Low Power Factor Wattmeter MCQ || Low Power Factor Wattmeter Questions and Answers

1. The low power factor wattmeter is used to measure

  1. Low value of power Factor Only
  2. High Value of Power Factor Only
  3. Both low value and High Value
  4. None of the above

Answer.1. Low value of power Factor Only

Explanation:

The ordinary electrodynamic wattmeters cannot be used for measuring power with precision in the circuits which have a low power factor (power factor less than 0.5) due to reasons given as follows.

  1. Due to low power factor, the deflecting torque on the moving system is very small, regardless of the fully excited pressure and current coils.
  2. At low power factors, the inductance of the pressure coil is large which induces errors.

The instrument that measures the low value of power factor accurately is known as the Low Power Factor Wattmeter (LPFW).

 

2.  In an ordinary dynamometer, the deflecting torque is _________

  1. Small
  2. Medium
  3. Large
  4. Very large

Answer.1. Small

Explanation:

In order to measure power in low power factor circuits, an ordinary electrodynamometer wattmeter is not suitable because of the following reasons:

(i) Even when the current coils and pressure coils are fully excited, the deflecting torque on the moving system is small.

(ii)) Large error is introduced due to the inductance of the pressure coil at a low power factor.

 

3. In Low Power Factor Wattmeter error in Pressure coil can be compensated by

  1. Using a series capacitor or compensating coil
  2. Using a parallel capacitor
  3. Using a series resistance
  4. Using a parallel resistance

Answer.2. Using a parallel capacitor

Explanation:

For low power factor circuits, the connection may result in very large errors. Due to the low power factor, the value of the current in the circuit is high while the power to be measured is small making the power loss considerably large. Thus, for low power factor wattmeters, the compensation of pressure coil current is mandatory.

Compensation for pressure coil inductance: Large errors are caused by pressure coil inductance in low power factor circuits. To compensate for these errors, a capacitance is connected in parallel to a part of the series resistance of the pressure coil circuit.

 

4. In Low Power Factor Wattmeter errors are introduced by ________

  1. Capacitance
  2. Inductance
  3. Resistance
  4. Impedance

Answer.2. Inductance

Explanation:

In order to measure power in low power factor circuits, an ordinary electrodynamometer wattmeter is not suitable because of the following reasons:

(i) Even when the current coils and pressure coils are fully excited, the deflecting torque on the moving system is small.

(ii)) Large error is introduced due to the inductance of the pressure coil at a low power factor.

With a low power factor, the value of φ is large and therefore error is large. Therefore, the error caused by the inductance of pressure coil is compensated by connecting a capacitor across a part of series resistance in the pressure coil circuit as shown in the

 

5. Pressure coil in Low Power Factor Wattmeter induced Error due to _____

  1. Capacitance
  2. Impedance
  3. Resistance
  4. Inductance

Answer.4. Inductance

Explanation:

Compensation for pressure coil inductance: Large errors are caused by pressure coil inductance in low power factor circuits. To compensate for these errors, a capacitance is connected in parallel to a part of the series resistance of the pressure coil circuit.

For low power factor circuits, the connection may result in very large errors. Due to the low power factor, the value of the current in the circuit is high while the power to be measured is small making the power loss considerably large. Thus, for low power factor wattmeters, the compensation of pressure coil current is mandatory.

 

6. Power coil in Low Power Factor Wattmeter has a low value of ________

  1. Inductance
  2. Capacitance
  3. Resistance
  4. Impedance

Answer.3. Resistance

Explanation:

The resistance of the pressure coil circuit is made low in order to make the pressure coil current large, resulting in increased operating torque. The pressure coil current in a low power factor wattmeter may be as high as 10 times the value of the current used in ordinary wattmeters.

 

7. In Low Power Factor Wattmeter Power loss in the pressure coil is ________

  1. Less
  2. More
  3. Intermediate
  4. Very less

Answer.2. More

Explanation:

  • A low power factor meter may result in very large errors.
  • Due to low power factor, the value of the current in the circuit is high while the power to be measured is small making the power loss considerably large.
  • Thus, for low power factor wattmeters, the compensation of pressure coil current is mandatory.

 

8. Resistance of pressure coil in a low power factor dynamometer type wattmeter is ________ value of current.

  1. Once time
  2. Three times
  3. Hundred times
  4. Ten times

Answer.4. Ten times

Explanation:

The resistance of the pressure coil circuit is made low in order to make the pressure coil current large, resulting in increased operating torque. The pressure coil current in a low power factor wattmeter may be as high as 10 times the value of the current used in ordinary wattmeters.

 

9. Low power factor wattmeters are designed to have ________

  1. Low torque
  2. High torque
  3. Medium torque
  4. No torque

Answer.1. Low torque

Explanation:

The control torque of a low power factor wattmeter must be very small such that for very low power (equal to 0.1), the instrument can give full-scale deflection.

 

10. The application of Low Power factor meters is

  1. Measurement of power in inductive loads
  2. Open circuit test on single-phase transformer
  3. Load test on induction motor
  4. All of the above

Answer.4. All of the above

Explanation:

Low power factor meter:

The instrument that measures the low value of power factor accurately is known as the Low Power Factor Wattmeter (LPFW). The low power factor meter is used for measuring the power of the highly inductive circuit.

Application:

As the LPF is most suitable for inductive loads, it is suitable for the measurement of power in inductive loads, open circuit test on single phase transformer, and load test on an induction motor.

It is not suitable for the measurement of power in resistive loads.

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