Electrical Measurements Questions and Answers Pdf

21. Which one of the following is the correct statement ? Due to inductance in the pressure coil the dynamometer wattmeter

(a) reads low on lagging pf and high on leading pf.
(b) reads high on lagging pf and low on leading pf.
(c) reading is not affected at all.
(d) always reads low.

22. Due to the effect of inductance in the pressure coil, a dynamometer type wattmeter

(a) reads low on lagging power factor and high on leading power factor.
(b) reads high on lagging power factor and low on leading power factor.
(c) reading is independent of the power factor.
(d) always reads lower than the actual value.

23. In a low pf wattmeter the compensating coil is connected

(a) in series with the current coil.
(b) in series with the pressure coil.
(c) across the current coil.
(d) across the potential coil.

24. Consider the following statements:

(i) The compensating coil of a low power factor wattmeter compensates the effect of the impedance of the current coil.
(ii) The compensating coil of a low power factor wattmeter compensates the effect of the impedance of the voltage coil circuit.
(a) (i) is true but (ii) is false.
(b) (i) is false but (ii) is true.
(c) both (i) and (ii) are true.
(d) both (i) and (ii) are false.

25. In a low power factor wattmeter, why is a compensating coil employed ?

(a) To neutralize the capacitive effect of pressure coil.
(b) To compensate for inductance of pressure coil.
(c) To compensate for the error caused by power loss in the pressure coil.
(d) To compensate for the error caused by eddy currents.

26. The error caused by pressure coil inductance in a wattmeter is compensated by connecting a

(a) capacitor across a part of series resistance in the pressure coil circuit.
(b) capacitor in series with the series resistance in the pressure coil circuit.
(c) capacitor across a part of a series inductor in the pressure coil circuit.
(d) capacitor in series with the series inductor in the pressure coil circuit.

27. Electrodynamic type wattmeter have large errors while measuring power in ac circuits at low power factor conditions, since the voltage across and the current through the

(a) current coil are not in phase.
(b) current coil are not in quadrature.
(c) pressure coil are not in phase.
(d) pressure coil are not in quadrature.

28. The current and potential coils of a wattmeter were accidentally interchanged while connecting. After energizing the circuit, it was observed that the wattmeter did not show the reading. This could be due to

(a) damage to the potential coil.
(b) damage to the current coil.
(c) damage to both potential and current coils.
(d) loose contact.

29. The magnetic field responsible for the production of the deflecting torque in an accurate dynamometer type wattmeter, being very weak, the ‘accuracy of the measurement can be increased by providing a

(a) Magnetic shield around the instrument.
(b) Compensating winding along with the pressure coil.
(c) Astatic arrangement to the moving system of the instrument.
(d) Capacitance shunt across a portion of the pressure coil.

30. Which one of the following methods decreases the error due to connections in a dynamometer type wattmeter ?

(a) Using bifilar compensating winding in place of current coil.
(b) Using non-inductive pressure coil circuit.
(c) Using a capacitor across a part of high resistance of pressure coil circuit.
(d) Using a swamping resistance.

31. How can the power supplied to a frequency heating system be measured?

(a) By dynamometer wattmeter.
(b) By induction wattmeter.
(c) By thermocouple type wattmeter.
(d) By moving iron
ammeter and voltmeter.

32. In the power measurement by ammeter-voltmeter method, if the voltmeter is connected across the load, then the value of the power will be

(a) the power consumed by the load.
(b) the sum of power consumed by the load and ammeter.
(c) the sum of power consumed by the load and voltmeter.
(d) the sum of power consumed by the load, ammeter and voltmeter.

33. In calibration of dynamometer wattmeter by potentiometer, phantom loading arrangement is used because

(a) the arrangement gives accurate results.
(b) the power consumed in calibration work is minimum.
(c) the method gives quick results.
(d) the on site calibration is possible.

34. The two wattmeter method is used to measure active power on a three-phase, three-wire system. If the phase voltage is unbalanced, then the power reading is

(a) affected by both negative sequence and zero sequence voltages.
(b) affected by negative sequence voltages but not by zero sequence voltages.
(c) affected by zero sequence voltages but not by negative sequence voltages.
(d) not affected by negative or zero sequence voltages.

35. In the case of power measurements by the two wattmeter method in a balanced 3-phase system with a pure inductive load,

(a) both the wattmeters will indicate the same value but of opposite sign.
(b) both the wattmeters will indicate zero.
(c) both the wattmeters will indicate the same value and of the same sign.
(d) one wattmeter will indicate zero and the other will indicate some non zero value.

36. If the power factor is below 0.5 in two wattmeter method of power measurement

(a) one of the wattmeter will give negative deflection.
(b) both the wattmeters will give negative deflection.
(c) both the wattmeters will give positive deflection.
(d) both the wattmeters will not give any deflection.

37. Consider the following statements regarding measurement of 3-phase power by two-wattmeter method; one of the wattmeter reads negative implying:

1. Power factor is less than 0.5.
2. Power flow is in the reverse direction.
3. Load power factor angle is greater than 60°.
4. Load is unbalanced.

Which of the above statements are correct?
(a) 1 and 2 only.
(c) 1 and 3 only.
(b) 2 and 3 only.
(d) 1, 2, 3 and 4.

38. If the readings of the two wattmeters are equal and positive in two wattmeter method, the load pf in a balanced 3-phase 3-wire circuit will be

(a) zero
(b) 0.5
(c) 0.866
(d) unity

39. The power of a three-wire balanced system was measured by two wattmeter method. The reading of one of the wattmeter was found to be double that of the other. What is the pf of the system ?

(a) 1.0
(b) 0.866
(c) 0.707
(d) 0.5

40. Which of the following statements are correct in case of a power factor meter ?

1. The deflection is proportional to the phase angle between field coil and crossed coil.
2. The restoring torque is provided by a controlling torque.
3. It consists of two coils mounted at right angles to each other.

Select the correct answer using the code given below :
(a) 1 and 2.
(b) 2 and 3.
(c) 1 and 3.
(d) 1, 2 and 3.