This set of Electrical Measurements & Measuring Instruments Multiple Choice Questions & Answers (MCQs) focuses on “Low Power Factor Wattmeter”.
1. In an ordinary dynamometer, the deflecting torque is _________
a) small
b) medium
c) large
d) very large
View Answer
Explanation: 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 ________
a) capacitance
b) inductance
c) resistance
d) impedance
View Answer
Explanation: 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 ________
a) inductance
b) capacitance
c) resistance
d) impedance
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Explanation: 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
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Explanation: 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
View Answer
Explanation: 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 ________
a) less
b) more
c) intermediate
d) very less
View Answer
Explanation: 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.
a) True
b) False
View Answer
Explanation: 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.
a) True
b) False
View Answer
Explanation: 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.
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