31. In a three-phase mercury arc rectifiers each anode conducts for
(a) one-third of a cycle
(b) one-fourth of a cycle
(c) one-half a cycle
(d) two-third of a cycle
Ans: a
32. In a mercury arc rectifier characteristic blue luminosity is due to
(a) colour of mercury
(b) ionization
(c) high temperature
(d) electron streams
Ans: b
33. Which of the following mercury arc rectifier will deliver least undulating current?
(a) Six-phase
(b) Three-phase
(c) Two-phase
(d) Single-phase
Ans: a
34. In a glass bulb mercury arc rectifier the maximum current rating is restricted to
(a) 2000 A
(b) 1500 A
(c) 1000 A
(d) 500 A
Ans: d
35. In a mercury arc rectifier______ flow from anode to cathode
(a) ions
(b) electrons
(c) ions and electrons
(d) any of the above
Ans: a
36. When a rectifier is loaded which of the following voltage drops take place ?
(a) Voltage drop in transformer reactance
(6) Voltage drop in resistance of transformer and smoothing chokes
(c) Arc voltage drop
(d) All of the above
Ans: d
37. On which of the following factors the number of phases for which a rectifier should be designed depend ?
(a) The voltage regulation of the rec¬tifier should be low
(b) In the output circuit there should be no harmonics
(c) The power factor of the system should be high
(d) The rectifier supply transformer should be utilized to the best ad-vantage
(e) all of the above
Ans: e
38. A mercury arc rectifier possesses ________ regulation characteristics
(a) straight line
(b) curved line
(c) exponential
(d) none of the above
Ans: d
39. It is the_______of the transformer on which the magnitude of angle of overlap depends.
(a) resistance
(b) capacitance
(c) leakage reactance
(d) any of the above
Ans: c
41. In a grid control of mercury arc rectifiers when the grid is made positive relative to cathode, then it the electrons on their may to anode.
(a) accelerates
(b) decelerates
(c) any of the above
(d) none of the above
Ans: a
42. In mercury arc rectifiers having grid, the arc can be struck between anode and cathode only when the grid attains a certain potential, this potential being known as
(a) maximum grid voltage
(b) critical grid voltage
(c) any of the above
(d) none of the above
Ans: b
43. In phase-shift control method the control is carried out by varying the of grid voltage.
(a) magnitude
(b) polarity
(c) phase
(d) any of the above
(e) none of the above
Ans: c
44. In a phase-shift control method, the phase shift between anode and grid voltages can be achieved by means of
(a) shunt motor
(6) synchronous motor
(c) induction regulator
(d) synchronous generator
Ans: c
45. The metal rectifiers are preferred to valve rectifiers due to which of the following advantages ?
(a) They are mechanically strong
(b) They do not require any voltage for filament heating
(c) Both (a) and (b)
(d) None of the above
Ans: c
46. Which of the following statement is incorrect ?
(a) Copper oxide rectifier is a linear device
(b) Copper oxide rectifier is not a perfect rectifier
(c) Copper oxide rectifier has a low efficiency
(d) Copper oxide rectifier finds use in control circuits
(e) Copper oxide rectifier is not stable during early life
Ans: a
47. The efficiency of the copper oxide rectifier seldom exceeds
(a) 90 to 95%
(b) 85 to 90%
(c) 80 to 85%
(d) 65 to 75%
Ans: d
48. Copper oxide rectifier is usually designed not to operate above
(a) 10°C
(b) 20°C
(c) 30°C
(d) 45°C
Ans: d
49. Selenium rectifier can be operated at temperatures as high as
(a) 25°C
(b) 40°C
(c) 60°C
(d) 75°C
Ans: d
50. In selenium rectifiers efficiencies ranging from ______ to ______ percent are attainable
(a) 25, 35
(b) 40, 50
(c) 60, 70
(d) 75, 85
Ans: d
51. Ageing of a selenium rectifier may change the output voltage by
(a) 5 to 10 per cent
(b) 15 to 20 per cent
(c) 25 to 30 per cent
(d) none of the above
Ans: a
52. The applications of selenium rectifiers are usually limited to potential of
(a) 10 V
(b) 30 V
(c) 60 V
(d) 100 V
(e) 200 V
Ans: d
53. Which of the following rectifiers have been used extensively in supplying direct current for electroplating ?
(a) Copper oxide rectifiers
(b) Selenium rectifiers
(c) Mercury arc rectifiers
(d) Mechanical rectifiers
(e) None of the above
Ans: b
54. A commutating rectifier consists of commutator driven by
(a) an induction motor
(b) a synchronous motor
(c) a D.C. series motor
(d) a D.C. shunt motor
Ans: b
55. Which of the following rectifiers are primarily used for charging of low voltage batteries from AC. supply ?
(a) Mechanical rectifiers
(b) Copper oxide rectifiers
(c) Selenium rectifiers
(d) Electrolytic rectifiers
(e) Mercury arc rectifiers
Ans: d
56. The efficiency of an electrolytic rectifier is nearly
(a) 80%
(b) 70%
(c) 60%
(d) 40%
Ans: c
57. Which of the following is the loss within the mercury arc rectifier chamber ?
(a) Voltage drop in arc
(6) Voltage drop at the anode
(c) Voltage drop at the cathode
(d) All of the above
Ans: d
58. The metal rectifiers, as compared to mercury arc rectifiers
(a) operate on low temperatures
(b) can operate on high voltages
(c) can operate on heavy loads
(d) give poor regulation
(e) none of the above
Ans: a
59. In a mercury arc rectifier, the anode is usually made of
(a) copper
(b) aluminium
(c) silver
(d) graphite
(e) tungsten
Ans: d
60. The ignited or auxiliary anode in mercury arc rectifier is made of
(a) graphite
(b) boron carbide
(c) aluminum
(d) copper
Ans: b