1) What is a Transformer ?
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 iscaused 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.