# MCQ in Transformers Part 4 | REE Board Exam

(Last Updated On: August 7, 2020) This is the Multiple Choice Questions Part 4 of the Series in Transformers as one of the Electrical Engineering topic. In Preparation for the REE Board Exam make sure to expose yourself and familiarize in each and every questions compiled here taken from various sources including but not limited to past Board Exam Questions in Electrical Engineering field, Electrical Engineering Books, Journals and other Electrical Engineering References.

#### Continue Practice Exam Test Questions Part 4 of the Series

Choose the letter of the best answer in each questions.

151. In a three-phase Y – Y transformer connection, neutral is fundamental to the

A. suppression of harmonies

B. passage of unbalanced currents due to loads

C. provision of dual electric service

D. balancing of phase voltages with respect to line voltages

Solution:

152. As compared to – bank, the capacity of the V – V bank of transformers is ____ percent.

A. 57.7

B. 66.7

C. 50

D. 86.6

Solution:

153. If three transformers in a – are delivering their rated load and one transformer is removed, then overload and each of the remaining transformer is ____ percent.

A. 66.7

B. 173.2

C. 73.2

D. 58

Solution:

154. When a V – V system is converted into a – system, increase in a capacity of the system is ____ percent.

A. 86.6

B. 66.7

C. 73.2

D. 50

Solution:

155. For supplying is a balanced 3- load of 40-kVA, rating of each transformer in V – V bank, should be nearly ____ kVA.

A. 20

B. 23

C. 34.6

D. 25

Solution:

156. When a closedbank is converted into an open- bank. Each of the two remaining transformer supplies ____ percent of the original load.

A. 66.7

B. 57.7

C. 50

D. 73.2

Solution:

157. If the load p.f. is 0.886, then the average p.f of the V-bank is

A. 0.886

B. 0.75

C. 0.51

D. 0.65

Solution:

158. A T – T connection has a higher ratio of utilization that a V – V connection only when

A. identical transformers are used

C. load power factor is unity

D. non-identical transformer are used

Solution:

159. The biggest advantage of T – T connection over the V – V connection for 3-phase power transformation is that it provides

A. a set of balanced voltages under load

B. a true 3-phase, 4 wire system

C. a higher ratio of utilization

D. more voltages

Solution:

160. One of the following statements concerning parallel operation of transformer, the one which is not correct is

A. transformers have equal voltage ratings

B. transformers must have same ratio of transformation

C. transformers must be operated at the same frequency

D. transformers must have equal

Solution:

161. Statement. An auto-transformer is more efficient in transferring energy from primary to secondary circuit.

Reason. Because it does so both inductively and conductively.

Key

A. statement is false, reason is correct and relevant

B. statement is correct, reason is correct but irrelevant

C. both statement and reason are correct and are connected to each other as cause and effect

D. both statement and reason are false

Solution:

162. Out of the following given choices for polyphase transformer connections which one will you select for threeto-two phase conversion?

A. Scott

B. star/star

C. double Scott

D. star/double-delta

Solution:

163. A T – T transformer cannot be paralleled with ____ transformer.

A. V – V

B. Y –

C. Y – Y

D. –

Solution:

164. Instrument transformers are used in a.c. circuits for extending the range of

A. ammeters

B. voltmeters

C. wattmeters

D. all of these

Solution:

165. Before removing the ammeter from the current transformer, its secondary must be short-circuited in order to avoid

A. excessive heating of the core

B. high secondary emf

C. increase in iron losses

D. all of these

Solution:

166. The core of a transformer is made of

A. silicon steel

B. annealed copper

C. seasoned copper

D. aluminum

Solution:

167. The core of a transformer is assembled with laminated sheets to reduce

A. hysteresis loss

B. eddy-current loss

C. magnetic noise

D. magnetizing current

Solution:

168. The use of grainoriented laminated sheets in building a transformer core

A. reduces magnetizing current

B. reduces eddy-current loss in the core

C. reduces hysteresis loss

D. increases the no-load power factor angle

Solution:

169. Cooling of transformers is necessary to

A. increase the efficiency

B. dissipate the heat generated in the windings

C. reduce the losses

D. reduce humming

Solution:

170. The emf induced in the windings of a transformer will

A. lag the core flux by 90 degrees

B. be in-phase with the core flux

C. be out-of-phase with the core flux

D. be independent of the core flux

Solution:

171. A 100-kVA, 1100/440-V, 50-Hz single-phase transformer has 100 turns on the secondary winding. The number of turns in the primary will be

A. 550

B. 275

C. 2750

D. 5500

Solution:

172. The emf induced in the secondary winding of a 50-Hz single-phase transformer having 100 turns on its secondary is 222 V. The maximum flux density in the core is 0.1 Wb/m2. The cross-sectional area of the core is

A. 0.1 m2

B. 0.01 m2

C. 1 m2

D. 0.001 m2

Solution:

173. The copper-loss and core-loss of a transformer of a transformer at various loads are as shown below. At what load will the efficiency of the transformer be a maximum?

A. Load = 50 kVA, Core loss = 320 W, Copper Loss = 500 W

B. Load = 40 kVA, Core loss = 320 W, Copper Loss = 320 W

C. Load = 30 kVA, Core loss = 320 W, Copper Loss = 180 W

D. Load = 20 kVA, Core loss = 320 W, Copper Loss = 80 W

Solution:

174. A transformer when supplying a load maintained 11 kV across terminals. When the load was switched off, the terminal voltage became 11550 V. What is the voltage regulation of the load?

