Wattmeter MCQ || Wattmeter Questions and Answers

11. A wattmeter is connected as shown in the figure. The wattmeter reads

 A wattmeter’s current coil is connected in series with the load, whereas its pressure coil is connected across Z2. T

  1. Zero always
  2. Total power consumed by Z1 and Z2
  3. Power consumed by Z1
  4. Power consumed by Z2

Answer.4. Power consumed by Z2

Explanation:

Wattmeter reads the power and it is given by

P = VPC ICC cos ϕ

VPC is the voltage across the pressure coil

ICC is current flows through the current coil

ϕ is the phase angle between VPC and ICC

Application:

The circuit representation of the given question is as shown below.

 A wattmeter’s current coil is connected in series with the load, whereas its pressure coil is connected across Z2. T

  • The potential coil is connected across Z2.
  • It reads the voltage across Z2 only.
  • So, Wattmeter reads only power consumed by Z2.

 

12. In two wattmeter method of measuring power, one of the wattmeters reads zero watts. The power factor of the load is

  1. Unity
  2. 0.8
  3. 0.5
  4. 0.6

Answer.3. 0.5

Explanation:

The reading of two wattmeters can be expressed as

W1 =  VLILcos(30 + φ)
W2 =  VLILcos(30 − φ)

(i) When PF is unity ( φ = 0°)

W1 =  VLILcos30°
W2 =  VLILcos30°

Both wattmeters read equal and positive reading i.e upscale reading

(ii) When PF is 0.5 (φ = 60°)

W1 =  VLILcos90° = 0
W2 =  VLILcos30°

Hence total power is measured by wattmeter W2 alone

(iii) When PF is less than 0.5 but greater than 0 i.e ( 90° > φ > 60°)

W1 = Negative
W2 = positive (since cos(−φ) = cosφ)

The wattmeter W2 reads positive (i.e.upscale) because for the given conditions (i.e. ( 90° > φ > 60°), the phase angle between voltage and current will be less than 90°.

However, in wattmeter W1, the phase angle between voltage and current shall be more than 90° and hence the wattmeter gives negative (i.e. downscale) reading.

Wattmeter cannot show negative reading as it has only a positive scale. An indication of negative reading is that the pointer tries to deflect in a negative direction i.e. to the left of zero.

In such a case, reading can be converted to positive by interchanging either pressure coil connections or by interchanging current coil connections. Remember that interchanging connections of both the coils will have no effect on wattmeter reading.

 (iv) When P.F reads zero (φ = 90°)

Such a case occurs when the load consists of pure inductance or pure capacitance

W1 =  VLILcos(30 + 90°) = – VLILsin30°
W2 =  VLILcos(30 − 90°) = VLILsin30°

In this condition, the two wattmeter reads equal and opposite i.e W1 + W2 = 0

 

13. The reading of wattmeter and ammeter is 1 kW and 10 A respectively in the three-phase circuit given below. What is the value of the power factor of the circuit, if the circuit is balanced?

  1. 0.5
  2. 0.65
  3. 0.74
  4. 0.86

Answer.4. 0.86

Explanation:

As per the given circuit diagram, a balanced load wattmeter reading will be

Iph = 10 A

Vph = V/√ 3 = 200/ √3 = 115.47

W = VphILCosφ

1000 = 115.47 × 10 × cosφ

Power factor cosφ = 0.866

 

14. While measuring power by two wattmeter method, the phase voltage is 3 V whereas the phase current is 0.5A. If the angle between phase current and phase voltage is 30 degrees, the total power consumed by the load will be

  1. 3.125 W
  2. 1.11 W
  3. 2.25 W
  4. 2.57 W

Answer.3. 2.25 W

Explanation:

VL = 3V

IL = √3 × 0.5

Total power consumed

W1 − W2 VL IL sinφ

Hence

√3 × 3 × 0.5× √3 × 0.5

W1 − W2 = 2.25 W

 

15. The two-wattmeter method is used to measure the power in a three-phase load. The wattmeter readings are 400 W and -35 W. What will be the total active power?

  1. 360 W
  2. 370 W
  3. 375 W
  4. 365 W

Answer.4. 365 W

Explanation:

Given

W1 = 400 watt

W2 = – 35 watt

Total power in the circuit (P) = W1 + W2

Total power = 400 + (-35) = 365 watt

 

16. A dynamometer type of wattmeter responds to

  1. R.M.S value of active power
  2. Average value of active power
  3. Average value of reactive power
  4. R.M.S value of reactive power

Answer.2. Average value of active power

Explanation:

The power measured by the dynamometer wattmeter at any instant is the instantaneous power absorbed by the three loads connected in three phases.  In fact, this power is the average power drawn by the load since the Wattmeter reads the average power because of the inertia of their moving system.

The power measured by each wattmeter varies from instant to instant. Due to the inertia of the moving system, the wattmeter measures the average power. The sum of the two wattmeter readings gives the total 3-phase power.

 

17. In the two-wattmeter method of 3ϕ power measurement, if the phase sequence of the supply is reversed:

  1. one of the meters will show a negative reading
  2. the meters will not read
  3. there won’t be a change in meter readings
  4. the reading of wattmeters will be interchanged

Answer.4. The reading of wattmeters will be interchanged

Explanation:

In the two-wattmeter method of 3ϕ power measurement, if the phase sequence of the supply is reversed then the reading of wattmeters will be interchanged.

Wattmeter cannot show negative reading as it has only a positive scale. An indication of negative reading is that the pointer tries to deflect in a negative direction i.e. to the left of zero.

In such a case, reading can be converted to positive by interchanging either pressure coil connections or by interchanging current coil connections.

Remember that interchanging connections of both the coils will have no effect on the overall wattmeter reading.

 

18. A wattmeter is being tested under phantom-loading conditions. If the wattmeter reading is 60W, the actual power consumed from the supply is

  1. Much higher than 60W
  2. 60W
  3. Much less than 60W
  4. 30W

Answer.3. Much less than 60W

Explanation:

Phantom Loading is a loading condition in which an energy meter is connected to a factious or phantom load for testing of an energy meter with a high current rating. Such loading is favorable to avoid wastage of energy during the test of the measurement instruments.

In phantom loading arrangement, energy consumption in the calibration test of wattmeter is reduced because of the absence of real load. Therefore power consumed will be much less than 60 W.

 

19. The wattmeter measures the angle between the current phasor detected by the _________ and the voltage phasor detected by the ____________.

  1. Ammeter, voltmeter
  2. Voltmeter, ammeter
  3. Voltage coil, current coil
  4. Current coil, voltage coil

Answer.4. Current coil, voltage coil

Explanation:

In the wattmeter, the arrangement of the current coil is in series with the load and the arrangement of the potential coil is across the load.

Hence the wattmeter measures the angle between the current phasor detected by the current coil and the voltage phasor detected by the voltage coil.

The wattmeter measures the average power and it is given by

P = VphIphCosφ

ϕ is the angle between Vph and Iph

 

20. Which power factor results in equal reading of both wattmeter in the two wattmeter method?

  1. Unity
  2. Half
  3. Zero
  4. None of the above

Answer.1. Unity

Explanation:

The reading of two wattmeters can be expressed as

W1 =  VLILcos(30 + φ)
W2 =  VLILcos(30 − φ)

When PF is unity ( φ = 0°)

W1 =  VLILcos30°
W2 =  VLILcos30°

Both wattmeters read equal and positive reading i.e upscale reading

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