11. The main advantages of Electrostatic wattmeter over Schering Bridge is _____
Good accuracy
Less expensive
Portable
Both 1 and 2
Answer.4. Both 1 and 2
Explanation:
The electrostatic wattmeter is basically a laboratory instrument and not a commercial instrument like a dynamometer and induction wattmeter.
It is mainly used for the measurement of small amounts of power when the voltage is high and the power factor is low.
It is also used for the measurement of dielectric loss in cables.
It largely supersedes the a.c. bridge methods, because it requires less expensive apparatus than does the Schering bridge and can provide very accurate results.
It is also useful in studying the behavior and error of electrodynamic wattmeters at power factors approaching zero.
12. The low power factor wattmeter is used to measure
Low value of power Factor Only
High Value of Power Factor Only
Both low value and High Value
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.
Due to low power factor, the deflecting torque on the moving system is very small, regardless of the fully excited pressure and current coils.
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).
13. In an ordinary dynamometer, the deflecting torque is _________
Small
Medium
Large
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.
14. In Low Power Factor Wattmeter error in Pressure coil can be compensated by
Using a series capacitor or compensating coil
Using a parallel capacitor
Using a series resistance
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.
15. In Low Power Factor Wattmeter errors are introduced by ________
Capacitance
Inductance
Resistance
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
16. Pressure coil in Low Power Factor Wattmeter induced Error due to _____
Capacitance
Impedance
Resistance
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.
17. Power coil in Low Power Factor Wattmeter has a low value of ________
Inductance
Capacitance
Resistance
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.
18. In Low Power Factor Wattmeter Power loss in the pressure coil is ________
Less
More
Intermediate
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.
19. Resistance of pressure coil in a low power factor dynamometer type wattmeter is ________ value of current.
Once time
Three times
Hundred times
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.
20. Low power factor wattmeters are designed to have ________
Low torque
High torque
Medium torque
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.
