Transformer interview questions with answer

A transformer is the most important machine for electrical engineering. Without it, we can't imagine electrical engineering.

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In this article we are going to discuss some important Transformer intreview questions and their most appropriate answers. If you are an electrical engineer and preparing for jobs interview 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 other 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.

Can we use a transformer in DC system?

No, we can't use a transformer in DC system. Because as we all know the transformer is a static AC machine which is used to change the voltage and current level without changing its frequency. So if we apply DC supply in the primary winding of the transformer then its winding will burn out.

What is the name of various types of transformer.?

A transformer is categorized in various types depending upon different aspects as mentioned below.

Based on the supply system

  • Single-phase transformer
  • Three-phase transformer

Based on the winding structure

  • Autotransformer ( 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 is suitable for a transformer?

Core type construction is mostly preferred for high voltage application of transformer such as distribution transformer, power transformer, and also in autotransformer.

Whereas shell-type construction is preferred mostly in the case of low voltage applications like transformer used in electronic circuits, converter circuits, etc.

What is a distribution transformer?

As the name suggests a Distribution transformer is 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 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 becomes high nearly about 98%.

What is an instrument transformer?

Instrument transformer is used in high voltage ac system for 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.?

As the rating of AC machine depends on the losses and losses depends 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 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 occurred in the primary circuit.

What is Stack factor.?

Stack factor is defined as the ratio of total cross section of iron to total cross section of core.

What is ideal transformer?

An ideal transformer is an imaginary transformer which has fallowing properties:

  • It's primary and secondary winding resistance are negligible.
  • The core has infinite permeability so that 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 transformer?

Lamination is used in the core of transformer in order to minimize Eddy current loss.

How to minimize the hysteresis losses in transformer.?

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

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

What is function of no load current in 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 transformer?

Basically iron losses occurs in core of transformer and It depends on supply frequency and maximum flux density in core. For all normal operation of transformer, the supply frequency and maximum flux density more or less remains constant. That's why iron losses in the transformer is considered as constant loss for all loading conditions.

Why open circuit ( OC ) test is performed in LV side of transformer.?

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

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

  • Top 20+ Current Transformer interview questions
  • Why SC test is generally performed on HV side of transformer.?

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

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

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

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

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