Interview questons and answers (sai saikumar jn)

 Previous Year Questions on Transformers 


[1] Why transformers rated in KVA instead of KW?
The copper loss (I2R) in the transformer depends on the current I through the winding while the iron or core loss depends on the voltage V as frequency of operation is constant. None of these losses depend on the power factor (cosφ) of the load. Hence losses decide the temperature rise and hence the rating of the transformer. As losses depends on V and I only, the rating of the transformer is specified as a product of these two parameters VxI. Thus the transformer rating is in kVA and not in kW.

[2] Why is the efficiency of transformers more than that of other rotating machines?
There are no moving parts in transformer hence the friction and mechanical losses are absent in transformer. Hence efficiency of the transformer is more than of other rotating machines.

[3] Define leakage inductance
Inductance offered by the winding due to the leakage flux associated with it is called leakage inductance. It is the ratio of leakage flux linkages with the winding to the current passing through the winding.

[4] What material is used in the transformer core?
High grade silicon steel laminations are used for the construction of the core.

[5] What is meant by leakage flux?
Part of the primary flux as well as the secondary flux completes the path through air and links with the respecting winding only. Such a flux is called leakage flux. Leakage fluxes link with the respective windings only and not to both the windings.

[6] Define voltage regulation of a transformer?
The regulation is defined as change in the magnitude of the secondary terminal voltage, when full load ie, rating load of specified power factor supplied at rated voltage is reduced to no load, with primary voltage maintained constant expressed as the percentage of the rated terminal voltage.

[7] What is an ideal transformer?
A transformer is said to be ideal if it satisfies following properties:

1. It has no losses
2. Its windings have zero resistance
3. Leakage flux is zero ie, 100% flux produced by primary links with the secondary
4. Permeability of core is so high that negligible current is required to establish the flux in it.