Electrical and Electronics Measurement Mcq

41. An advantage of a PMMC instrument is that it is

(a) free from friction error.
(b) has high (torque/weight of the moving parts) ratio.
(c) has low (torque/weight of the moving parts) ratio.
(d) can be used on both ac and dc.

42. The term artificial aging in instrument is associated with

(a) springs.
(b) permanent magnets.
(c) controlling torques.
(d) damping.

43. Moving iron and PMMC instruments can be distinguished from each other by looking at

(a) pointer.
(b) terminal size.
(c) scale.
(d) scale range.

44. The disadvantage of PMMC instrument is

(a) high power consumption.
(b) high cost relative to moving iron instruments.
(c) low torque/weight ratio.
(d) absence of effective and efficient eddy current damping.

45. A high frequency ac signal is applied to a PMMC instrument. If the rms value of ac signal is 2V, the reading of the instrument will be

(a) zero.
(b) 2 V.
(c) 2 .5 V
(d) 4 V.

46. In a PMMC instrument, if the control spring is replaced by another one having a higher spring constant, then the natural frequency and damping ratio will

(a) decrease.
(b) increase and decrease respectively.
(c) decrease and increase respectively.
(d) increase.

47. A sinusoidal voltage of 1 V_rms value at 10 Hz is applied across the two terminals of a PMMC type of voltmeter. What is the deflection of the pointer ?

(a) zero volt.
(b) 1 V
(c) √2 V
(d) The pointer oscillates around zero volt.

48. Which of the following factors limit the deflection of the pointer of a PMMC instrument of about 90° ?

1. Its damping mechanism.
2. Linearity of the magnetic field in which the coil moves.
3. Control spring arrangement.
4. Shape of the pole shoe of the horseshoe magnet.

Select the correct answer using the code given below : Code:
(a) Only 1 and 3.
(b) Only 2 and 4.
(c) Only 2 and 3.
(d) Only 1 and 4.

49. In a PMMC instrument, the central spring stiffness and the strength of the magnet decreases by 0.04% and 0.02% respectively due to a rise in temperature by 1°C. With a rise in temperature of 10°C, the instrument reading will

(a) increase by 0.2%.
(b) decrease by 0.2%.
(c) increase by 0.6%.
(d) decrease by 0.6%.

50. A PMMC type ammeter and a moving iron type ammeter are connected in series in a resistive circuit fed from output of a half wave rectifier voltage source. If the moving iron type reads 5A, the PMMC type instrument is likely to read

(a) zero
(b) 2.5 A
(c) 3.18 A
(d) 5 A

51. The scale of a dynamometer type instrument marked in terms of rms value would be

(a) uniform throughout.
(b) non-uniform crowded near full scale.
(c) non-uniform crowded at the beginning.
(d) non-uniform crowded around midscale.

52. In an electrodynamometer type instrument an astatic movement is provided in order to

(a) eliminate error owing to stray magnetic fields.
(b) provide damping torque.
(c) increase the instrument operating torque.
(d) eliminate temperature error.

53. In a dynamometer type moving coil instrument a swamping resistance is provided in order to

(a) control the deflecting torque.
(b) reduce the bulk of the moving system.
(c) reduce the current flowing through the moving coil.
(d) provide equal time constant for moving coil and fixed coil, when used for ac measurement.

54. Which one of the following statements is correct ? The deflection of hot-wire instrument depends on

(a) rms value of the ac current.
(b) rms value of the ac voltage.
(c) average value of the ac current.
(d) average value of the ac voltage.

55. Consider the following statements about hot-wire instruments:

1. They read equally well on dc and/or ac circuits.
2. They are simple and robust in construction and power consumption is low.
3. They are quite suitable for measurement of currents at very high frequencies.

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

56. Which of the following statements is not correct for thermocouple measuring instruments ?

(a) They are very useful as transfer instruments.
(b) They are incapable of taking any overload.
(c) Power losses are very low.
(d) Their calibration does not change with time or temperature.

57. Which of the following are the characteristics of a thermocouple type of indicating instrument ?

1. Its accuracy is very high, as high as about 1 percent.
2. It has a linear scale because a D’Arsonval movement is used for measuring the output.
3. It is an RF instrument and can be used for frequency up to about 50 MHz.
4. It cannot be damaged by overloads.

Select the correct answer from the codes given below.
(a) 1 and 2.
(b) 2 and 3.
(c) 3 and 4.
(d) 1 and 3.

58. Consider the following statements in respect of the thermoelectric instruments.

1. They indicate the rms value of current or voltage.
2. They suffer from waveform errors.
3. They can be used for frequency ranges of the order of MHz.
4. They have a low overload capacity.

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

59. Rectifier moving coil instruments respond to

(a) peak value, irrespective of the nature of the waveform.
(b) average value, for all waveforms.
(c) rms value for all waveforms.
(d) rms value, for symmetrical square waveforms.

60. An ac voltmeter using full-wave rectification and having a sinusoidal input has an ac sensitivity equal to

(a) 1.414 times dc sensitivity.
(b) dc sensitivity.
(c) 0.9 times dc sensitivity.
(d) 0.707 times dc sensitivity.