Transformer FAQ | Interview Questions- Part -1

What is a transformer?

By definition, a transformer is a static electrical device that transforms electric energy from one level to another level. Normally transformers are used to step up or step down voltage and current levels, impendence matching and providing electrical isolation

What are the losses occurring in a transformer?

The losses occurring in a transformer, by its nature can be classified into the following types:

  • Core loss
  • Copper loss
  • Dielectric loss and
  • Stray losses.

Copper loss is caused due to the resistance offered by the conductors used in the transformer windings against the flow of current through it and the core loss is due to the eddy currents and hysteresis set up on the transformer core by the magnetic flux. Core loss is always constant.

What are the tests conducted on a transformer?

The tests conducted on a transformer are

a) Open circuit test/ no-load test- the secondary terminals of the transformer are left open-circuited.

b) Short circuit test (secondary terminals of the transformer are short-circuited).

c) Sumpner’s test.

What is the purpose of conducting open circuit test on a transformer?

The purposes of conduction the open circuit test on the transformer is:

a) In order to determine the constant losses (core losses)

b) Magnetizing component Im and core loss component I0 can also be determined by performing an open circuit test on transformers.

c) In order to determine the shunt branch parameters of the equivalent circuit (conductance and susceptance).

What is the purpose of conducting open circuit test on a transformer?

The purpose of conducting a short circuit test on transformer is:

a) To determine the full load copper loss (variable loss) of the transformer.

b) To determine equivalent resistance and reactance of the transformer.

What is the purpose of conducting sumpner’s test on a transformer?

Sumpner’s test is conducted on transformers to determine the steady-state temperature rise in it when it is operated continuously for a long time. This test requires two identical transformers that are connected back to back. Any other transformer parameters cannot be determined by using this test. This test is also called “Heat run test”.

Why are transformers rated in KVA?

The losses occurring in the transformer (core loss and copper loss) are independent of its power factor. Hence it is rated in KVA and not in KW.

What is magnetizing current?

Magnetizing current is the constant current drawn by the transformers from the supply in order to produce magnetic flux. Magnetizing current is always constant for a given transformer irrespective of the nature of load it is supplying. It remains constant from no load to full load.

When can we obtain the maximum efficiency from a transformer?

The transformer can supply at its maximum efficiency when its core loss is equal to its copper loss.

What are the stray losses?

Stray losses result from the eddy currents set up by the leakage flux in the transformer walls and conductors.

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