EE 7 - Transformers, Alternators, Synchronous Motors, Induction Motor

1. The principle of working of a transformer is

 A. static induction

B. dynamic induction

C. mutual induction

D. self induction

C. mutual induction

2. Transformer is used to change the values of  

A. frequency

B. voltage

C. power

D. power factor

B. voltage

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

A. low reactance

B. high reactance

C. high conductivity

D. low resistance

A. low reactance

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

A. physically

B. magnetically

C. electrically

D. electromagnetically

B. magnetically

5.  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

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

6. The laminations are made from  

A. low carbon steel

B. silicon steel sheet

C. nickel alloy steel stampings

D. chrome steel sheets

B. silicon steel sheet

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

A. low permeability and high hysteresis

B. low permeability and low hysteresis

C. high permeability and low hysteresis

D. high permeability and high hysteresis loss

C. high permeability and low hysteresis

8. The special silicon steel is used for laminations because  

A. hysteresis losses are reduced

B. eddy current losses are reduced

C. both the above losses are reduced

D. none of these

A. hysteresis losses are reduced

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

A. electric circuit

B. magnetic circuit

C. winding wire gauge

D. dielectric circuit

B. magnetic circuit

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

A. provide strength to the windings

B. to decrease the hysteresis loss

C. decrease the reluctance of the magnetic path

D. reduce eddy current losses

C. decrease the reluctance of the magnetic path

11. 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

A. as low as possible

12. Ideal transformer assumptions do not include  

A. zero reactance of windings

B. zero resistance of windings

C. no leakage flux

D. no saturation of core

A. zero reactance of windings

13. The reactance of a transformer is determined by its  

A. leakage flux

B. common core flux

C. size of the core

D. permeability of the material of the core

A. leakage flux

14. The efficiency of a transformer is normally in the range of  

A. 50% to 70%

B. 60% to 75%

C. 80% to 90%

D. 90% to 98%

D. 90% to 98%

15. The resistance of low voltage side of a transformer  

A. is equal to 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. B or C

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

16. Eddy current losses in transformer core are reduced by  

A. increasing the thickness of laminations

B. decreasing the thickness of laminations

C. taking the wire of higher gauge for winding the transformer

D. decreasing the air gap in the magnetic circuit

B. decreasing the thickness of laminations

17. The primary and the secondary voltages are  

A. 180 degrees out of phase in a transformer

B. always in phase in a transformer

C. 90 degrees out of phase in a transformer

D. 30 degrees or 60 degrees our of phase in a transformer

A. 180 degrees out of phase in a transformer

18. The basic property of the transformer is that it 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

D. without changing power, frequency or shape

19. Eddy current losses in a transformer are minimized by laminating the core, the lamination being insulated from each other by  

A. a light coat of core plate varnish

B. thin sheet of mica

C. thick paper of insulation

D. any one of the above

A. a light coat of core plate varnish

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

A. maximum flux in core and frequencies only

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

C. frequency of the supply only

D. number of turns in secondary only

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

21. 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

A. Faraday’s law

B. Joule’s law

C. Lenz’s law

D. Coulomb’s law

C. Lenz’s law

22. The voltage transformer ratio is  

A. E1/E2

B. N1/N2

C. E2/E1

D. N2/N1

A. E1/E2

23. The magnitude of 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

C. same at all levels

24. 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

A. LT winding placed next to core

25. 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. low voltage winding of large rating transformers

A. high voltage winding of small rating transformers

26. 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

C. the secondary induced emf 

27. Transformers are rated in  

A. kW

B. kV

C. kWH

D. kVA

D. kVA

28. 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

A. air core

29. If in a transformer the secondary turns are doubled and as the same time the primary voltage is reduced by half, then the secondary voltage will  

