41. Arc drawn between the two electrodes produces heat and has a temperature
(a) between 0 – 600°C.
(b) between 500°C – 1,000°C.
(c) between 1,000°C – 3,500°C.
(d) of the order of 4,000°C.
Answer: (c) between 1,000°C – 3,500°C.
42. The arc furnaces of conical shapes have the advantage(s) of
(a) large surface area per unit bath volume.
(b) low power consumption.
(c) reduced radiation losses.
(d) all of the above.
Answer: (d) all of the above.
43. In arc furnace, energy losses that take place in the furnace are
(a) losses through walls.
(b) losses through escape gasses.
(c) losses through water cooling.
(d) all of the above.
Answer: (d) all of the above.
44. In submerged arc furnaces the power is controlled by
(a) varying the spacing between the electrodes.
(b) varying the voltage applied to the electrodes.
(c) either (a) or (b).
(d) varying the arc length.
Answer: (c) either (a) or (b).
45. For arc furnaces the low voltage high current power supply is needed because
(a) heavy currents produce large amount of heat resulting in higher temperatures.
(b) maximum secondary voltage is also limited to 275 V (line-to-line on open circuit) owing to insulation and safety consideration.
(c) life of the roof refractory is increased.
(d) all of the above.
Answer: (d) all of the above.
46. The main application of indirect arc furnace is to melt
(a) iron.
(b) steel.
(c) non-ferrous metals.
(d) none of the above.
Answer: (c) non-ferrous metals.
47. For power transformers employed for arc furnaces, it is desirable to arrange the furnace and the transformer in such a way that leads are
(a) shorter in length and placed distant apart.
(b) shorter in length and placed close together.
(c) longer in length and placed close together.
(d) in any arrangement.
Answer: (b) shorter in length and placed close together.
48. In induction heating
(a) heat is produced due to currents induced in the charge by electromagnetic action.
(b) the resistance of the charge must be low and voltage applied must be high in order to produce sufficient heat.
(c) magnetic materials can be easily treated in comparison to non-magnetic materials.
(d) all of the above.
Answer: (d) all of the above.
49. In heating the ferromagnetic materials by induction heating, heat is produced owing to
(a) flow of induced current through the charge.
(b) hysteresis loss occurring below Curie temperature.
(c) hysteresis loss as well as eddy current loss occurring in the charge.
(d) any of the above factors.
Answer: (c) hysteresis loss as well as eddy current loss occurring in the charge.
50. Induction heating takes place in
(a) insulating materials.
(b) conducting and magnetic materials.
(c) conducting but non-magnetic materials.
(d) conducting materials may be magnetic or non-magnetic.
Answer: (d) conducting materials may be magnetic or non-magnetic.
51. Induction hardening is possible in case of
(a) dc supply only.
(b) ac supply only.
(c) ferrous materials only.
(d) non-conducting materials only.
Answer: (b) ac supply only.
52. Low frequency supply is necessary for direct core type induction furnaces because
(a) magnetic coupling between the primary and secondary circuit is poor.
(b) with normal frequency supply the electromagnetic forces cause severe stirring action in the molten metal.
(c) both (a) and (b).
(d) none of (a) and (b).
Answer: (c) both (a) and (b).
53. In induction heating, which of the following is of high value?
(a) Frequency.
(b) Current.
(c) Voltage.
(d) Power factor.
Answer: (a) Frequency.
54. Induction furnaces are used for
(a) heat treatment of castings.
(d) heating of insulators.
(c) melting of aluminum.
(d) all of the above.
Answer: (a) heat treatment of castings.
55. In induction healing, the depth up to which current will penetrate is proportional to
(a) 1/(frequency)1/2.
(b) 1/frequency.
(c) frequency.
(d) (frequency)2.
Answer: (a) 1/(frequency)1/2.
56. The advantage(s) of eddy current heating is/are
(a) easy temperature control, little wastage of heat and possibility of heating in vacuum or other special atmosphere.
(b) heat can be made to penetrate into metal surface to any desired depth.
(c) the area of surface over which heat is produced can be accurately controlled.
(d) all of the above.
Answer: (d) all of the above.
57. The supply frequency usually employed for high frequency eddy current heating is
(a) 10 MHz
(b) 10 kHz to 400 kHz
(c) 5 kHz
(d) 1 kHz
Answer: (b) 10 kHz to 400 kHz
58. In case of core-loss induction furnace, the charge should be in
(a) solid state.
(b) molten state.
(c) either of (a) or (b).
Answer: (c) either of (a) or (b).
59. High frequency induction heating is used for
(a) ferrous metals only.
(b) non-ferrous metals only.
(c) both ferrous and non-ferrous metals.
Answer: (c) both ferrous and non-ferrous metals.
60. In dielectric heating current flows through
(a) air.
(b) dielectric.
(c) metallic conductor.
(d) ionic discharge between dielectric medium and metallic conductor.
Answer: (b) dielectric.