In two wattmeter method of 3 phase power measurement, when does one wattmeter re

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Question
In two wattmeter method of 3 phase power measurement, when does one wattmeter read negative
Options
A. when power factor is less than 0.5 lagging
B. when power factor is greater than 0.5 lagging
C. when power factor is less than 0.5
D. when power factor is unity

Correct Answer

when power factor is less than 0.5

Explanation

When p.f. is less than 0.5 (leading or lagging) one of the wattmeters reads negative.

Measurements and Instrumentation problems

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1. An LVDT is used to measure displacement. The LVDT feeds a Voltmeter of 0-5 V range through a 250 gain amplifier. For a displacement 0.5 mm the output of LVDT is 2 mV. The sensitivity of instrument is
Options
A. 0.1 V/mm
B. 0.5 V/mm
C. 1 V/mm
D. 5 V/mm

2. An ammeter of 0-25 A range has a guaranteed accuracy of 1% of full scale reading. The current measured is 5 A. The limiting error is
Options
A. 2%
B. 2.5%
C. 4%
D. 5%

3. A piezoelectric force transducer has a charge sensitivity of 20 pC/N. It is connected to a charge amplifier and overall gain of transducer and amplifier is 50 mV/N. The gain of amplifier is
Options
A. 1 mV/pC
B. 1.5 mV/pC
C. 2.5 mV/pC
D. 4 mV/pC

5. A moving coil instrument has a resistance of 0.5 ? and a full scale deflection of 0.1 A. To convert it into an ammeter of 0-10 A the shunt resistance should be
Options
A. 0.004 ?
B. 0.005 ?
C. 0.05 ?
D. 0.1 ?

8. A standard cell of 1.0185 V is used with a slide wire potentiometer. The balance was obtained at 60 cm. When an unknown emf was connected, the balance was obtained at 82 cm. The magnitude of unknown emf is
Options
A. 1.39 V
B. 0.75 V
C. 13.9 V
D. 7.45 V

9. To increase Q factor of a coil, the wire should be
Options
A. long
B. thin
C. thick
D. long and thin

10. The Lissajous pattern observed on screen of CRO is a straight line inclined at 45� to x axis. If X-plate input is 2 sin ?t, the Y-plate input is
Options
A. 2 sin ?t
B. 2 sin (?t + 45�)
C. 2 sin (?t - 45�)
D. 22 sin ?t + 45�)

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