Electrical Measurements & Measuring Instruments Multiple Choice Questions on “Dynamometer Type Wattmeter”.
1. Power is ___________
a) rate of doing work
b) rate of producing voltage
c) rate of generating current
d) rate of overcoming friction
Answer: a
Clarification: Power is defined as the rate of doing work. The unit of power is watt. In D.C. circuits, power is the product of the voltage consumed and the current flowing through a circuit. P = VI watts.
2. In A.C. circuits, power consumed is ________
a) product of voltage and current
b) it depends on the p.f. of the circuit in addition to voltage and current
c) it depends on the supply voltage
d) it depends on the magnitude of the circuit current
Answer: b
Clarification: In a A.C. circuit, the power consumption is given by the expression,
P = VI cos∅
where, V is the voltage across the circuit
I is the current flowing through the circuit
cos∅ is the power factor of the circuit.
3. In D.C. circuits, power is measured using ________
a) ohmmeter and galvanometer
b) ohmmeter and voltmeter
c) ammeter and voltmeter
d) ammeter and galvanometer
Answer: c
Clarification: An ammeter is used to measure current flowing through a circuit, while a voltmeter is used to measure the voltage across the circuit. Hence in D.C. circuits, ammeter and voltmeter are used to measure power.
4. In A.C. circuits, power is measured using ________
a) voltmeter
b) ammeter
c) ohmmeter
d) wattmeter
Answer: d
Clarification: A.C. circuits make use of power factor of the circuit in addition to the current flowing through the circuit and the voltage across the circuit. As a result, a wattmeter is used to measure A.C. power.
5. A wattmeter consists of a current coil and a potential coil.
a) True
b) False
Answer: a
Clarification: Power in single phase A.C. circuits are measured by making use of a wattmeter. It consists of a current coil (cc) and a potential coil (pc). Current coil is connected with the load while the potential coil is connected across the supply.
6. A dynamometer type wattmeter consists of ________
a) only potential coil
b) potential and current coils
c) only current coil
d) no coils
Answer: b
Clarification: Dynamometer type wattmeter is used for the measurement of A.C. as well as D.C. power. It consists of a fixed coil forming the current coil whereas the moving coil forms the potential coil.
7. Controlling torque is provided by gold springs.
a) True
b) False
Answer: b
Clarification: In a Dynamometer type wattmeter, controlling torque is provided by two phosphor bronze hair springs. They act as leads to the current flowing through the coil. Also air friction damping is used.
8. In a Dynamometer type wattmeter, the fixed coil is split into ________
a) 4
b) 3
c) 2
d) 1
Answer: c
Clarification: When a Dynamometer type wattmeter is used for the measurement of A.C. power, the fixed coil is split into two equal parts. The two parts are air-cored to avoid hysteresis loss.
9. When a current carrying coil is placed in the magnetic field?
a) no force is exerted
b) voltage is produced
c) power is generated
d) a force is exerted
Answer: d
Clarification: When the current carrying coil of a Dynamometer type wattmeter is placed in the magnetic field of another current carrying coil, the moving coil experiences a force. As a result a deflection torque is generated and the moving coil undergoes deflection.
10. When the moving coil in a Dynamometer type wattmeter deflects ________
a) pointer moves
b) pointer doesn’t move
c) current flows
d) voltage is generated
Answer: a
Clarification: In a Dynamometer type wattmeter, when the moving coil deflects the pointer moves over the scale. The pointer then comes back to rest at a point where the deflecting torque equals the controlling torque.
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