The transformer is a device which transfer electrical energy from one circuit to another without changing the frequency by the principle electromagnetic induction. The transformer has two windings primary winding and secondary winding. These two windings are connected by a common magnetic circuit.

The transformer operates only in AC source. There are two main reasons why transformer works on AC only.

1. When When a AC source is applied to the transformer an alternating flux is produce in the winding of the transformer which links with secondary winding of the transformer and emf is induced in the secondary winding by principle of electromagnetic induction. Thus power is transferred from one circuit to another.

2. The alternating flux generated in the primary winding by AC source, generates the induced emf in primary wind called self-induced emf. The self-induced emf opposes the applied voltage to the primary winding which results in keeping the current in the winding to a safe value.

When transformer is connected to the DC source, two problems are created.

1. The emf in the secondary winding is not generated because the flux produce in by the DC source is not varying but remains constant.

2. In the primary winding due constant magnetic flux self-induced emf is not generated thus there will no opposition to the applied voltage. The resistance of the winding is very low thus heavy current will flow through primary winding which may result in burning of primary winding.This is reason why transformer is not connected to DC Source.

This two points can also mathematically have proven.

1. When DC sources is connected to the primary winding, the flux (Φ) produced is Constant. The emf induced in the winding is

E=n\frac{d\phi }{dt}

where \phi\;\;\; is constant, then

E = N (0) = 0

Thus no induced emf in the secondary winding

2. Suppose we connect transformer to AC source having Primary Voltage( V_{1} ) = 230V Resistance( R_{1} )= 10 Ω, Inductance(L )= 0.4 Source Frequency (*f*) = 50HzThe inductive reactance of the primary winding will be

X_{L }= 2π*f*L

X_{L }= 2 x 3.1415 x 50Hz x 0.4H

X_{L }= 125.67Ω

Now impedance will be

Z=\sqrt{(R_{L}^{2}+X_{L}^{2})^{2}}

Putting the value of R and X_{L } in the above equation

Z = 126.1 Ω

Current in the primary winding will be

I = V/Z = 230/126.1 = 1.82A

Now we connected same rated DC Voltage to the Primary winding of Transformer.For DC Source, Frequency, *f *= 0

Thus

X_{L }= 2π*f*L = 0

And

Z=\sqrt{(R_{L}^{2}+X_{L}^{2})^{2}}

Curren will be

I = V/Z = 230 / 10 = 12.61 A

This current will be 6.9 times than the current when transformer connected in AC Source. This heavy current burning the winding.

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