Transformers-obj-questions.pdf

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Electrical Machines, Second Edition > 1 Transformers > MULTIPLE-CHOICE QUESTIONS AND ANSWERS : Safari Books Online

MULTIPLE-CHOICE QUESTIONS AND ANSWERS 1. In a transform e r

(a) All turns are equally insulated

(b) The end turns are more strongly insulated

(c) The end turns are closely wound (d) The end turns are widely separated 2. Lam inate d insulations coate d with varnish are norm ally use d in the transform e r

(a) To reduce reluctance of magnetic path (b) To reduce the effect of eddy current (c) To increase the reluctance of magnetic path (d) To reduce the hysteresis effect 3. The re quire d thick ne ss of lam ination in a transform e r de cre ase s whe n

(a) The applied frequency increases (b) The applied frequency decreases (c) The applied voltage increases

(d) The applied voltage decreases

4. O il in transform e r is use d to

(a) Transfer electrical energy (b) Insulate the windings (c) Cool the windings

(d) None of the above

5. The following arrange m e nt will re duce the re luctance of m agne tic path of the transform e r.

6. The windings of a transform e r are divide d into se ve ral coils be cause

(a) It is difficult to wind as one coil (b) It reduces voltage per coil (c) It requires less insulation

(d) None of the above

7. The size and construction of bushings in a transform e r de pe nd upon the

(a) Size of winding (b) Size of tank (c) Current flowing (d)Voltage supplied 8. Transform e r hum m ing sound is re duce d by the

(a) Proper bracing of transformers assemblies (b) Proper insulation (c) Proper design (d) Proper design of winding 9. Sludge in transform e r oil is due to

(a) Decomposition of oil

(b) Decomposition of insulation

(c) Moisture content in oil (d) None of the above 10. A transform e r use d only for e le ctrical isolation be twe e n two circuits has turns ratio which is http://my.safaribooksonline.com/book/electrical-engineering/9788131760901/1-transformers/navpoint-99

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(a) More than unity (b) Less than unity (c) Equal to unity (d) More than 0.5 11. If 90 pe r ce nt of norm al voltage and 90 pe r ce nt of norm al fre que ncy are applie d to a transform e r, the pe r ce nt charge in hyste re sis losse s will be

(a) 20% (b) 4.7% (c) 19% (d) 21% 12. If 110 pe r ce nt of norm al voltage and 110 pe r ce nt of norm al fre que ncy is applie d to a transform e r, the pe rce ntage change of e ddy curre nt losse s will be

(a) 10% (b) 20% (c) 25% (d) 21% 13. A transform e r has two 2,400 V prim ary coils and two 240 V coils. By prope r conne ction of the windings, the transform ation ratio that can be obtaine d is

(a) 10 (b) 5 (c) 20 (d) 9 14. A single -phase , 2,200/200 V transform e r tak e s 1 A at the HT side or no load at a powe r factor of 0.385 lagging. The iron losse s are

(a) 167 W (b) 77 W (c) 88 W (d) 98 W 15. Ne gle cting re sistance , at constant flux de nsity, the powe r re quire d pe r k ilogram to m agne tize the iron core of a transform e r is 0.8 W at 25 Hz and 2.04 W at 60 Hz. The powe r re quire d pe r k ilogram for 100 Hz is

(a) 3.8 W (b) 3.63 W (c) 3.4 W (d) 5.2 W 16. Se le ct the corre ct state m e nt:

(a) emf per turn of both the windings are equal (b) emf per turn in HV winding is more than the emf per turn in LV winding (c) emf per turn in HV winding is less than the emf per turn in LV winding 17. The flux involve d in the e m f e quation of a transform e r has

(a) rms value (b) Average value (c) Total value (d) Maximum value 18. The no-load curre nt in a transform e r lags the applie d voltage by

(a) 90° – 95° (b) About 80° – 85° (c) 0° – 15° (d) About 110° 19. High le ak age im pe dance transform e rs are use d for applications such as

