The transformers are designed for either single-phase supply or three-phase supply. It may be either step up or step down transformer. However, the working principle of step down transformer and all these transformers are the same and that is electromagnetic induction.

A transformer consists of two highly inductive coils (windings) wound on a steel or iron core. The winding connected to the AC supply is known as primary winding whereas the winding connected to the load is known as secondary winding.

The primary and secondary windings are isolated from each other as well as from the iron core electrically. The electrical power is transferred from primary circuit to secondary circuit by magnetic flux. The symbolic representation of the transformer is shown in the figure.

# Working Principle of Step Down Transformer

• When the primary winding is connected to the AC supply an AC current starts flowing through it.
• The AC current of primary winding produces an alternating flux φ in the core.
• Most of this alternating flux links with secondary winding through the core.
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• This alternating flux induces a voltage into the secondary winding according to the Faraday’s law of electromagnetic induction.
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• The EMF is induced in the secondary winding is due to mutual induction hence it is known as mutually induced EMF.

The induced EMF in the secondary and primary depends upon the rate of change of flux linkages (Ndφ/dt).

The rate of change of flux in secondary and primary circuit is the same. Therefore, induced EMF in secondary is proportional to number of turns of the secondary winding (E2 α N2) and in primary is proportional to number of turns of primary (E1 α N1).

If the secondary turns (N2) are less than primary turns (N1), the secondary induced EMF will be less than primary and transformer is called step down transformer. Whereas if N2 > N1, the secondary induced EMF will be more than primary and transformer is called step up transformer.

A transformer changes only current and voltage levels of AC supply. It does not have any effect on the frequency of AC supply. It can be operated only on AC supply.

If a transformer is connected to the DC supply, a large amount of current will flow through the primary winding and it can damage the transformer winding.

## Transformer | Objective Type Question Answers

### #1 In a transformer, the magnetic coupling between primary and secondary circuit can be increased by

using the magnetic core of low reluctance

### #2 If the flux density in the core of a transformer is increased

size of transformer can be reduced

### #3 At no load, the current taken by a transformer

lags behind the applied voltage by 80 degree

50

400 kVA

### #6 In a power transformer

low voltage winding is closer to core

### #7 If the supply frequency is increased keeping voltage constant

eddy current losses will remain unchanged

### #8 At no load, the current taken by a transformer

lags behind the applied voltage by 80 degree

### #9 Leakage flux of a transformer may be minimized by

sectionalizing and interleaving the primary and the secondary windings

### #11 In a transformer it is difficult to measure the efficiency by output-input measurement method because

efficiency of transformer is usually high and hence extremely accurate measurements will be necessary

0.4 webers/m^2

180 degree

### #15 The desireable properties of transformer core material is

high permeability and low hysteresis loss

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Categories: Transformer