A. be halved

B. not change

C. be four times as high

D. be reduced to a quarter

B. not change

30. 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%

A. 1 to 3%

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

A. air cooling

B. air-blast cooling

C. oil cooling

D. natural cooling

C. oil cooling

32. Which of the following is a correct statement about eddy currents?  

A. Eddy currents improve the efficiency of a motor.

B. Eddy currents heat up the metal parts.

C. Eddy currents do not influence the movement.

D. Eddy currents are used for arc welding.

B. Eddy currents heat up the metal parts.

33. Which of the statement given below is true about autotransformer?  

A. It has two separate windings connected in series externally.

B. It can only step down the voltage.

C. It has only one winding.

D. It is most suitable for power transformation.

C. It has only one winding.

34. 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

A. decreases the weight per kVA

35. 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

D. about the same

36. The transformer oil used in transformers provide  

A. cooling and lubrication

B. insulation and lubrication

C. insulation and cooling

D. insulation, cooling and lubrication

C. insulation and cooling

37. What is the typical use of an autotransformer?  

A. Toy transformer

B. Control transformer

C. Variable Transformer

D. Isolating Transformer

C. Variable Transformer

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

A. always different

B. always same

C. always in ratio of k

D. sometimes same

B. always same

39. The full load copper loss in a transformer is 400 watts. At half load, the copper loss will be  

A. 400 watts

B. 100 watts

C. 200 watts

D. 50 watts

B. 100 watts

40. A transformer is working at its maximum efficiency. Its iron loss is 500 watts. Its copper loss will be  

A. 300 watts

B. 250 watts

C. 350 watts

D. 500 watts

D. 500 watts

41. Power transformers are designed to have maximum efficiency at  

A. no load

B. half load

C. near full load

D. little more than full load

C. near full load

42. As compared to an amplifier a transformer cannot  

A. increase output power

B. increase the output current

C. increase the output voltage

D. decrease the output current

A. increase output power

43. Distribution transformers have good all-day efficiency due to  

A. low copper loss

B. low iron loss

C. low copper as well as iron loss

D. low power loss

B. low iron loss

44. 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

A. extract moisture of the air

45. Two transformers when operating in parallel will share the load depending upon their  

A. magnetizing current

B. leakage reactance

C. per unit impedance

D. efficiency

C. per unit impedance

46. The magnetic coupling between the primary and secondary windings of a transformer may be increased by  

A. increasing the number of laminations of core material

B. using the magnetic core of low reactance

C. changing the turns ratio of the transformer

D. no means, because it is constant

B. using the magnetic core of low reactance

47. Conservator consists of  

A. an air-tight metal drum fixed at the top of the tank

B. drum placed at the bottom of the tank

C. overload protection

D. none of these

A. an air-tight metal drum fixed at the top of the tank

48. For a three phase transformer, turns ratio k is given as  

A. k = (sqrt of 3) ×  line voltage ratio

B. k =  line voltage ratio

C. k = (sqrt of 3) × phase voltage ratio

D. k =  phase voltage ratio

B. k =  line voltage ratio

49. In a power transformer, the winding which is nearer to the iron core of the transformer  

A. is a H.V. winding

B. is L.V. winding

C. may be H.V. or L.V. winding

D. is sandwiching of the two windings

B. is L.V. winding

50. All day efficiency of a transformer η_A  

A. η_A  = electrical efficiency of transformer

B. η_A  > electrical efficiency of transformer

C. η_A < electrical efficiency of transformer

D. none these may be true

C. η_A < electrical efficiency of transformer

51. If we increase the flux density in the case of a transformer  

A. the size of the transformer will reduce

B. the distortion in the wave shape will reduce

C. the hysteresis and eddy current losses will reduce

D. none of the above is true

A. the size of the transformer will reduce

52. A good transformer oil should be absolutely free from  

A. sulphur

B. alkalies

C. moisture

D. all of the above

D. all of the above

53. In an ideal transformer on no load, the voltage applied across the primary winding of the transformer is balanced by  

A. the emf induced in the primary winding

B. the drop across the impedance of primary side

C. the drop across the impedance of secondary side

D. the secondary induced emf

D. the secondary induced emf

54. No load primary input is practically equal to the iron losses in the transformer because  

A. primary current is small

B. secondary current is small

C. both the currents are small

D. no load is not equal to iron losses

A. primary current is small

55. The transformation of energy from primary of the transformer to secondary takes place due to  

A. the flux linkage between the two windings

B. the difference in number of turns of primary and secondary windings

C. the changing current in two windings

D. none of these

A. the flux linkage between the two windings

56. If Rs is the resistance of secondary winding of the transformer and k is transformation ratio then the equivalent secondary resistance referred to primary side will be

A. R_s/k²

B. R_s/k

C. R_s/k³

D. R_s/(sqrt of k)

A. R_s/k²

57. Mark the correct statement.  

A. The percentages of resistance, reactance and impedance have the same value whether referred to primary or secondary of the transformer.

B.  A good transformer must have regulation as high as possible.

C. If the power factor is leading the regulation of a good transformer will be higher than when it is lagging.

D. None of the above is correct.

A. The percentages of resistance, reactance and impedance have the same value whether referred to primary or secondary of the transformer.

