This is the Multiple Choice Questions Part 1 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.

#### Start Practice Exam Test Questions Part 1 of the Series

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1. REE Board Exam April 1997

A single-phase transformer is rated 4,160-V primary and 250-V secondary. It has 1,500 turns in the primary and voltage regulation of 8%. What should be the number of turns in the secondary winding?

A. 88 turns

B. 92 turns

C. 90 turns

D. 86 turns

Answer: **Option C**

Solution:

2. EE Board Exam April 1991

A certain transformer has a ratio of 8 is to 1. The primary winding resistance is 0.00283 Ω per turn and has 1,155 more turns than the secondary winding. If the copper loss in the primary is 20% less than the secondary, find the resistance of the secondary winding.

A. 0.073 ohm

B. 0.090 ohm

C. 0.068 ohm

D. 0.084 ohm

Answer: **Option A**

Solution:

3. EE Board Exam October 1991

A 10-kVA; 2,400/240 V single-phase transformer has the following resistances and leakage reactances. Find the primary voltage required to produce 240-V at the secondary terminals at fullload when the load power factor is 0.8 lagging.

r_{1} = 3 ohms r_{2} = 0.03 ohm

x_{1} = 15 ohms x_{2} = 0.15 ohm

A. 2,450 volts

B. 2,575 volts

C. 2,496 volts

D. 2,632 volts

Answer: **Option C**

Solution:

4. EE Board Exam April 1983

A single-phase transformer rated 75 kVA, 8000 volts primary and 240 volts secondary is given the short circuit test. With the secondary terminals short circuited, 440 volts (60 Hz) is impressed on the primary, which then draws rated current and 1125 watts. Solve for the percentage impedance of the transformer.

A. 5.29%

B. 4.93%

C. 5.50%

D. 5.13%

Answer: **Option C**

Solution:

5. EE Board Exam April 1993

A short circuit test was performed upon a 10-kVA, 2300/230 volt transformer with the following results: Esc = 137 volts; Psc = 192 W; Isc = 4.34 A. Calculate in primary terms the reactance of the transformer.

A. 31.56 ohms

B. 29.88 ohms

C. 10.19 ohms

D. 12.45 ohms

Answer: **Option B**

Solution:

6. EE Board Exam October 1984

A single-phase, 25 kVA, 4800/240 volt transformer was given the short circuit test. With the secondary terminals short-circuited, 93.75 volts was applied on the primary so that the ammeter reads rated current and the wattmeter reads 162.74 watts, all on the primary side. Solve for the percentage resistance of the transformer

A. 1.95%

B. 3.39%

C. 3.02%

D. 2.23%

Answer: **Option B**

Solution:

7. EE Board Exam April 1990

The following data were obtained when a short circuit test was performed upon a 100 kVA, 2400/240 volts distribution transformer: Esc = 72 volts; Isc = 41.6 A; Psc = 1,180 W. All instruments are on the high side during the short circuit test. Calculate the equivalent resistance and reactance of the transformer.

A. 0.682 ohm, 1.59 ohms

B. 0.682 ohm, 1.73 ohms

C. 0.752 ohm, 1.73 ohms

D. 0.752 ohm, 1.59 ohms

Answer: **Option A**

Solution:

8. REE Board Exam March 1998

A 13.8 kV/480 V, 10 MVA three-phase transformer has 5% impedance. What is the impedance in ohms referred to the primary?

A. 0.952 ohm

B. 0.03 ohm

C. 5.125 ohms

D. 9.01 ohms

Answer: **Option A**

Solution:

9. EE Board Exam April 1990 B.

The following data were obtained when a short circuit test was performed upon a 100 kVA, 2400/240 volts distribution transformer: Esc = 72 volts; Isc = 41.6 A; Psc = 1,180 W. All instruments are on the high side during the short circuit test. Calculate the percent regulation at a power factor of 0.75 lagging.

A. 2.54%

B. 2.86%

C. 2.62%

D. 2.71%

Answer: **Option D**

Solution:

10. EE Board Exam October 1992

A 200-kVA transformer with impedance of 5% and the power factor of the load is 85%. The primary voltage is 6,000 volts while the copper loss is 5 kW. Find the percentage regulation.

A. 5.03%

B. 5.21%

C. 4.43%

D. 4.03%

Answer: **Option C**

Solution:

11. EE Board Exam April 1993

A short circuit test was performed upon a 10-kVA, 2300/230 volt transformer with the following results: Esc = 137 volts; Psc = 192 W; Isc = 4.34 A. Calculate the percent regulation at a lagging power factor of 0.707.

A. 5.28%

B. 5.55%

C. 5.37%

D. 5.72%

Answer: **Option C**

Solution:

12. EE Board Exam October 1992

In a 440 V, 50 c/s transformer, the total iron loss is 2500 W. When the supplied p.d. is 220 V at 25 c/s, the corresponding loss is 850 W. Calculate the eddy current loss at normal frequency and p.d.

