Important questions on Transformer?


 • Why transformers are rated in kVA, not in kW?

  Power rating of a transformer or any electric machines indicates the maximum value of voltage and current it can carry without over-heating.

Power  = Voltage(V) × Current(A) × Power factor

  As transformer is consider as a power source while the induction motor is considered as a load.
Manufacturers of Transformers cannot predict the power factor because power factor depends upon the load connected and load may be resistive, capacitive, and inductive.
Since transformer does not depend on power factor, therefore power factor is not consider while designing a transformer.
Power = Voltage(V) × Current(A)

Also, there are two major losses in transformer
1. copper losses
2. iron losses or core losses

Copper losses depends on the current passing through transformer winding while iron losses or core losses depends on voltage. Both losses are independent of power factor, so transformers are designs for rated voltage and rated current.
That’s why the transformer are rated in in kVA, not in kW.

  What are the properties of an ideal transformer?

The efficiency of an ideal transformer is considered as 100%. The properties of an ideal transformer low are
1. no iron losses
2. no winding losses
3. no magnetic leakage
4. zero magnetising current
5. infinite permeability of the core

  What is leakage flux in transformer?

In a transformer main flux links with both primary and secondary winding. Some portion of the main flux generated by primary winding does not links with the secondary winding. This portions of flux which does not do any useful work is known as leakage flux.

 In short, the flux which links only with one side of winding and does not do any useful work is called leakage flux.

  What happen when DC supply is given to a transformer?

Transformer work on the principle of mutual induction. It requires alternative source of supply. When a DC supply is given to transformer, the rate of change of flux is zero which means a constant flux is links with primary and secondary winding. Therefore the output voltage is zero.

Secondly, as the rate of change of flux is zero, there will be no back EMF induced in the primary winding to oppose the applied voltage and since the resistance of primary winding is quite low because inductive reactance (XL) is zero due to the inductive reactance formula ( XL = 2πfl ) where the frequency is zero for DC. Therefore primary will draw Heavy current that may cause burning out of the primary winding.

In short, when DC supply is given to a transformer its secondary output will zero and primary winding may burn out.

 • What is the conditions for maximum efficiency of a transformer?

Before starting it you must remind that there are two main losses in transformer, copper losses and iron losses.

Iron losses are fixed losses and does not change with change in load. But the copper losses are variable losses and increases with increase in laod. At no load condition copper losses are almost negligible. As the load on transformers increases its copper losses also increase. So there will be load at which copper losses becomes equal to variable losses. At that condition when the copper losses become equal to iron loss, the efficiency of Transformer will maximum.

In short, the efficiency of Transformer will maximum when its copper losses are equal to iron losses.

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