58. If the supply frequency to the transformer is increase, the iron loss will  

A. decrease

B. increase

C. do not change

D. vary

B. increase

59. The size of a transformer core will depend on  

A. frequency

B. flux density of the core material

C. area of the core

D. A and B both

D. A and B both

60. Two transformers operating in parallel will share the load depending upon their  

A. rating

B. leakage reactance

C. efficiency

D. per unit impedance

A. rating

61. Three 10 : 1 single phase, step-down transformers are connected in wye-delta. For a primary line-to-line voltage of 3980 V, find the corresponding line voltage on the secondary side.  

A. 398 V

B. 230 V 

C. 3980 V

D. 2300 V

B. 230 V 

62. While conducting short-circuit test on a transformer, the following side is short-circuited:  

A. H.V. side

B. L.V. side

C. Primary side

D. Secondary side

B. L.V. side

63. Current transformer secondary is connected to an ammeter of 5 A range to measure line current of 1000 A. The ammeter is to be serviced. It can be removed  

A. without interfering with supply

B. with interfering with supply

C. only after switching off the main current path

D. only before switching off the main current path

C. only after switching off the main current path

1. In huge alternators, the moving part is  

A. brushes

B. armature

C. poles

D. rotor

C. poles

2. In case of alternator, the stator frame serves  

A. to protect the whole machine

B. as a return path for the flux

C. to hold the armature stampings

D. to verticate the armature

B. as a return path for the flux

3.  Alternators mostly work on rotating field type principle which is different from d.c. machines where the field is stationary. Therefore alternators  

A. don’t obey Faraday’s law

B. work on Faraday’s law

C. do not obey Lenz’s law

D. don’t obey Ohm’s law

B. work on Faraday’s law

4. High speed alternators have a rotor construction  

A. similar to dc machines

B. of non-salient type

C. of salient type

D. of silent type

B. of non-salient type

5. The voltage applied to the field of a rotating cylindrical rotor type alternators  

A. high dc voltage

B. high ac voltage

C. low dc voltage

D. low ac voltage

C. low dc voltage

6.  Alternators are usually designed to generate  

A. variable frequency

B. definite currents

C. definite frequencies

D. definite power factor

C. definite frequencies

7. The standard practice, nowadays in alternators is to have  

A. rotating field

B. rotating armature

C. either rotating field or rotating armature

D. none of the above

A. rotating field

8. In power generating stations (thermal stations) the usual number of poles employed in alternators are  

A. twenty four

B. forty eight

C. thirty two

D. none of these

D. none of these

9. The diameter of high speed turbo alternators as compared to hydroelectric ones are  

A. smaller because speed is high

B. smaller because speed is less

C. larger because speed is high

D. larger because speed is less

A. smaller because speed is high

10. The slip rings employed in a three-phase alternator in hydrostation are insulated for  

A. extra high tension voltage

B. low voltage

C. full armature voltage

D. very high voltage

B. low voltage

11. When the speed of an alternator increases, the frequency  

A. decreases

B. remains the same

C. increases

D. changes

12. Steam-turbo alternators are much smaller in size than water-turbine alternators for a given output. This is so because  

A. steam-turbo alternators are built with smaller capacities

B. steam-turbo alternators run at high speed

C. steam-turbo alternators have long rotors

D. all of these

B. steam-turbo alternators run at high speed

13. For a coil having a span of 2/3 or pole pitch, the coil span factor is  

A. 0.8

B. 0.866

C. 0.786

D. 0.966

B. 0.866

14. Distributing the armature winding of alternator is more than one number of slots per pole per phase results in  

A. reduction of irregularities produced in the waveform

B. economy of materials used in winding

C. less weight of the entire armature

D. increase of generated emf per phase

A. reduction of irregularities produced in the waveform

15. When the power factor of load is unity, the armature flux of an alternator will be  

A. demagnetizing

B. square wave form

C. cross magnetizing

D. none of these

C. cross magnetizing

16. An alternator is capable of delivering power at a particular frequency. The frequency can be increased by  

A. increasing the armature speed

B. reversing the armature rotation

C. increasing the current supplied to the field electromagnets

D. reversing the field polarity

A. increasing the armature speed

17. If the input of the prime mover of an alternator is kept constant but excitation is increased then  

A. kVA will be leading

B. kVA will be lagging

C. kW will be changed

D. the power factor of the load remains constant

B. kVA will be lagging

18. For two alternators operating in parallel, if the load shared by one of them is to be increased, its field excitation is  

A. to be strengthened keeping input torque the same

B. to be weakened keeping input torque the same

C. to be kept constant but input torque should be increased

D. to be kept constant but input torque should be decreased

C. to be kept constant but input torque should be increased

19. If P = number of pair poles, f = the frequency in Hz, S = speed in rpm, the relationship between the three for synchronous alternators is given as  

