Measurements and Instrumentation - Section 1 (2)

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Measurements and Instrumentation - Section 1

Which of the following statement is correct for two wattmeters method of power measurement in 3 phase circuit?

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.

Answer: Option D

Explanation:

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

10.

Which of the following measurements using a dc potentiometer do not require standardisation of potentiometer?

A.	Measurement of unknown voltage
B.	Measurement of resistance
C.	Measurement of unknown current
D.	Calibration of voltmeter

Answer: Option B

Explanation:

Unknown resistance is compared with a standard resistance. Therefore standardisation of potentiometer is not necessary.

11.

A voltmeter using thermocouples measures

A.	rms value
B.	peak value
C.	average value
D.	peak to peak value

Answer: Option A

Explanation:

Since the thermocouple is based on heating effect, rms value is measured.

12.

The speed transducer using a drag cup rotor has the main advantage of

A.	very high accuracy
B.	low energy requirement
C.	cheap construction
D.	all of the above

Answer: Option B

Explanation:

A drag cup rotor has low inertia. Therefore it requires little amount of energy.

13.

A dynamometer wattmeter is connected in ac circuit. The reading will be

A.	V-A product
B.	average power
C.	peak power
D.	instantaneous power

Answer: Option B

Explanation:

Wattmeter measures average power over a cycle.

14.

A digital frequency counter can be converted to DVM by addition of a suitable

A.	voltage controlled oscillator
B.	D/A converter
C.	power amplifier
D.	operational amplifier

Answer: Option B

Explanation:

D/A converter changes digital output to analog output.

15.

Which of the following are needed both for protection and metering?

A.	Wattmeters
B.	Instrument transformers
C.	Energy meters
D.	Power factor meters

Answer: Option B

Explanation:

Instrument transformers are used to measure high currents and voltages. They also feed the relays for protection.

16.

Which of the following is true about an impulse voltage?

A.	It is a unidirectional voltage
B.	Chopped impulse flashover does not occur
C.	Time taken to rise is equal to time taken to fall
D.	RMS value is always less than 50% of average value

Answer: Option A

Explanation:

Impulse is unidirectional (either positive or negative).

Measurements and Instrumentation - Section 1 (2)

Measurements and Instrumentation - Section 1

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