# Transformer FAQ and Transformer Interview Questions

## How does a transformer work?

A transformer is an electric machine that converts AC voltage from one level to another level with minimum power loss. It is based on the principle of electromagnetic induction. A typical power transformer consists of two sets of coils – Primary coil and secondary coil. The rate of voltage transformation is proportional to the ratio of the number of turns of primary and secondary coils.

## What is the difference between a power transformer and an instrument transformer?

A power transformer is used either for stepping up or stepping down a voltage level, primarily in power transmission and distribution. An instrument transformer is used in a circuit for the measurement of voltage or current. Current transformers and potential transformers are examples of instrument transformers.

## Why do transformers hum?

Transformer hum or transformer noise is caused due to the repeated expansion and contraction of the steel core inside the transformer. The expansion of core depends on the amount of flux, which in turn depends on the applied voltage and number of turns of transformer coils. The magnetic flux passing through the transformer core varies continuously which expands and contracts the core.

## What is a tap changer?

A tap changer is used to regulate the output voltage of a transformer. The variations in load on the transformer can result in minor variations in the output voltage of a transformer. This can be rectified using a tap changer. The output voltage can be either increased or reduced by a margin of 10% using tap changer.

## Why are transformers rated in KVA?

All transformers are rated in kVA for the following two reasons:

• The transformers do not alter the power factor of the supply. The power factor of the input supply and that of the output are always the same.
• All losses occurring in the transformers are independent of the power factor of the input supply.

Know More: Why is a transformer rated in KVA?

## What is inrush current in a transformer?

When a transformer is turned on for the first time it draws a current several times its rated current. It is instantaneous and last for a few cycles of input supply until the steady-state flux is reached.

## What is core loss in transformers?

Core losses, also known as constant losses are those losses occurring in the laminated steel core of the transformer. These losses do not vary upon load and remain constant for any particular transformer. These losses result from the variation of magnetization of core throughout the cycle of AC supply. Core losses are sometimes referred to as iron losses and two primary kinds of core losses are Hysteresis losses and eddy current losses.

## What are eddy currents?

When a conductor placed in a varying magnetic field, circulating currents are produced in the conductor. These currents are called eddy currents. In transformers, the magnetic field produced by the primary winding induces eddy currents in the transformer core. This heats up the transformer core.

## What is hysteresis loss?

A considerable amount of energy is lost in the transformer core due to hysteresis. This caused due to the lag in a decrease of magnetic flux in the core when the magnetic field decreases when the AC cycle moves from positive maximum to negative maximum of voltage.

## How to minimize core losses?

In order to reduce core losses, transformers are designed using better grade steel (normally Cold Rolled  Grain Oriented –CRCO). Also using laminated cores especially, thinner core laminations can help in achieving better transformer efficiency by reducing core losses.