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# MCQ in Transformers Part 5 | REE Board Exam

This is the Multiple Choice Questions Part 5 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 5 of the Series

Choose the letter of the best answer in each questions.

201. The basic property of the transformer is that is changes the voltage level of an a.c. signal

A. without changing the power

B. without changing its shape

C. without changing its frequency

D. without changing power, frequency or shape

Solution:

202. The induced emf in the transformer secondary will depend upon

A. maximum flux in core and frequency only

B. frequency, flux and number of turns in the secondary

C. frequency of the supply only

D. number of turns in the secondary only

Solution:

203. At every instant the direction of secondary current in a transformer must be such as to oppose any change in flux. This is in accordance with

B. Jouleโs law

C. Lenzโs law

D. Coulombโs law

Solution:

204. The voltage transformation ratio is

A. E1/E2

B. N1/N2

C. E2/E1

D. N2/N1

Solution:

205. The magnitude of the mutual flux in a transformer is

A. low at low loads and high at high loads

B. high at low loads and low at high loads

C. same at all levels

D. varies at low loads and constant at high loads

Solution:

206. The concentric windings are used in core type transformer with

A. LT winding placed next to core

B. HT winding placed next to core

C. LT winding on the outer side

D. HT winding on the outer side

Solution:

207. Cross-over windings are used for

A. high voltage winding of small rating transformers

B. low voltage winding of small rating transformers

C. high voltage winding of large rating transformers

D. none of these

Solution:

208. In an ideal transformer on no-load, the primary applied voltage is balanced by

A. the secondary voltage

B. the drop across resistance and reactances

C. the secondary induced emf

D. the primary induced emf

Solution:

209. Transformers are rated in

A. kW

B. kV

C. kWh

D. kVA

Solution:

210. What type of core is used for a high frequency transformer?

A. air core

B. closed iron core

C. aluminum core

D. open iron core

Solution:

211. If in a transformer the secondary winding are doubled and at the same time the primary voltage is reduced to half, then secondary voltage will

A. be halved

B. not change

C. be four times as much

D. be reduced to a quarter

Solution:

212. The no-load current of a transformer in terms of full-load current is usually

A. 1 to 3%

B. 3 to 9%

C. 9 to 12%

D. 12 to 20%

Solution:

213. Which is the common method of cooling a power transformer?

A. air cooling

B. air blast cooling

C. oil cooling

D. natural cooling

Solution:

214. The use of higher flux density in transformer design

A. decreases the weight per kVA

B. increases the weight per kVA

C. increases the weight per kW

D. decreases the weight per kW

Solution:

215. What is the efficiency of transformer compared with that of electrical motors of the same power?

A. much smaller

B. somewhat smaller

C. much higher

D. about the same

Solution:

216. The transformer oil used in transformer provide

A. cooling and lubrication

B. insulation and lubrication

C. insulation and cooling

D. insulation, cooling and lubrication

Solution:

217. In any transformer the voltage per turn in primary and secondary remains

A. always different

B. always same

C. always in the ratio of a

D. sometime same

Solution:

218. In transformer the purpose of breather is to

A. extract moisture of the air

B. to take insulating oil from conservator

C. to provide cooling to the winding

D. to provide insulation to the winding

Solution:

219. 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 area of the core in cm2.

A. 292

B. 375

C. 288

D. 362

Solution:

220. 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:

221. 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 flows through it?

A. 10 A

B. 12 A

C. 20 A

D. 15 A

Solution:

222. The eddy current and hysteresis losses of a 4,400-V, 60-Hz singlephase distribution transformer are 280 and 560 watts, respectively. Determine the core loss when the transformer is connected across a 4,600-volt, 50-Hz source.

A. 977 W

B. 983 W

C. 994 W

D. 967 W

Solution:

223. A 20-kVA, 500-V load is to be supplied by an ideal step-up autotransformer from a 400-V source. Find the current in the common winding.

