Which of the following statement is correct for two wattmeters method of power m

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Question
Which of the following statement is correct for two wattmeters method of power measurement in 3 phase circuit?
Options
A. Power can be measured by this method only for star connected loads
B. When two wattmeters have equal readings power factor is 0.5.
C. When power factor is 1, one of the wattmeters readings is zero
D. When the two wattmeter read equal and opposite, p.f. is zero.

Correct Answer

When the two wattmeter read equal and opposite, p.f. is zero.

Explanation

When p.f. is zero, active power is zero and the two redaings are equal and opposite.

Measurements and Instrumentation problems

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1. A voltmeter using thermocouples measures
Options
A. rms value
B. peak value
C. average value
D. peak to peak value

2. The angle ? for a mica capacitor is about
Options
A. 25�
B. 15�
C. 5�
D. 1�

3. 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�)

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

5. 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

6. In a CRO which of the following is not a part of electron gun
Options
A. cathode
B. grid
C. accelerating anode
D. X - Y plates

7. The coil of a moving iron instrument has a resistance of 500 ? and an inductance of 1 H. It reads 250 V when a 250 V dc is applied. If series resistance is 2000 ?, its reading when fed by 250 V, 50 Hz ac will be
Options
A. 260 V
B. 252 V
C. 250 V
D. 248 V

8. In a ballistic galvanometer, a charge of 100 ?C gives a first swing of 25�. The charge required to cause first swing of 50� is
Options
A. 200 ?C
B. 400 ?C
C. 50 ?C
D. 25 ?C

9. In a strain measuring device using a strain gauge, the output quantity is
Options
A. voltage
B. resistance
C. impedance
D. either (a) or (b)

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