Advanced Miscellaneous Problems on Measurement of Power

Electrical Measurements

This set of Advanced Electrical Measurements Questions and Answers focuses on “Advanced Miscellaneous Problems on Measurement of Power”.

1. A coil (which can be modelled as a series RL circuit) has been designed for high Q performance. The voltage is rated at and a specified frequency. If the frequency of operation is increased 10 times and the coil is operated at the same rated voltage. The new value of Q factor and the active power P will be?
a) P is doubled and Q is halved
b) P is halved and Q is doubled
c) P remains constant and Q is doubled
d) P decreases 100 times and Q is increased 10 times

2. Two watt-meters, which are connected to measure the total power on a three-phase system supplying a balanced load, read 20.5 kW and -3.5 kW respectively. The total power is?
a) 13.0 kW
b) 17 kW
c) 15 kW
d) 19 kW

3. A circuit is used to measure the power consumed by the load. The current coil and the voltage coil of the watt-meter have 0.02 Ω and 1000 Ω resistances respectively. The measured power (as compared to the load power) will be?
a) 0.4 % less
b) 0.2 % less
c) 0.2 % more
d) 0.4 % more

4. A sampling watt-meter is used to measure the average power of a load. The peak to peak voltage of the square wave is 10 V and the current is a triangular wave of 5A p-p. The reading in watt will be?
a) Zero
b) 25 W
c) 50 W
d) 100 W

5. The line to line input voltage to the 3-phase, 50 Hz, AC circuit, is 100 V rms. Assuming that the phase sequence is RYB; the watt-meters would read?
a) W1 = 886 W and W2 = 886 W
b) W1 = 500 W and W2 = 500 W
c) W1 = 0 W and W2 = 1000 W
d) W1 = 250 W and W2 = 750 W

6. The meter constant of a single-phase, 100 V induction watt-meter is 600 rev/kW-h. The speed of the meter disc for a current of 15 A at 0.8 power factor lagging will be?
a) 30.3 rpm
b) 16 rpm
c) 12 rpm
d) 33.1 rpm

7. A 3-phase, 600 V motor, the load having 0.6 power factor uses two watt-meter to measure the power. If the power measured be 45 kW, then the reading of each instrument will be?
a) P1 = 35 kW, P2 = 10 kW
b) P1 = 47.25 kW, P2 = -2.25 kW
c) P1 = 39.82 kW, P2 = 5.179 kW
d) P1 = 45 kW, P2 = 0 kW

8. An average-reading digital Multimeter reads 10 V when fed with a triangular wave, symmetric about the time-axis. If the input is same, the rms reading meter will read?
a) 20/3–√
b) -10/3–√
c) -20/3–√
d) 10/3–√

9. In the Two wattmeter method of measuring power in a balanced three-phase circuit, one wattmeter shows zero and the other positive maximum. The load power factor is?
a) 0
b) 0.5
c) 0.866
d) 1.0

10. Two watt-meters connected to measure the total power on a three-phase system supplying a balanced load reads 10.5 kW and -2.5 kW respectively. Then the total power factor is?
a) 0.334
b) 0.684
c) 0.52
d) 0.334

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