Electrical Power Engineering Multiple Choice Questions and Answers (MCQs) Part 4:
152. For arc heating, the electrodes used are made of
(a) copper.
(b) graphite.
(c) tungsten.
(d) aluminium.
Answer : (b) graphite.
153. In an arc furnace, the choke is provided to
(a) reduce the surge severity.
(b) stabilize the arc.
(c) improve the power factor.
(d) all of the above.
Answer : (b) stabilize the arc.
154. It is desirable to operate the arc furnaces at a power factor of
(a) zero.
(b) unity.
(c) 0.707 lagging.
(d) 0.707 leading.
Answer : (c) 0.707 lagging.
155. It is desirable to keep the arc length short in order to
(a) have better heating.
(b) have better stirring action and reduce oxidation problem.
(c) increase the life of roof refractory.
(d) all of the above.
Answer : (d) all of the above.
156. 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.
157. 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.
158. 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.
159. 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).
160. 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.
161. 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.
162. 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.
163. 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.
164. 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.
165. 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.
166. 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.
167. 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).
168. In induction heating, which of the following is of high value?
(a) Frequency.
(b) Current.
(c) Voltage.
(d) Power factor.
Answer : (a) Frequency.
169. Induction furnaces are used for
(a) heat treatment of castings.
(b) heating of insulators.
(c) melting of aluminium.
(d) all of the above.
Answer : (a) heat treatment of castings.
170. In induction heating, 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.
171. 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.
172. 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
173. 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).
174. 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.
175. 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.
176. Dielectric loss is proportional to
(a) frequency.
(b) (frequency)2.
(c) (frequency)3.
(d) (frequency)1/2.
Answer : (a) frequency.
177. The dielectric loss in a dielectric is proportional to
(a) voltage impressed on the dielectric.
(b) the square of the voltage impressed on the dielectric.
(c) the square root of the voltage impressed on the dielectric.
(d) none of the above.
Answer : (b) the square of the voltage impressed on the dielectric.
178. For heating of plywood, the frequency should be
(a) 1-2 MHz
(b) 10-25 kHz
(c) 1 kHz
(d) 100 Hz
Answer : (a) 1-2 MHz
179. The normal voltage used in dielectric heating is
(a) 1.5 kV
(b) 15 kV
(c) 33 kV
(d) 66 kV
Answer : (a) 1.5 kV
180. For dielectric heating, with the increase of mass of workpiece, the optimum frequency ………. for maximum power transfer.
(a) increases
(b) decreases
(c) independent of mass
Answer : (b) decreases
181. For dielectric heating the range of frequency normally employed is
(a) 10 kHz to 100 kHz
(b) 100 kHz to 10 MHz
(c) 1 MHz to 10 MHz
(d) 10 MHz to 40 MHz
Answer : (d) 10 MHz to 40 MHz
182. The power factor will be maximum in case of
(a) electric arc heating.
(b) resistance heating.
(c) induction heating.
(d) dielectric heating.
Answer : (b) resistance heating.
183. Furnaces used for cremation are
(a) electric resistance heating.
(b) electric arc heating.
(c) dielectric heating.
(d) high frequency eddy current heating.
Answer : (a) electric resistance heating.
184. In an electric room heat convector the method of heating used is
(a) arc heating.
(b) resistance heating.
(c) induction heating.
(d) infrared heating.
Answer : (b) resistance heating.
185. When the composition of non-ferrous metal is to be varied frequently or when heating is intermittent ……… furnace is desirably used.
(a) direct resistance
(b) direct arc
(c) indirect resistance
(d) Ajax Wyatt
Answer : (c) indirect resistance
186. Hysteresis loss and eddy current loss are used in
(a) resistance heating.
(b) dielectric heating.
(c) induction heating of steel.
(d) induction heating of brass.
Answer : (c) induction heating of steel.
187. The power factor will be leading in case of
(a) dielectric heating.
(b) induction heating.
(c) electric arc heating.
(d) resistance heating.
Answer : (a) dielectric heating.
188. The method of heating used for non-conducting materials is
(a) induction heating.
(b) dielectric heating.
(c) electric resistance heating.
(d) electric arc heating.
Answer : (b) dielectric heating.
189. The method appropriate for heating of non-ferrous metals is
(a) indirect resistance heating.
(b) radiant heating.
(c) indirect arc heating.
(d) dielectric heating.
Answer : (c) indirect arc heating.
190. The method suitable for heating of conducting medium is
(a) induction heating.
(b) indirect arc heating.
(c) eddy current heating.
(d) radiant heating.
Answer : (a) induction heating.
191. The most modern method for food processing is
(a) induction heating.
(b) resistance heating.
(c) dielectric heating.
(d) eddy current heating.
Answer : (c) dielectric heating.
192. The main advantage of dielectric heating is that
(a) it can be used for drying the explosives.
(b) heating occurs in the material itself.
(c) heating occurs due to the high frequency.
(d) none of the above.
Answer : (b) heating occurs in the material itself.
193. ………. is not an application of dielectric heating.
(a) Food processing.
(b) Soldering.
(c) Gluing of wood.
(d) Diathermy.
Answer : (b) Soldering.
194. Which of the following falls under the category of plastic or non-fusion welding ?
(a) Resistance welding.
(b) Electron beam welding.
(c) Electro-slag welding.
(d) Arc welding.
Answer : (a) Resistance welding.
195. Which of the following falls under the category of fusion or non-pressure welding?
(a) Resistance welding.
(b) Metal-arc welding.
(c) Ultrasonic welding.
(d) Explosive welding.
Answer : (b) Metal-arc welding.
196. Proper selection of welding depends upon, in addition to cost involved,
(a) kinds of metals to be joined.
(b) nature of products to be fabricated.
(c) production technique used.
(d) all of the above.
Answer : (d) all of the above.
197. The metal surfaces for electrical resistance welding must be
(a) cleaned.
(b) lubricated.
(c) moistened.
(d) rough.
Answer : (a) cleaned.
198. Resistance welding cannot be used for
(a) ferrous materials.
(b) non-ferrous materials.
(c) dielectrics.
(d) any of the above.
Answer : (c) dielectrics.
199. In electrical resistance welding the greatest resistance is offered by
(a) metal surface.
(b) contact layer of metals to be welded.
(c) contact point of electrode with metal top.
(d) contact point of electrode with metal bottom.
Answer : (b) contact layer of metals to be welded.
200. In electric resistance welding
(a) the current required exceeds 100 A.
(b) the voltage required ranges from 4 to 12 V.
(c) the amount of power supplied to the weld usually ranges from 60 watts to 80 watts for each square mm of area.
(d) all of the above.
Answer : (d) all of the above.
201. Resistance to the flow of current is made of
(a) resistance of current path in the work.
(b) resistance between the contact surfaces of the parts being welded.
(c) resistance between the electrodes and the surface of the parts being welded.
(d) all of the above.
Answer : (d) all of the above.