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Transformer’s interview questions paper-3

34. What are the types of transformers according to transformation?

Ans: (1) Step up transformer & (2) Step down transformer.

35. What are the types of the transformer according to the arrangement of iron cores ?

Ans : there are three types :- (a) Core type (b) Shell type (c) Distributed core or Berry type.

36. What is the difference between a core type and a shell type transformer?

Ans :In core type transformer the primary and secondary winding are placed on each leg having one magnetic circuit while in shell type transformer both the winding are placed on a common central core having two magnetic circuits. In core type transformer the iron loss will be less and copper loss will be more as compared with a shell-type transformer .

37. What magnetic circuit is formed in Berry type construction and why ?

Ans : Distributed magnetic circuits are formed in Berry type construction because of its distributed cores.

38. What is called limb?

Ans: the vertical portion of the iron core where the windings are placed is called limb.

39. Why are the primary and secondary windings placed on each limb in core type transformer ?

Ans: To reduce the leakage flux.

40. Why is L.T winding placed near the cores?

Ans: To reduce the total dielectric strength of the insulating materials provided on the winding.

41. What is meant by leakage reactance of a transformer ?

Ans : Practically the flux of primary winding does not link fully the secondary winding . A portion of primary flux without linking the secondary winding passes through the air path. This flux opposes the main circuit current and behaves in the same way as in inductance . The effect is same as that of a reactance acting in series to the primary circuit .

42. What are the types of transformers according to winding and supply?

Ans : (a) The single phase transformers ,(b) The three phase transformers and (c)The polyphase transformers.

43. What are the types of winding according to the construction?

Ans: Sandwich-type and Cylindrical type.

44. How is the winding named according to the voltage ?

Ans:High voltage winding and Low voltage winding.

45. What is the difference between cylindrical type and sandwich type winding ?

Ans: In cylindrical type winding the length of the coil is equal to the length of the core limb and the primary and secondary windings are placed one over the other placing low voltage winding nearer the core ,while in sandwitch type winding the H.T and L.T winding are placed on limb lengthwise one above the other link the sandwitch.

46. Name the different types of coils used in the transformer winding and state there used.

Ans: There are four types of coils e.g.
(a)Spiral type coil - This consists of layers wound in a continuous length from top to bottom of the coils ,the conductors of which consist of a number of square or rectangular strips in parallel. These coils are only used for low voltage winding carrying a heavy current of more than 100 amps .
(b)Cross over type coil - this type of consists of a number of layers having a number of turns per layer, the conductor being round wire or a strip insulated with a paper covering .Cross over coils are wound on formers . These coils are used for high voltage winding of distribution transformer and current up to about 20 amps .
(c) Helical type coil - This type of coil is wound in the form of a helix and consists of a number of rectangular strip wound in parallel radially so that each separate turn occupies the total radial depth of the winding . These coils are used for lower voltage winding of the larger transformer at voltages 11KV to 33KV . Multilayer helix can be used for higher voltage winding up to 500KV.
(d)Continuous disc-type coil – This coil consists of a number of discs wound continuously from a single wire or a number of strips in parallel . In each disc, a number of turns are wound radially over one another and the conductor passes from disc to disc uninterruptedly . These coils are used for both higher voltage winding from 33KV to 500KV and lower voltage winding from 11KV to 66KV above the rating of 1 MVA.

47. What insulation is largely used in the windings of a high voltage transformer ?

Ans: Varnish coated paper insulation is largely used for the winding of a high voltage transformer .

48. What do you mean by uniform insulation ?

Ans: Where the insulation to earth at all points of a winding can withstand the separate source test voltage appropriate to the line end then it is called as uniform insulation . This is generally used up to maximum system voltage of 72.5 KV (r.m.s).

49. What is meant by graded insulation?

Ans: In high voltage transformers or banks of transformers where the high voltage winding is star connected and permanently and solidly grounded, the insulation to ground is reduced in steps or graded from the high voltage terminal to the grounded terminal .It is called graded insulation and generally used for highest system voltage of 72.5KV (r.m.s) and above.

50. What are the types of transformers according to its location ?

Ans:(1) Indoor type transformer & (2) Outdoor type transformer.

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