# Transformer Questions and Answers Interview Viva

What is the operating principle of a Transformer?

A transformer works based on the principle of mutual induction. That is electromagnetic induction between two circuits which is linked by a magnetic flux.

What are the two types of transformers based on the manner in which the primary and secondary coils are placed on the core?

1. Core-type transformer.
2. Shell-type transformer.
1. 2. What are the few main differences between the shell type and core type transformer?

In a core-type transformer, the core is surrounded by the winding. The cores used are L-L lamination. It has only one magnetic flux path. Flux leakage is high.

In a shell-type transformer, the winding is surrounded by the core. E-I lamination cores are used. It has two magnetic flux paths. Low flux leakage.

What is the transformation ratio of a transformer?

Transformation Ratio K is defined as the ratio of the EMF in the secondary winding to that in the primary winding.

Ep or VP – Primary Voltage

Es or VS – Secondary voltage

IP – Primary current

IS – Secondary current

Transformation ratio, K = VS/VP = IP/IS

The current is the inverse ratio of the voltage transformation ratio, which is 1/k.

What is the formula for calculating transformer winding turns ratio?

NP – Number of turns in the primary winding

NS – Number of turns in the secondary winding

f- Frequency

Φm- Maximum value of flux

Equation for induced emf, E = 4.44* Φm *f * N

K = VS/VP = Es/Ep = 4.44* Φm *f * NS / 4.44* Φm *f * NP

K = VS/VP = NS / NP

Hence, the transformation ratio k is also the ratio of the number of turns in the secondary winding to the number of turns in the primary winding.

What are the two types of power losses in a transformer?

Power loss in transformers can be categorized into two main types: copper losses and iron losses.

What is copper loss?

Copper loss is due to the power dissipation as heat in the primary and secondary windings due to the winding resistance.

What is Iron loss?

Iron loss or core loss occurs in the core of the transformer due to alternating flux in the core. It consists of eddy current loss and hysteresis loss which depends upon the core material, supply frequency, and flux density.

What is hysteresis loss?

Hysteresis loss is caused due to magnetization and demagnetization in the transformer core. During the reversal of the magnetic field, some amount of energy is lost in the form of heat in the core, called hysteresis loss.

What is eddy current loss?

The magnetic flux produces an induced EMF in the transformer core which forms small circulating currents. Some amount of energy will be dissipated in the form of heat due to these current loops and it is called an eddy current loss.

What is the method used to reduce the eddy current loss?

To reduce eddy current loss, the core resistance needs to be increased to reduce the amount of circulating current. So, in order to increase core resistance, the transformer core will be made of laminated thin sheets of steel.

What are the different methods used for transformer cooling?

Air Cooling – Air cooling methods are used for dry-type transformers, it can be either air natural or forced. Natural Air cooling is where the air carries the heat produced by the transformer. A forced-air also is used for better heat transfer.

Oil cooling – In an oil-cooled transformer, the primary and secondary windings of the transformer are immersed in oil. Oil transfers the heat from the winding to the surface or fins and it also provides electrical insulation between windings. Both air and oil could be natural or forced type.

Oil-immersed Water Cooling – In this method, water is used to dissipate heat from the oil through heat exchangers.

Why power transformer windings are immersed in oil?

Power transformers are often designed with windings (the coil of wire around the core) immersed in oil. There are several reasons for this:

1. Cooling: The oil helps to dissipate the heat generated by the transformer, which is important for preventing damage to the windings and improving the transformer’s overall efficiency.
2. Insulation: The oil helps to insulate the windings from one another, which is important for preventing short circuits and ensuring that the transformer operates correctly.
3. Protection: The oil helps to protect the windings from damage, such as from moisture, dust, or other contaminants.
4. Stability: The oil helps to keep the windings in place, which is important for maintaining the transformer’s mechanical stability.

Overall, the use of oil in power transformers helps to improve their performance and reliability, making them more suitable for use in a wide range of applications.

What is an autotransformer?

