Transformer Interview Questions


In this article, we will look at some transformer interview questions & Answers which will helps you to know some more about transformers. Lets take a look at it.

Q : 1  What are Ferromagnetic Materials ?

Ferromagnetic materials have strong magnetic properties due to which these materials form a very strong magnetic field when subjected to an external magnetic field. Moreover, these materials remain magnetize for some time even after the external magnetizing field is being removed. Ferromagnetic materials are used as core of transformer. Iron is ferromagnetic material.

Q : 2  What is the main purpose of using an iron core in a transformer ?

Power from primary to secondary circuit in a transformer is transferred through magnetic coupling which is provided by the core. The main purpose of using an iron core is to provide low reluctance path to the magnetic flux that links from primary to secondary.

Q : 3  Does the transformer draw any current when its secondary is open and why ?

When no load is connected to the secondary of transformer, current drawn by the transformer from the primary side should be zero. However, this doesn’t happen practically. A small amount of current is always required to setup the required magnetic flux in the core of transformer. Hence no load current which is about 3- 5% of the full load current will always flow from the primary side in order to magnetize the core of transformer even at no load.

Q: 4  Why the tappings are provided on high voltage winding ?

Tappings in transformer are generally provided on the high voltage side due to the following reasons :

HV side is generally wound over the low voltage for insulation purpose and it is outside the transformer. Hence it is easy to place the tappings at high voltage side as compared to the low voltage side.

More number of turns on the high voltage side gives an opportunity to place large number of tap changer contacts. In this way, we can regulate the voltage even with a small difference. Moreover, there will be low current on the high voltage side which results in a less sparks.

Q : 5  What will happen if DC supply is given to a transformer ?

When DC source is connected to the primary of transformer, there is no change in magnetic flux linking to the coil due to which no emf will be produced. This simply means that high electric current will flow through the primary since there is no induced emf to opposes the supply voltage. As a result, heavy short circuit current overheat the coils and may burn out the insulation and windings of transformer.

It is to be noted that this will happen only when the applied dc voltage is same as that of the peak AC voltage. If the value of applied dc is low, the current in that case is not sufficient to damage the windings of transformer.

Q: 6  What is meant by an Auto Transformer ?

Auto transformer is a kind of electrical transformer where primary and secondary side shares same common winding. It is basically one winding transformer wound on a common laminated core. An  Auto transformer works as a voltage regulator. It is smaller in size as compare to the normal transformer.

Q :7 Why Core loss is constant in a transformer ?

Hysteresis loss and eddy current loss occur in the core of transformer are known as iron or core losses in a transformer. Core losses are affected by maximum magnetic flux density B in the core,  supply frequency F and constant value K. Since transformer is connected to constant frequency F and given constant voltage supply, hence core losses are constant at all load. Moreover, iron or core loss can determined by open circuit test.

 Q :8 What are the losses that change with the load on the transformer ?

There are two major types of losses in a transformer which are termed as :

Iron or Core Losses ( Constant losses answered above)

Copper loss

These losses are proportional to the square of load current. Copper loss formula is represented by Eddy current loss formula. Assuming R to be kept constant. This means if load current is doubled, copper loss in a transformer will become four times. Similarly, copper losses decreases with the decrease of load since load current also becomes less in that case. Hence we can say that copper loss are the losses that changes with the change of load on the transformer. For Full detail of losses in transformer, kindly read the following article :

Types of Power losses in Transformers ↗

Q: 9 What is magnetic permeability ?

Magnetic permeability is the measure of the ability of magnetic material to support the formation of magnetic field within itself.

Q :10 Why is a transformer rated in KVA ?

Ans :  As we know,    KW= KVA Power Factor.

Transformer doesn’t have their own power factor. It is the type of load connected to the transformer which defines the power factor of transformer. Hence transformers operates at a power factor determined by their loads. Since manufacturer doesn’t know what kind of load ( its power factor) is connected to the transformer. Hence transformers are rated in apparent power (KVA) instead of active power ( KW ).

Q: 11  What is Leakage flux in a transformer ?

Flux which only links with one winding is called leakage flux in a transformer. The flux which links with the primary coil is called primary leakage flux whereas the flux which only links with the secondary winding is called secondary leakage flux. Adding both sides of flux will give us total leakage flux in a transformer. Leakage flux depends on load current and it increases with the increase of load current.

Q: 12  Which winding of transformer has more cross sectional area ?

Cross sectional area of winding depend on the amount of carrying current. Since LV( low voltage) winding carries high amount of current, hence low voltage side has more cross sectional area as compared to the HV winding.

Q : 13  What is the common in the two windings of a transformer ?

Two things which are same for primary and secondary of transformer are Ampere turns and volts per turn.

Q : 14  Why open circuit test is performed  on LV side whereas short circuit test is performed at the  HV side of transformer ?

Ans : For that answer, plz read the following article :

Open Circuit and Short Circuit test on Transformer ↗


That’s all. Hope this will helps you.

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