(a) Power distribution (b) Electrical toys (c) Arc welding

(d) Fluorescent lamps

20. A transform e r 2,000 k VA, 250 Hz is ope rate d at 50 Hz. Its k VA rating should be re vise d to

(a) 400 kVA

(b) 10,000 kVA

(c) 2,000 kVA (d) Cannot be revised 21. The transform e r which will have the large st size is

(a) 100 kVA, 25 Hz (b) 100 kVA, 100 Hz (c) 100 kVA, 50 Hz (d) 100 kVA, 60 Hz 22. The m agne tic flux of a transform e r follows

(a) High reluctance path

(b) Low reluctance path

(c) Low conductivity path (d) High conductivity path 23. The e fficie ncy of a transform e r is usually in the range of

(a) 50–60% (b) 65–75% (c) 70–90% (d) 90–98% 24. Transform e r is lam inate d to re duce http://my.safaribooksonline.com/book/electrical-engineering/9788131760901/1-transformers/navpoint-99

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(a) Hystersis loss

(b) Eddy current loss

(c) Partly (a) and partly (b) (d) Increases exciting current 25. Transform e r change s the value of

(a) Power

(b) Frequency

(c) Voltage (d) Current 26. Transfe r of e le ctrical powe r from prim ary to se condary in a transform e r tak e s place

(a) Electrically

(b) Electromagnetically

(c) Magnetically (d) None of these 27. The thick ne ss of a 50 Hz transform e r lam ination is

(a) 0.35 cm (b) 0.35 mm (c) 0.33 m

(d) 0.30 cm

28. A powe r transform e r is a constant

(a) Current device (b) Voltage device (c) Power device

(d) Main flux device

29. The value of flux in the e m f e quation of a transform e r is

(a) rms

(b) Average

(c) Maximum (d) Integral wave cycle 30. The le ak age flux in a transform e r de pe nds upon the value of

(a) Load current (b) Supply frequency (c) Mutual flux

(d) None of these

31. The voltage re gulation of a transform e r at full-load 0.8 powe r factor lag is 6 pe r ce nt. Its voltage re gulation at full-load 0.8 powe r factor le ad will be

(a) Negative (b) 54% (c) Positive (d) Zero 32. The full-load e fficie ncy of a transform e r at 0.85 p.f. lag is 97 pe r ce nt. Its e fficie ncy at full load 0.85 bpowe r factor le ad will be

(a) 99% (b) 96% (c) 97% (d) 98% 33. If a load on se condary side of a transform e r incre ase s, the curre nt on the prim ary side

(a) Remains constant (b) Increases (c) Decreases (d) None of these 34. If a sinusoidal e x citing curre nt is applie d to a transform e r, the m utual flux produce d is

(a) Negative (b) Sinusoidal (c) Zero (d) Flat top 35. To ge t sinusoidal flux in a transform e r the m utual flux produce d is

(a) Non-sinusoidal (b) Sinusoidal (c) Direct current (d) Zero 36. A 220V, 150V bulb is conne cte d in se rie s with the prim ary of a 220/1,100 V, 50 Hz transform e r. If the load on the se condary side is disconne cte d, the brightne ss of the bulb will

(a) Decrease

(b) Increase

(c) Be unaffected (d) Decrease to a very low value 37. The ope n circuit te st of a transform e r give s inform ation about

(a) Core losses of the transformer (b) Cu losses of the transformer (c) Exciting current

(d) None of these

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38. The short circuit te st of a transform e r give s the inform ation of

(a) Cu losses of the transformer (b) Core losses of the transformer (c) Winding circuit impedance

(d) None of these

39. During te sting of a transform e r

(a) Both SC and OC tests are performed at rated current (b) Both SC and OC tests are performed at rated voltage (c) OC test is performed at rated voltage (d) SC test is performed at rated current 40. All-day e fficie ncy of a transform e r is

