Transformer interview questions paper-1

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In this page , we have dicussed about lots of transformer interview question.All transformer interview question have been collected from exprience Electrical Engineers.There are 13 nos. page of transformer interview question.Hope transformer interview question should be help you for preparing your interview.

1) What is a Transformer ?

Ans : A transformer is a static piece of apparatus which transforms electrical energy from one Circuit to another circuit without changing the frequency either stepping up or stepping down the voltage .

2 .On what theory the principal of operation of a transformer is based ?

Ans : The principle of operation of a transformer is based upon the theory of mutual induction Between two electrical linked by a common magnetic flux .

3 . What do you understand by the rating of a transformer ?

Ans: The rating of a transformer is the maximum power which can be drawn from it without the temperature rise in the winding exceeding the safe limits for the particular class of insulation employed .

4.How is the rated capacity of a transformer expressed and why ?

Ans: The rated capacity of a transformer is expressed in KVA not in KW. Generally the rating of a transformer is determined by its temperature rise . The temperature rise is caused by the losses in the machine . Copper loss depends on the value of the load current and iron loss on voltage . Therefore ,total loss of a transformer depends on volt-ampere (VA) and is independent of load power Factor. A certain amount of current will produce the same I2 R loss at any value of power factor . This loss limits the output of the machine .The output in kilowatts is proportional to the power factor . For a given KW load if the power factor decreases, the load current increases proportionately causing more losses and temperature rise in the machine . For the above reason, transformers are generally rated in KVA, not in KW .

5. What is the power factor of a transformer ?

Ans : The power factor of a transformer is very low and lagging on no load . But the power factor on load is nearly equal to the power factor of the load which it is carrying .

6. What is the normal phase difference between the voltage and the on load current in a transformer ?

Ans :The no load current in a transformer normally lags behind the voltage by about 70.

7.What are the essential parts of a transformer ?

Ans: The essential parts are as follows :- a)Magnetic circuit consisting of laminated iron core and clamping structures. b)Primary winding . c)Secondary winding . d)Tank filled with insulating oil . e)H.T terminals with bushing . f)L.T terminals with bushing . g)Conservator tank. h)Breather. i)Vent pipe . j)WTI ( Winding Temperature Indicator ) & OTI ( Oil Temperature Indicator). k)Radiator.

8.What is the name of the winding to which the supply is given ?

Ans : Primary winding .

9 . What is the name of the winding from which the supply is taken for load connecting ?

Ans: Secondary winding .

10. What material is used for the cores of a transformer and why ?

Ans: Lamination of specially alloyed silicon steel (silicon content 4 to 5%) are used due to its high electrical resistance ,high permeability non-ageing characteristics and minimum iron loss .

11. What is the use of iron core in a transformer ?

Ans :The iron core is used in a transformer to provide a continuous easy magnetic path of low reluctance .

12.How is magnetic leakage reduced ?

Ans: The magnetic leakage is reduced to a minimum by sectionalizing & interleaving the primary and secondary windings .

13. Why should the joints in iron core be staggered ?

Ans: The joints in iron core should be staggered to avoid clear air gap in the magnetic circuit as the air gap reduces the magnetic flux due to its high reluctance .

14.Why are iron cores made laminated in the transformer ?

Ans: Iron cores are made laminated to reduce the eddy current loss .

15. What determines the thickness of the lamina or stamping ?

Ans: Frequency.

16.Why is the lamination insulated from each other ?

Ans: The laminations are insulated from each other by insulating varnish or thin paper to break the path of eddy currents thus to reduce the eddy current loss .

17. What is stacking factor ? what is its approximate value ?

Ans: Stacking factor is the ratio of iron content in the laminated varnished core by volume. Its value is about 90% i.e. 10% volume is occupied by the varnish and air .

18. What is called grain oriented laminations ?

Ans: The grain oriented laminations are such cold rolled laminations specially annealed to orient the iron crystals i.e the grains in a uniform direction in the direction of rolling which produces loss . These laminations are punched and assembled in such a way that the grains of different lamina line in the same direction as the magnetic field and not across it .

19. What is the permissible maximum flux density in transformer core ?

Ans: 1.6 to 1.8 weber per meter .

20. Why is special care taken during tightening of iron cores ?

Ans : To make it free from vibration and humming sound.

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