Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Low Power Factor Wattmeter”.
1. In an ordinary dynamometer, the deflecting torque is _________
d) very large
Clarification: The deflecting torque in an ordinary dynamometer is small. The current and pressure coils are excited. The measurement of power factor is inaccurate.
2. Errors are introduced by ________
Clarification: At low power factors in an ordinary dynamometer type wattmeter, the errors are caused due to inductance of the pressure coil.
3. Power coil has a low value of ________
Clarification: In a low power factor dynamometer type wattmeter, the pressure coil is designed in order to have a low value of resistance. The current flowing through the pressure coil is increased in order to raise the operating torque.
4. Resistance of pressure coil in a low power factor dynamometer type wattmeter is ________
a) once time
b) three times
c) hundred times
d) ten times
Clarification: In a low power factor dynamometer type wattmeter, pressure coil has a resistance value that is one tenth of the actual with respect to unity power wattmeters. This is done in order to ensure a reasonable amount of torque at low power factors.
5. Low power factor wattmeters are designed to ________
a) have a low torque
b) have a high torque
c) have a medium torque
d) have no torque
Clarification: Low power factor wattmeters have a low value of control torque. They provide full scale deflection for low values of power factors of the order of 0.1.
6. Power loss in the current coil is ________
d) very less
Clarification: Low power factor circuits have low power. Current coil constitutes a high current. As a result, the power loss in the current coil is high. The reading obtained from the wattmeter is prone to errors.
7. Pressure coil has no error.
Clarification: Pressure coil has an error induced due to inductance. It is given by the relation EI sinΦ tanβ. As value of Φ is large for low value of power factor, the error is high in a pressure coil.
8. Error in a pressure coil can be compensated.
Clarification: In a low power factor wattmeter, by connecting a capacitor of value C across a portion of the resistance R in the circuit the error in the pressure coil can be compensated.