A. 2337 W

B. 2165 W

C. 2010 W

D. 2216 W

Answer: **Option B**

Solution:

13. EE Board Exam April 1990

A 50 kVA, 2300/230 volts, 60 cycle transformer is tested in the laboratory so that its characteristics maybe determined. The standard test requires an open circuit test and short circuit test.

Open Circuit Test:

I = 6.5 A E = 230 V P = 187 W

Short Circuit Test

I = 21.7 A E = 115 V P = 570 W

Determine the efficiency of the transformer at half load, unity pf.

A. 97.55%

B. 98.70%

C. 98.45%

D. 97.80%

Answer: **Option B**

Solution:

14. EE Board Exam April 1992

Given that the full load copper losses are exactly twice the iron losses in a 50 kVA transformer, and that the quarter load efficiency is 96.5%, calculate the full load efficiency at unity power factor.

A. 97.48%

B. 98.12%

C. 97.64%

D. 96.25%

Answer: **Option C**

Solution:

15. EE Board Exam April 1995

The core loss of a 5 kVA single –phase transformer with normal voltage applied to the primary is 75 watts. The maximum efficiency occurs at 60% of full-load kVA. What is the fullload efficiency of the transformer at 0.80 power factor?

A. 95.16%

B. 89.52%

C. 93.38%

D. 91.36%

Answer: **Option C**

Solution:

16. EE Board Exam October 1980, October 1982

A 25-kVA distribution transformer operates at 120% load, 0.3 pf lagging for 1 hour; art full-load, 0.9 lagging pf for 3 hours; 50% load at unity pf for 8 hours; and 10% load at 0.8 leading pf for the rest of the day. If core loss is 150 watts and full load copper loss is 500 W, what is the all-day efficiency of the transformer?

A. 94.23%

B. 92.86%

C. 97.14%

D. 96.68%

Answer: **Option D**

Solution:

17. EE Board Exam October 1990

Find the all-day efficiency of a transformer having maximum efficiency of 98% at 15-kVA at unity power factor and loaded as follows:

12 hours 2-kW at 0.5 pf lagging

6 hours 12-kW at 0.8 pf lagging

6 hours at no load

A. 95.3%

B. 91.7%

C. 94.6%

D. 93.3%

Answer: **Option A**

Solution:

18. EE Board Exam October 1991

A 30-kVA, 2,400/240 volts, 60-Hz transformer has a full-load power of unity over the period of 24 hrs. The maximum efficiency is 95% and it occurs at full-load. Calculate the all-day efficiency if loaded as follows:

6-hours at full-load

6 hours at quarter load

A. 91.53%

B. 92.45%

C. 90.37%

D. 93.86%

Answer: **Option C**

Solution:

19. EE Board Exam April 1995

Ideal transformer assumptions do not include

A. no leakage flux

B. no saturation of core

C. zero resistance of the winding

D. zero reactance of the winding

Answer: **Option D**

Solution:

20. EE Board Exam October 1993

A transformer is designed to carry 30 times its rated current for 1 second. The length of time that a current of 20 times the rated current can be allowed to flow is

A. 0.67 sec

B. 2 sec

C. 1.5 sec

D. 2.25 sec

Answer: **Option D**

Solution:

21. REE Board Exam October 1998

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

Answer: **Option C**

Solution:

22. REE Board Exam October 1997

A small single transformer has 10.2 watts no load loss. The core has a volume of 750 cu. cm. The maximum flux density is 10,000 gauss and the hysteresis constant of core is 5 x 10-4. Using the Steinmetz law to find the hysteresis, determine the eddy current loss.

A. 4.55 watts

B. 5.55 watts

C. 3.55 watts

D. 2.55 watts

Answer: **Option A**

Solution:

23. EE Board Exam April 1992

When the emf in the two windings of a transformer are opposite in direction, the polarity is

A. positive

B. additive

C. negative

D. subtractive

Answer: **Option D**

Solution:

24. EE Board Exam October 1991

The copper loss of the transformer will

A. not be affected by a change in the power factor

B. decrease with a lowering of the power factor

C. increase with an increase of the power factor

D. increase with the lowering of the power factor

Answer: **Option D**

Solution:

25. EE Board Exam June 1990

Power lost due to alternating magnetization and demagnetization of the core material

A. copper loss

B. eddy current loss

C. hysteresis loss

D. rheostat loss

Answer: **Option C**

Solution:

26. EE Board Exam June 1990

In the open circuit test of the transformer the wattmeter reading is equal to

A. eddy current loss

B. current loss

C. hysteresis loss

D. core loss

Answer: **Option D**

Solution:

27. EE Board Exam October 1991

A 30-kVA; 2,400/240 volts, 60-Hz transformer has a full-load power of unity over the period of 24 hrs. The maximum efficiency is 95% and it occurs at full-load. Calculate the all-day efficiency if loaded 6-hours at full-load.

A. 78.90%

B. 88.38%

C. 89.67%

D. 92.45%

Answer: **Option B**

Solution:

28. EE Board Exam April 1991

A certain transformer has a ratio of 8:1. The primary winding resistance is 0.00283 Ω and has 1,165 more turns than the secondary winding. If the copper loss in the primary is 20% less than the secondary, calculate the resistance of the secondary winding.

