Transformer Interview Question Answer - 2

Why the secondary amp – turns are called as demagnetizing ampere - turns in the transformer?

Demagnetizing ampere turns

When the current is passing through secondary winding of the transformer, secondary flux is produced which oppose the main flux therefore the main flux is reduced. As the secondary amp – turns reduces the main flux, it is called as demagnetizing ampere – turns.

Why core losses are constant in the transformer under all load conditions?

Core losses

The core losses depend upon the supply voltage. As the supply voltage remains constant, the core loss remains constant in spite of any load conditions.

What is meaning of the leakage flux?

Leakage flux

The flux which does not follow its main path is known as leakage flux.

            OR

The flux which completes its path through air is called as leakage flux.

Describe the effect of load current on the leakage flux.

Effect of load current on leakage flux

The primary and secondary ampere – turns is small at no load condition hence leakage fluxes are negligible. The leakage fluxes associated with primary and secondary current increases as the load current is increased.

What is approximately percentage leakage reactance drop of the supply voltage in the transformer?

The percentage leakage reactance drop is approximately 1 to 2% of the supply voltage in the transformer.

Which parameters can find out from open circuit test and short circuit test?

The following parameters can find out from

Open circuit test :

No load power factor cos Ф_o

Iron loss W_i

Magnetizing current I_µ

Working component of the no load current I_w

No load reactance X_o and resistance R_o

Short circuit test

Copper loss at full load as well as other load condition

Equivalent impedance Z₀₁ or Z₀₂

Which winding is generally kept open in the open circuit test in the transformer?

The high voltage winding is generally kept open in the open circuit test in the transformer.

Which loss is neglected in the open circuit test of the transformer?

Open circuit test

The normal (or name plate) voltage is applied to the primary of the transformer during open circuit test. This will set up normal flux in the core of the transformer hence the wattmeter indicates total iron loss.

However, the no load current is very small (1 to 5% of full load current) therefore copper loss is negligible.

How to find out voltage transformation ratio by performing open circuit test in the transformer?

Open circuit test

The high resistance voltmeter is connected across secondary winding of the transformer. The voltage transformation ratio of the transformer is finding out by measuring the voltage across primary and secondary winding.

Which winding is generally short circuited in the short circuit test in the transformer?

The low voltage winding is generally short-circuited test in the transformer and the supply is given to the high voltage winding.

What should be applied voltage to the transformer winding during short circuit test?

The voltage is applied to the primary of the transformer in the short circuit test such that full load current flows in the either primary or secondary winding. This voltage is generally 5 to 10% of the supply voltage.

Why the rating of the transformer is given in kVA?

Rating of transformer

The copper loss in the transformer depends upon current and iron loss depends upon voltage therefore total loss in the transformer depends upon the product of VA therefore the rating of the transformer is given in kVA. The manufacturer of the transformer does not know the power factor of the load therefore the rating of the transformer is given in the kVA not in KW (kVA × Power factor).

Why do you mean by ‘voltage regulation’? Describe its significance.

Voltage regulation

The voltage regulation of the transformer is defined as difference between no load voltage to the full load voltage as a percentage of no-load voltage.                         

% Voltage regulation = ( Vo – V_f ) / V_o × 100%

                Where V_o = No load voltage

                                             V_f = Full load voltage

The voltage regulation indicates percentage of voltage drop at full load condition. Lower value of the voltage regulation indicates better transformer than that of higher value of the voltage regulation. Higher value of voltage regulation means lower value of short circuit current and vice versa.

Explain the terms: Per unit resistance and Per unit reactance

Per unit resistance

It is resistance drop with respect to primary / secondary voltage when full load current flows through it.

           %R = I₁R₀₁ / V₁   

                  = I₂R₀₂ / V₂ 

Per unit reactance

It is reactance drop with respect to primary / secondary voltage when full load current flows through it.

           %X = I₁X₀₁ / V₁   

                  = I₂X₀₂ / V₂ 

At what value of power factor, the voltage regulation of the transformer becomes minimum?

The voltage regulation of the transformer has

Positive value for lagging power factor

Negative value for leading power factor and

Minimum value at unity power factor

Which parameters can find out by performing back-to-back test in the transformer?

Back-to-back test : Efficiency at any load, Voltage regulation &

Temperature rise

Why efficiency of the transformer is higher than that of rotating machines?

There are no rotating losses in the transformer therefore the efficiency of the transformer is higher than that of the induction motor.

Which parameters greatly affect the value of copper loss? Voltage or current?

Copper loss

The current in the winding greatly affect the value of the copper loss ( Copper loss in the winding = I²r , where I is the current passing through the winding and r is the resistance of the winding. )

What is condition for maximum efficiency in the transformer?

The iron loss is equal to copper loss is the condition for maximum efficiency in the transformer.

Why the distribution transformer is designed for low value of core loss?

Distribution transformer

The distribution transformer primary is supplied throughout the day (24 hours) therefore the iron losses occur throughout the day whereas the copper losses depend upon the duration and amount of load.

Therefore, the design of the distribution transformer is made such that iron losses become minimum.

Define: All day efficiency

All day efficiency

It is defined as the output in the kWh to the input in kWh. The all-day efficiency depends upon the load cycle.

Describe the condition for zero regulation in the transformer.

Condition for zero regulation in the transformer

tan φ = r / x

Where r = Resistance of the winding and

             x = Reactance of the winding

 

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