(a) Equal to its power efficiency

(b) Less than its power efficiency

(c) More than its power efficiency (d) None of these 41. R outine e fficie ncy of a transform e r de pe nds upon the value of

(a) Load current

(b) Supply frequency

(c) Power factor of load (d) Both (a) and (b) 42. A fe rrite core has le ss e ddy curre nt loss than an iron core be cause fe rrite s have

(a) Lower permeability

(b) High hysteresis

(c) Alnico as the main constituent (d) High resistance 43. The m ax im um re gulation of a transform e r occurs at a powe r factor of

(a) Unity (b)

(c)

(d)

44. The ze ro of re gulation of a transform e r occurs at a powe r factor of

(a) Zero (b)

(c)

(d)

45. C ircular coil se ctions are use d be cause the y

(a) Reduce iron material

(b) Reduce copper material

(c) Have the strongest mechanical shape (d) All of these 46. A transform e r is conne cte d to a constant voltage source . If the supply fre que ncy de cre ase s, the m agne tic flux in the core will

(a) Increase towards saturation (b) Decrease (c) Remain unchanged

(d) None of these

47. The powe r in autotransform e r is transfe rre d through

(a) Inductive process

(b) Convection process

(c) Conduction process (d) All of these 48. The cross-se ctional are a of the com m on portion of an autotransform e r is

(a) Kept constant

(b) Proportionally decreased

(c) Proportionally increased (d) None of these http://my.safaribooksonline.com/book/electrical-engineering/9788131760901/1-transformers/navpoint-99

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49. The e fficie ncy of an autotransform e r for the sam e output com pare d to a two-winding transform e r is

(a) Greater (b) Lesser (c) Poor (d) None of these 50. The condition for succe ssful paralle l ope ration of transform e r is

(a) Correct polarity (b) Per unit impedance based on their rating should be equal (c) Identical voltage and frequency rating (d) Equal ratio of equivalent resistance to reactance (e) All of these 51. During paralle l ope ration of transform e rs incorre ct polarity will re sult in

(a) Open circuit (b) Dead short circuit (c) Regeneration of power (d) Power factor of transformer will be different from that of the connected load

52. In an autotransform e r of voltage ratio

(a)

(b)

(c)

and V1 > V2, the fraction of powe r transfe rre d inductive ly is

(d)

53. A 10 k VA, 400/200 V sinlge -phase transform e r with 10 pe r ce nt im pe dance draws a ste ady short circuit line curre nt of

(a) 50 A (b) 150 A (c) 250 A (d) 350 A 54. A 400/100 V, 10 k VA two-winding transform e r is re conne cte d as an autotransform e r across a suitable voltage source . The m ax im um k VA is

(a) 50 kVA (b) 15 kVA (c) 12.5 kVA (d) 8.75 kVA 55. An autotransform e r having a transform ation ratio of 0.8 supplie s a load of 10 k W . The powe r transfe rre d inductive ly from the prim ary to the se condary is

(a) 10 kW (b) 8 kW (c) 2 kW (d) Zero 56. A single -phase induction re gulator is a constant input transform e r to obtain sm ooth variation of the output voltage by varying the

(a) Ratio of turns between primary and secondary winding (b) Frequency (c) Flux-density in the core (d) Angle between the magnetic forces of the primary and secondary windings 57. In ope rating a 400 Hz transform e r at 50 Hz

(a) Only voltage is reduced in the same proportion as the frequency (b) Only kVA is reduced in the same proportion as the frequency (c) Both voltage and kVA rating are reduced in the same proportion as the frequency (d) None of the above http://my.safaribooksonline.com/book/electrical-engineering/9788131760901/1-transformers/navpoint-99

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58. In pe rform ing the short circuit te st of a transform e r

(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 59. In pe rform ing ope n circuit te st of a transform e r

(a) High voltage is usually kept open (b) Low voltage is usually kept open (c) Any side can be kept open