A. 11.55%

B. 5.5%

C. 5%

D. 55%

Solution:

175. Power lost in the open-circuit and shiort0-circuit tests on a transformer gives approximately an account of the following losses:

A. Core-losses and copper losses respectively

B. Copper-losses and core-losses respectively

C. Eddy-current loss and hysteresis loss respectively

D. Hysteresis loss and eddy-current loss respectively

Solution:

176. Maximum efficiency of a transformer occurs when

A. hysteresis loss and eddy current loss are minimum

B. the sum of hysteresis loss and eddy current loss is equal to copper loss in the windings

D. hysteresis loss is equal to eddy-current loss

Solution:

177. The all-day efficiency of a transformer is the ratio of

A. kWh output and kWh input per day

B. kWh output and kWh input in a day

C. output power and input power

D. input power and output power

Solution:

178. The ratio of the primary to secondary voltage of a transformer is 2:1. The saving in terms of weight of copper required if an autotransformer is used instead of a two-winding transformer will be

A. 50%

B. 33.33%

C. 66.67%

D. 97%

Solution:

179. For satisfactory parallel operation of two single-phase transformers a number of conditions are to be fulfilled. A number of conditions are written below. Indicate which of these is not required to be fulfilled.

A. kVA ratings of the two transformers should be equal

B. Transformers should be properly connected with regard to their polarity

C. Voltage ratings of the primary windings should be suitable for supply system voltage and frequency. The turns ratio of the transformer should be equal.

D. The percentage impedance of the two transformers should be equal.

Solution:

180. An additional condition for parallel operation of three-phase transformers over single-phase transformers is that

A. the transformers should belong to the same vector group

B. ratios of winding resistances to resistances for the transformers should be equal

C. the transformers should have the same kva ratings

D. the transformers should not belong to the same vector group

Solution:

181. The maximum flux density in the core of a 3000/240-V, 50 Hz singlephase distribution transformer is 1.25 Tesla. If the voltage induced per turn is 8 volts, determine the cross-sectional core in cm2.

A. 292

B. 375

C. 288

D. 362

Solution:

182. The net cross section of a transformer is 20 square inches. The maximum flux density is 11 kilogausses. The frequency is 60 Hz. If there are 1,000 turns in the primary, determine the primary induced emf.

A. 3780 V

B. 3945 V

C. 3590 V

D. 3850 V

Solution:

183. _____current are wasteful currents which flow in cores of transformers and produced heat.

A. Residual

B. Eddy

C. Sneak

D. Magnetizing

Solution:

184. Calculate the total magnetic flux in a 60-cycle transformer in which the induced emf per turn of the winding is equal to 2 V.

A. 7.50 mWb

B. 7.02 mWb

C. 7.60 mWb

D. 7.42 mWb

Solution:

185. The low side of a certain welding transformer has 2 turns and delivers 300 A. If the high voltage side has 30 turns, how much current lows through it?

A. 10 A

B. 12 A

C. 20 A

D. 15 A

Solution:

186. The principle of working of working of the transformer is

A. static induction

B. dynamic induction

C. mutual induction

D. self-induction

Solution:

187. Transformer is used to change the value of

A. frequency

B. voltage

C. power

D. power factor

Solution:

188. The path of the magnetic flux in a transformer has

A, low reactance

B. high reactance

C. high conductivity

D. low resistance

Solution:

189. Electric power is transformed from one coil to the other in a transformer

A. physically

B. magnetically

C. electrically

D. electromagnetically

Solution:

190. A transformer operates

A. always at unity power factor

B. has its own power factor

C. at power factor below a particular value

D. at power factor depending on the power factor of the load

Solution:

191. The laminations are made from

A. low carbon steel

B. silicon steel sheet

C. nickel alloy steel stampings

D. chrome steel sheets

Solution:

192. The steel for construction of transformer core is made so as to made

A. low permeability and high hysteresis

B. low permeability and low hysteresis

C. high permeability and low hysteresis

D. high permeability and high hysteresis

Solution:

193. The special silicon steel is used for laminations because

A. hysteresis losses are reduced

B. eddy current losses are reduced

C. both of the above losses are reduced

D. none of these

Solution:

194. What is common in the two windings of a transformer?

A. electric circuit

B. magnetic circuit

C. winding wire gauge

D. none of these

Solution:

195. The main function of the iron core in a transformer is to

A. provide strength to the windings

B. to decrease the hysteresis losses

C. decrease the reluctance of the magnetic path

D. reduce eddy current losses

Solution:

196. Preferably, the resistance between the primary and the secondary of a transformer should be

A. as low as possible

B. as high as possible

C. low or high depending upon whether it is step up or step down respectively

D. high or low depending upon whether it is step up or step down respectively

Solution:

197. Ideal transformer assumptions do not include

A. zero reactance of windings

B. zero resistance of windings

C. no leakage flux

D. no saturation of flux

Solution:

198. The reactance of a transformer is determined by its

A. leakage flux

B. common core flux

C. size of core

D. permeability of the material of the core

Solution:

199. The efficiency of the transformer is normally in the range of

A. 50% to 70%

B. 60% to 75%

C. 80% to 90%

D. 90% to 98%

Solution:

200. The resistance of the low voltage winding of a transformer

A. is equal to the resistance of its high voltage side

B. is more than the resistance of its high voltage side

C. is less than the resistance of its high voltage side

D. either B or C

Solution:

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