A. f = PS/60 Hz

B. P = fS/120 poles

C. f = PS/120 Hz

D. S = fP/120 rpm

C. f = PS/120 Hz

20. If Ө is the angle measured in electrical degrees between any point on the pole face and center of the pole then for setting a sinusoidal wave as output, the air gap at that point should be such that  

A. air gap α sin θ

B. air gap α cos θ

C. air gap α (1/sinθ)

D. air gap α (1/cosθ)

D. air gap α (1/cosθ)

21. The sinusoidal voltage developed per pole per phase in a group of coils is defined as  

A. E_gap = 4.44ΦNnfk_dk_pV 

B. E_gap = 4.44ΦNfk_dk_pV

C. E_gap = 4.44ΦNk_ffV

D. none of these

Where N = no. of turns per coil n = no. of coils per phase per pole

A. E_gap = 4.44ΦNnfk_dk_pV 

22. The armature reaction of an alternator will be cross-magnetizing of the power factor of the load is  

A. unity

B. less than unity

C. more than unity

D. zero leading

A. unity

23. If X_s = the synchronous reactance of the machine, X_e = leakage reactance and X_a = reactance due to armature flux then  

A. X_a = X_s – X_e

B. X_a = X_s + X_e

C. X_a = X_e – X_s

D. X_e = X_aX_s

A. X_a = X_s – X_e

24. The best and the most accurate method for determining the voltage regulation is  

A. synchronous impedance method

B. mmf method

C. emf method

D. Potier triangle method

D. Potier triangle method

25. An alternator is said to be over-excited if it is operating at  

A. leading p.f.

B. lagging p.f.

C. unity p.f.

D. zero voltage regulation

A. leading p.f.

26. From what source does a rotating field type alternator receive its exciter voltage?

 A. from an externally controlled ac voltage

B. from an externally controlled dc voltage

C. from a self excited source like shunt generator

D. from an auxiliary generator driven by main generator shaf t

D. from an auxiliary generator driven by main generator shaf t

27. High voltage alternators are usually of the rotating field type. This means the generated voltage is connected  

A. through slip rings to the load

B. through slip rings of the rotating field

C. directly to the load

D. directly to the rotor

C. directly to the load

28. The maximum current that can be supplied by alternators depends upon the  

A. strength of the excite voltage

B. number of poles

C. strength of the magnetic field

D. maximum heat dissipation

C. strength of the magnetic field

29. One of the advantages of distributing the winding in alternators is to  

A. reduce harmonics

B. improve voltage wave form

C. reduce the amount of copper

D. decrease the value of the voltage

B. improve voltage wave form

30. Distribution factor K₄ is defined as the ration of emf’s of  

A. distributed winding to full pitch winding

B. concentrated winding to distributed winding

C. distributed winding to concentrated winding

D. full pitch winding to distributed winding

C. distributed winding to concentrated winding

31. If n = number of slots per pole per phase and α = slot angle, the distribution factor is

A. (nsin α/2) / (sin α/2)

B. (sin nα/2) / (nsin α/2)

C. (sin n²) / (nsin α/2)

D. (sin nα/2) / (nsin α/2)

D. (sin nα/2) / (nsin α/2)

32. For a uniformly distributed winding the value of distribution factor is  

A. unity

B. 0.96

C. 0.866

D. 0.955

D. 0.955

33. One of the advantages of a short pitch winding lies in  

A. elimination or suppression of harmonics

B. reduction of voltage

C. increasing the inductance

D. reduction of the circulating current

A. elimination or suppression of harmonics

34. Pitch factor is defined as the ratio of the emf’s of  

A. distributed winding to full pitch winding

B. short pitch coil to full pitch coil

C. full pitch winding to short pitch winding

D. full pitch winding to concentrated winding

B. short pitch coil to full pitch coil

35. A winding is short pitched by 60 degrees electrical. Its pitch factor is  

A. 0.5

B. 0.866

C. 0.707

D. 0.966

B. 0.866

36. Synchronous impedance of an alternator is defined as the ratio of  

A. short circuit voltage to short circuit current

B. open circuit voltage to short circuit current

C. short circuit current to no load current

D. open circuit voltage to short circuit current for same value of excitation

D. open circuit voltage to short circuit current for same value of excitation

37. Synchronous reactance consists of two components, namely  

A. armature resistance and leakage reactance

B. leakage reactance and fictitious reactance

C. armature resistance and fictitious reactance

D. leakage reactance and capacitive reactance

B. leakage reactance and fictitious reactance