y. So total power = WR+WY = -3000+8000 = 5000W

7. The readings of the two watt meters used to measure power in a capacitive load are -3000W and 8000W respectively. Find the power factor.
a) 0.25
b) 0.5
c) 0.75
d) 1
View Answer

Answer: a
Explanation: As the load is capacitive, the wattmeter connected in the leading phase gives less value. WR=-3000. WY=8000. tanØ = √3 (8000-(-3000))/5000=3.81 => Ø = 75.29⁰ => cosØ = 0.25.
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8. The wattmeter reading while measuring the reactive power with wattmeter is?
a) VLILsecØ
b) VLILsinØ
c) VLILtanØ
d) VLILcosØ
View Answer

Answer: b
Explanation: The wattmeter reading while measuring the reactive power with wattmeter is wattmeter reading = VLILsinØ VAR.

9. The total reactive power in the load while measuring the reactive power with wattmeter is?
a) √3VLILcosØ
b) √3VLILtanØ
c) √3VLILsinØ
d) √3 VLILsecØ
View Answer

Answer: c
Explanation: To obtain the reactive power, wattmeter reading is to be multiplied by √3. Total reactive power = √3VLILsinØ.
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10. A single wattmeter is connected to measure the reactive power of a three-phase, three-wire balanced load. The line current is 17A and the line voltage is 440V. Calculate the power factor of the load if the reading of the wattmeter is 4488 VAR.
a) 0.6
b) 0.8
c) 1
d) 1.2
View Answer

Answer: b
Explanation: Wattmeter reading = VLILsinØ => 4488 = 440 x 17sinØ => sinØ = 0.6. Power factor = cosØ = 0.8.

Sanfoundry Global Education & Learning Series – Network Theory.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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