# Transformer Interview Questions with Answer

In this article, we are going to discuss some important Transformer interview questions and their most appropriate answers. If you are an electrical engineer and preparing for job interviews in core electrical companies then these questions are very helpful to you. Just go through it and revise your knowledge of the transformer.

If we talk about transformer then we feel that it is the backbone of electrical engineering. A transformer is the most important machine for electrical engineering. Without it, we can’t imagine electrical engineering.

## What is a Transformer?

A transformer is a static electrical machine that transfers electrical power from one electrical circuit to another without changing its frequency. It works on the electromagnetic induction principle.

## What is the working principle of a transformer?

Transformer works on the principle of Faraday’s law of electromagnetism and mutual induction. When primary winding is energized by supply voltage then corresponding secondary voltage will be induced in the secondary winding of a transformer which is electrically isolated and linked magnetically with the primary winding.

## What is the function of a Transformer?

The main function of a Transformer is to increase (step up) or decrease (step down) the voltage level keeping power and frequency constant.

## Can we use a transformer in the DC system?

No, Transformers do not work on DC supply. Because when a DC voltage is applied to the primary transformer. Then primary winding of the transformer will draw a constant current, hence magnetic flux produced will be constant. As a result, there will be no mutual induction between primary and secondary winding, and hence no EMF will induced in the secondary of transformer. Also due to DC voltage in primary, primary winding drow heavy current and hence due to low impedance (only resistance) of windings, it will be burnt out. So if we apply DC supply to transformer then it will not work and its winding will burn out.

## What is the name of various types of transformers?

A transformer is categorized into various types depending on different aspects as mentioned below.

Based on the supply system

• Single-phase transformer
• Three-phase transformer
Based on the winding structure
• Auto-transformer ( single winding )
• Two winding transformer
• Three winding transformer
• Six winding transformer
Based on construction and design
• Core type transformer
• Shell type transformer
Based on service
• Distribution transformer
• Power transformer
Based on measurements
• Current transformer (CT )
• Voltage transformer OR potential transformer ( PT )
• Capacitive voltage transformer ( CVT )

Based on function

• Step-up transformer
• Step down transformer
• Isolation transformer

Based on the cooling system

• Oil cooled transformer
• Air-cooled transformer

## Where are core-type and shell-type construction suitable for a transformer?

Core-type construction is mostly preferred for high-voltage applications of transformers such as distribution transformers, power transformers, and also in auto-transformer. Whereas shell-type construction is preferred mostly in the case of low-voltage applications like transformers used in electronic circuits, converter circuits, etc.

## What is a distribution transformer?

As the name suggests a Distribution transformer is the step-down high-voltage transformer used for the distribution of electrical energy of voltage up to 33kV at the distribution substation. It is a small-sized transformer designed for maximum efficiency at 70% loading.

## What is a power transformer?

A power transformer is an extra-high voltage step-up transformer used at the generating station and transmitting power station for transmission of electrical power at a voltage greater than 33 kV. It is a large-sized transformer generally rated in MVA. Its efficiency is high nearly about 98%.

## What is an instrument transformer?

Instrument transformer is used in high-voltage AC systems for the measurement of electrical quantities such as current, voltage, power, frequency, power factor, etc. The most frequently used instrument transformers are the current transformer (CT ) and potential transformer (PT).

## Why transformer is rated in kVA, not in KW?

The rating of an AC machine depends on the losses, and losses depend on voltage and current. In the case of a transformer, both iron losses and copper losses don’t depend on the power factor of the load. Hence the rating of the transformer is also independent of the load power factor. That’s why the transformer is rated in kVA but not in kW.

## What is the all-day efficiency of a transformer?

The term all-day efficiency is defined for distribution transformers. In all electrical machines, We find efficiency but in the case of the distribution transformer, we calculate all-day efficiency because the distribution transformer works for 24 hours at different loading conditions. All-day efficiency is also known as the energy efficiency of the transformer and it is defined as the ratio of total output energy in kWh to the input energy in kWh for a certain period of time generally 24 hours.

## What is a step-up and step-down transformer?

In step-up transformer

Secondary voltage > primary voltage

Turns ratio > 1

Secondary current < Primary current

In step-down transformer

Secondary voltage < primary voltage

Turns ratio < 1

Secondary current > Primary current

## What is an isolation transformer?

As the name suggests it is a transformer used to isolate two electrical circuits. In this transformer, the primary and secondary turns become the same and hence its turn ratio will be equal to 1. It protects the secondary circuit when a fault occurs in the primary circuit.

## What is the Stack factor?

The stack factor is defined as the ratio of the total cross-section of iron to the total cross-section of the core.

## What is an ideal transformer?

An ideal transformer is an imaginary transformer that has the following properties:

• Its primary and secondary winding resistance is negligible.
• The core has infinite permeability so negligible mmf is required to establish the flux in the core.
• It’s leakage flux and leakage inductance is zero
• There are no losses due to resistance, hysteresis, and Eddy current, thus efficiency is 100%.

## Why lamination is used in the core of the transformer?

In order to minimize the Eddy current losses, Coils in the windings of the core of the transformer are laminated with each other.

## How to minimize the hysteresis losses in the transformer?

Hysteresis losses are important part of constant losses in transformers. It occurs all the time even in no load condition. So it is very important that it should be as minimum as possible for better efficiency of the transformer. Hence different techniques are used to minimize the hysteresis losses in the transformer. These techniques are:

• By using good magnetic material (higher permeability ) in the core of the transformer.
• By using less magnetic flux density in the core of the transformer.

## What is the function of no-load current in a transformer.?

The main function of no-load current is to set up working flux in the core and also it is responsible for supplying no-load losses ( Eddy current loss and hysteresis loss ).

## Why iron losses are considered as constant losses in transformers?

Basically iron losses occur in the core of the transformer and It depends on supply frequency and maximum flux density in the core. For all normal operations of the transformer, the supply frequency and maximum flux density more or less remain constant. That’s why iron losses in the transformer are considered as constant losses for all loading conditions.

## Why open circuit ( OC ) test is performed on the LV side of the transformer.?

An open circuit ( OC ) is performed to find out the core losses in the transformer. In this testing, all the measuring equipment is placed on the LV side. So there are two reasons of conducting the OC test on the LV side, these are:

• If LV winding is preferred then the voltage ( rated ) to be supplied is less as compared to the HV side.
• And also when we prefer LV winding for the OC test then a small rating measuring instrument will be required.

## Why SC test is generally performed on the HV side of the transformer.?

In the SC test usually, the LV winding is short-circuited with a thick conductor and all measurements are taken at the HV side. The main reasons behind this are:

• The rated current on the HV side is lower than that of the LV side, and it can be safely measured with the help of an available laboratory ammeter.
• In this case, since the applied voltage is less than 5% of the rated voltage of winding, better accuracy in the reading of the voltmeter is possible when the HV side is used as primary.

## In a transformer, Why is low-voltage winding placed near the core?

In transformer LV and HV windings are sandwiched on the core with the help of proper insulation. When we placed LV winding near the core we found that less insulation is required in between LV and HV winding as compared to another case. 