Transformer Interview Questions and Answers (2)

Transformers Interview Questions:

What is excitation current or magnetising current in Transformer?

Answer: The no load primary current provided to the transformer is called magnetising current or excitation current. This current is used to produce the flux in the transformer core and overcome the no load losses (core losses) of the transformer. This excitation current will be of the range from 2 to 5 percent of the rated full load primary current.

Why power factor of the transformer under no load is very low?

Answer: During no load condition of transformer, the excitation or magnetizing current flow in the primary winding of the transformer. This excitation current is made up of large component of magnetizing component of current (Im) which is in quadrature with the applied voltage and comparatively small in phase component of current (Ie) which is in phase with the applied voltage. Thus due to large magnetizing component, power factor of the transformer will be very low. Power factor will be usually 0.1 to 0.2 lag.

What is the relation between applied voltage and emf induced with respect to flux wave?

Answer: Under no load condition and considering no losses

• Induced emf in both primary and secondary lags behind the main flux by 90^o.
• Applied voltage to primary winding leads the main flux by  90^o and is in phase opposition with the induced emf in the primary winding (V1= -e1)
• Secondary voltage V2=e2 as there is no voltage drop in the secondary.

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What are the different losses of Transformer?

Answer: Transformer has two types of losses 1. Iron losses 2. Copper losses

Iron losses or core losses: Iron losses are the no load losses in transformer. This losses remain unchanged at any load of the transformer. Iron losses are categorized as Hysteresis loss and Eddy current loss.

Copper loss: This loss is due to the I²R loss in the windings of the transformer. This is a variable loss varies with the load on the transformer.

Other losses include stray load losses and dielectric losses.

If supply frequency f transformer is made double keeping voltage unchanged what happens?

Answer: Hysteresis loss will decrease and eddy current loss remains unchanged.