A. 30 A

B. 40 A

C. 50 A

D. none of these

Solution:

224. 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. 7.50 mWb

D. 7.42 mWb

Solution:

225. The current densities in the primary and secondary windings of a step don transformer are 1250 and 1500 A per square inch, respectively. The ratio of transformation is 10:1 and the mean length per turn of the primary is 15% greater than that of the secondary. If the primary winding resistance is 20 ohms, calculate the resistance of the secondary windings.

A. 0.202 ohm

B. 0.219 ohm

C. 0.209 ohm

D. 0.220 ohm

Solution:

226. A 10-kVA, 2000/400 V, single-phase transformer has primary and secondary winding impedances of 5.0 + j10 and 0.2 + j0.45 ohms, respectively. Calculate the voltage across the secondary terminals at full load, 80% power actor lagging, when the primary is supplied with 2,000 volts.

A. 400 V

B. 394 V

C. 385 V

D. 379 V

Solution:

227. An open delta bank, consisting of two-single phase transformers is operating with balanced three-phase load of 50 kVA, 440 V at 0.8 p lagging and a single phase load of 10 kW resistive connected across c-a leg. Determine the minimum ratings of the transformers. Assume a phase sequence of a-bc.

A. 38.80 kVA, 34.06 kVA

B. 34.32 kVA, 33.40 kVA

C. 35.48 kVA, 36.34 kVA

D. 30.83 kVA, 39.52 kVA

Solution:

228. An open delta, three-phase transformer consisting of two singlephase transformers is operating with a balanced three-phase load of 50 kVA, 440-V at a lagging pf of 0.8. Determine the operating pf of each transformer.

A. 0.393 lagging, 0.993 lagging

B. 0.383 lagging, 0.887 lagging

C. 0.383 lagging, 0.993 lagging

D. 0.393 lagging, 0.887 lagging

Solution:

229. Which of the following given below is the eddy current loss not proportional to

A. square of the thickness

B. square of the frequency

C. square of the load current of the laminated core

D. square of the flux density

Solution:

230. A transformer will have negative voltage regulation when its load is

A. capacitive

B. resistive

C. inductive

D. both B and C

Solution:

231. EE Board Exam April 1990

Copper loss of a transformer are determined by means of a

A. polarity test

B. short circuit test

C. open circuit test

D. inductive circuit test

Solution:

232. The secondary of a transformer is connected wye-grounded. The voltage from line to neutral is 132 volts. If the load current is 40 amperes, calculate the load in kVA.

A. 5.28 kVA

B. 9.15 kVA

C. 20.11 kVA

D. 15.84 kVA

Solution:

233. The current that flows through the common winding of a 20- kV/7.97 kV autotransformer is 200 amperes. Determine the secondary current.

A. 330 amperes

B. 220 amperes

C. 200 amperes

D. 150 amperes

Solution:

234. The secondary current of a 20 kV/7.97-kV autotransformer is 100 amperes. Determine the current that flows through the common winding.

A. 60 amperes

B. 100 amperes

C. 70 amperes

D. 50 amperes

Solution:

235. Two transformer are connected in parallel supplying a load of 2,500 kVA. The first transformer has an impedance of 3% and a rating of 1,250 kVA while the second has an impedance of 3% and a rating of 1,500 kVA. Calculate the load shared by the second transformer.

A. 1453.19 kVA

B. 1792.51 kVA

C. 1363.64 kVA

D. 1227.53 kVA

Solution:

236. A transformer has a core loss of 64 W and a copper loss of 144 W when it is carrying 20% overload current. The load at which this transformer will operate at the maximum efficiency is ____ percent.

A. 80

B. 66

C. 120

D. 75

Solution:

237. A 100 ohms load has a pf of 0.8. A 1 k generator supplies power to the load through a transformer. Best matching will be produced when transformer ratio is approximately

A. 3:1

B. 8:1

C. 10:1

D. 2:1

Solution:

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