A. 5.53 x 10^{-5} ohm

B. 2.56 x 10^{-4} ohm

C. 6.78 x 10^{-7} ohm

D. 4.18 x 10^{-5} ohm

Answer: **Option A**

Solution:

29. EE Board Exam October 1992

A 200-kVA transformer has an impedance of 15% and the power factor of the load is 85%. The primary voltage is 6,000 volts while the copper loss is 15 kW. Find the percentage voltage regulation.

A. 13.49%

B. 15.78%

C. 8.91%

D. 23.67%

Answer: **Option A**

Solution:

30. EE Board Exam June 1990

Transformer connection that can transform threephase system to two-phase system and vice versa

A. Scott or T-connection

B. open delta

C. autotransformer connection

D. wye-delta connection

Answer: **Option A**

Solution:

31. EE Board Exam June 1990

Which transformer has only one winding?

A. distribution transformer

B. power transformer

C. autotransformer

D. isolated winding transformer

Answer: **Option C**

Solution:

32. EE Board Exam April 1990

When the primary and secondary windings take the form of a common ring which is encircled by two or more rings of magnetic materials distributed around its periphery, the transformer is termed as

A. grounding transformer

B. regulating transformer

C. core type transformer

D. shell type transformer

Answer: **Option D**

Solution:

33. A transformer transforms

A. frequency

B. voltage

C. current

D. voltage and current

Answer: **Option D**

Solution:

34. Which of the following is not a basic element of a transformer?

A. Core

B. primary winding

C. secondary winding

D. mutual flux

Answer: **Option D**

Solution:

35. In an ideal transformer

A. windings have no resistance

B. core has no losses

C. core has infinite permeability

D. all of these

Answer: **Option D**

Solution:

36. The main purpose of using core in a transformer is to

A. decrease iron losses

B. prevent eddy current loss

C. eliminate magnetic hysteresis

D. decrease reluctance of the common magnetic circuit

Answer: **Option D**

Solution:

37. Transformer cores are laminated in order to

A. simplify its construction

B. minimize eddy current loss

C. reduce cost

D. reduce hysteresis

Answer: **Option B**

Solution:

38. A transformer having 1000 primary turns is connected to a 250-V a.c supply. For a secondary voltage of 400 V, the number of secondary turns should be

A. 1600

B. 250

C. 400

D. 1250

Answer: **Option A**

Solution:

39. The primary and secondary induce e.m.fs. E1 and E2 in a two-winding transformer are always

A. equal in magnitude

B. anti-phase with each other

C. in-phase with each other

D. determined by load on transformer secondary

Answer: **Option C **

Solution:

40. A step-up transformer increases

A. voltage

B. current

C. power

D. frequency

Answer: **Option A**

Solution:

41. The primary and secondary windings of an ordinary 2-winding transformer always have

A. different number of turns

B. same size of copper wire

C. a common magnetic circuit

D. separate magnetic circuits

Answer: **Option C**

Solution:

42. In a transformer, the leakage flux of each winding is proportional to the current in that winding because

A. Ohm’s law applies to magnetic circuits

B. leakage paths do not saturate

C. the two windings are electrically isolated

D. mutual flux is confined to the core

Answer: **Option B**

Solution:

43. In a two-winding transformer, the e.m.f. per turn in secondary winding is always ____ the induced e.m.f. power turn in primary.

A. equal to K times

B. equal to 1/K times

C. equal to

D. greater than

Answer: **Option C**

Solution:

44. In relation to a transformer, the ratio 20:1 indicates that

A. there are 20 turns on primary one turn on secondary

B. secondary voltage is 1/20th of primary voltage

C. primary current is 20 times greater than the secondary current

D. for every 20 turns on primary, there is one turn on secondary

Answer: **Option D**

Solution:

45. In performing the short circuit test of a transformer

A. high voltage side is usually short circuited

B. low voltage side is usually short circuited

C. any side is short circuited with preference

D. none of these

Answer: **Option B**

Solution:

46. The equivalent resistance of the primary of a transformer having K = 5 and R_{1} = 0.1 ohm when referred to secondary becomes ____ ohm.

A. 0.5

B. 0.02

C. 0.004

D. 2.5

Answer: **Option D**

Solution:

47. A transformer has negative voltage regulation when its power factor is

A. zero

B. unity

C. leading

D. lagging

Answer: **Option C**

Solution:

48. The primary reason why open-circuit test is performed on the transformer is that it

A. draws sufficiently large no-load current for convenient reading

B. requires least voltage to perform the test

C. needs minimum power input

D. involves less core loss

Answer: **Option A**

Solution:

49. No-load test on a transformer is carried out to determine

A. copper loss

B. magnetizing current

C. magnetizing current and no-load loss

D. efficiency of the transformer

Answer: **Option C**

Solution:

50. The main purpose of performing open-circuit test on a transformer is to measure its

A. Cu loss

B. core loss

C. total loss

D. insulation resistance

Answer: **Option B**

Solution:

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