(d) None of the above

60. Transform e r core usually use s grain-orie nte d lam inate d she e ts. The grain orie ntation re duce s

(a) Copper loss

(b) Eddy current loss

(c) Hysteresis loss (d) None of the above 61. In the circuit m ode l of a transform e r, the core loss is re pre se nte d as

(a) Series resistance (b) Series inductance (c) Shunt resistance (d) Shunt inductance 62. W hile pe rform ing short circuit te st on a transform e r, the im pre sse d voltage m agnitude is k e pt constant but the fre que ncy is incre ase d. The short circuit curre nt will

(a) Increase1

(b) Decrease

(c) Remain the same (d) None of the above 63. A transform e r has a hyste re sis loss of 30 W at 240 V, 60 Hz. The hyste re sis loss at 200 V, 50 Hz will be

(a) 20.8 W (b) 25 W (c) 30 W (d) 36 W 64. In a transform e r, the e x citing curre nt will be in phase quadrature with the im pre sse d voltage provide d

(a) Only the leakage impedance drop is ignored (b) Only the core loss is ignored (c) Both the leakage and impedance drop and the core loss are ignored (d) Only no-load copper loss is ignored 65. Magne tizing im pe dance of a transform e r is de te rm ine d by

(a) Short circuit test (b) Open circuit test (c) Both open circuit and short circuit tests (d) None of these 66. A transform e r is to be te ste d at full-load conditions consum ing only losse s from the m ains. W e do

(a) Load test

(b) Open circuit and short circuit tests

(c) Back-to-back test (d) None of these 67. In a transform e r ope rating at constant voltage if the input fre que ncy incre ase s the core loss will

(a) Increase

(b) Decrease

(c) Remain constant (d) Increase as the square of the frequency 68. Grain-orie nte d lam inate d she e t ste e l in a transform e r re duce s http://my.safaribooksonline.com/book/electrical-engineering/9788131760901/1-transformers/navpoint-99

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Electrical Machines, Second Edition > 1 Transformers > MULTIPLE-CHOICE QUESTIONS AND ANSWERS : Safari Books Online

(a) Copper loss

(b) Eddy current loss

(c) Hysteresis loss (d) None of above 69. R1 is the re sistance of the prim ary winding of a transform e r. The turns ratio in te rm s of prim ary to se condary is a. The e quivale nt re sistance of the prim ary re fe rre d to as se condary is

(a) R1/a (b) a2 R1 (c) R1/a2 (d) None of these 70. C hoose the corre ct state m e nt:

(a) emf per turn in high-voltage winding is more than the emf per turn in low-voltage winding (b) emf per turn in both the finding are equal (c) emf per turn in both the windings are not equal (d) None of these 71. If E2 and V2 be the induce d e m f in se condary winding and V2 be the te rm inal voltage during load, the re gulation of the transform e r is give n by

(a)

(b)

(c)

(d)

72. O ne transform e r has le ak age im pe dance of 1 + j 4 Ω and 3 + j 4 Ω for its prim ary and se condary windings, re spe ctive ly. The transform e r has

(a) Low voltage primary

(b) High voltage primary

(c) Medium voltage primary (d) Insufficient data 73. A single -phase transform e r has its m ax im um e fficie ncy at 60 pe r ce nt of full load. At full load coppe r loss will be

(a) Equal to core loss

(b) Less than core loss

(c) More than core loss (d) Zero 74. The flux in a m agne tic core is alte rnating sinusoidally at a fre que ncy of 600 Hz. The m ax im um flux de nsity is 2 T and e ddy curre nt loss is 15 W . If the fre que ncy is raise d to 800 Hz and m ax im um flux de nsity re duce d to 1.5 T, the e ddy curre nt loss will be

(a) Same

(b) Reduced to half

(c) Doubled (d) Increased by 50% 75. The core flux in a transform e r de pe nds m ainly on

(a) Supply voltage (b) Supply voltage and frequency (c) Supply voltage, frequency and load (d) Supply voltage and load but independent of frequency A nswers

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