It is a type of transformer with a single winding on a core. That is the part of the winding that is shared and common to both primary and secondary winding. The primary voltage will be connected across the two end terminals of the total winding. And the secondary winding is actually connected across a point tapped between the single winding and one of the end terminals, which is connected to the neutral. This is the case of a step-down autotransformer, for a step-up just the opposite the primary would be across the tapped side.

What are the instrument transformers?

Instrument transformers are high-accuracy transformers that are used in electrical power system used to transform voltage or current for measuring electrical quantities, operate instruments, provide isolation and protection, etc…

What are a current transformer and a potential transformer?

A current transformer (CT) or Potential transformer (PT) is a type of transformer that scales the larger value of an electrical quantity into small values that can be used to measure in its secondary. A CT produces current and PT produces a voltage at its secondary which is proportional to the current and voltage in its primary. It converts the actual supply to a measurable scale that is suitable for the instrument (voltmeter, Ammeter, …) ratings.

Define the Impedance ratio of a transformer?

It is the ratio of the secondary impedance Zs to that of the primary impedance Zp, that is Zs/Zp.

The impedance of the primary winding, Zp = Vp/Ip

Secondary winding impedance, Zs = Vs/Is

Impedance Ration Zs / Zp = (VS / Vp ) * ( Ip / Is ) = K * K

Zs / Zp =K2

From the above equation, it is clear that the impedance ratio is the square of the voltage transformation ratio.

Why the transformer is rated in kVA, not in kW?

The transformer is rated in kVA is due to the reasons like power loss, load type, relation to the amount of heating, etc. The core loss of a transformer depends on the voltage and copper loss depends on the current. The total loss is independent of the power factor of the load and it depends only on the Volt-Ampere (VA).

What are the open circuit and short circuit tests?

Open circuit test – In this test, the High Voltage (HV) side is kept open, and the voltage is applied to the Low Voltage (LV) side of the transformer. The instruments connected to the primary side measure the voltage by a voltmeter, no load current by an ammeter, and no-load power by a wattmeter. It is conducted to determine no-load parameters such as no-load current and losses (iron loss) of the transformer.

Short circuit test – Here the LV side is short-circuited and the voltage is applied to the HV side. The voltmeter, ammeter, and wattmeter connected to the HV side measure the electrical quantities in full load condition. It helps to determine the copper loss, winding resistance, impedance, reactance, etc…

Why is in the open circuit test the HV side open and in the short circuit test the LV side is shorted?

In an open circuit test if the LV side is kept open and measurements are conducted on the HV side the no-load current will be inconveniently small and the voltage becomes extremely large which is not within a suitable range for measurements.
If the HV side is short-circuited in a short circuit test the primary current becomes extremely large and the voltage becomes low, which is also inconvenient to measure.

What is an ideal transformer?

An ideal transformer is a theoretical transformer with Zero loss. That is no copper loss which is zero winding resistance, no core loss, no leakage flux, etc… Ideally, it gives 100 percent efficiency.

Refer for more about: Ideal transformer

What Causes the buzzing or Humming Noise in transformers?

It is due to Magnetostriction, a property of ferromagnetic materials which causes them to expand or contract in response to a magnetic field that results in a humming sound.

### 11 Responses

1. Nishant says:

why does a transformer make a buzzing noise or humming sound and how to reduce that?

• Stephen D'cruz says:

It is due to phenomenon called magnetostriction. It is the property of Ferromagnetic materials or the iron core that are exposed to alternating magnetic field causes expansion and contraction which produces an audible humming or buzzing sound.

This cannot be eliminated completely but can be controlled by the proper transformer design and installation.

• gil says:

Magnetostriction.

2. hitesh says:

Nice info! thanks for sharing

• Aafhia says:

Thank you so much

What is step-up transformer?

A step-up transformer increases the output voltage, where a step-down transformer decreases the output voltage.
The secondary voltage of a step-up transformer will be greater than its primary voltage (V2 > V1), hence the number of winding of the secondary side need to have a greater value than the primary side proportional to the voltage ratio (N2 > N1).

4. Vikas Dev Pandey says:

My college viva done basis of your blog thanku so much bro