# Why does not Transformer Work with DC Supply?

Last Updated on October 31, 2021 by Electricalvolt

Therefore,The transformer functions in the Faraday’s Law of electromagnetic induction. When AC is applied to primary of the transformer EMF induced in the primary which opposes the voltage. The back EMF induced in the primary is;

Ep = – Np dΦ/dt
Ep = 4.44 Φ f Tph

The flux produced in the primary gets linked to the secondary turns of the Transformer and voltage is induced in the secondary.

If DC is fed to transformer, the voltage induced in the primary is;

Ep = – Np dΦ/dt

Rate of change of flux in case of the DC is zero.Therefore,

Ep = – Np dΦ/dt
Ep = – Np x 0
Ep = 0

The current flowing in the primary is;

Ip = (Vp- Ep)/ Zp
Ep = 0  [ In case of DC ]
Ip = (Vp- 0)/ Zp
Ip = Vp/ Zp
The impedance of the primary winding in case of DC supply is applied to primary is;

Zp =√ ( R2p + X2p)

Where, Xp = 2 πf Lp

In case of DC supply, f=0
Xp = 2 πf Lp
Xp = 2 πx 0 xLp
Xp =0

Therefore,

Zp =√ ( R2p + X2p)
Zp =√ ( R2p + 0)
Zp =√  R2p
Zp =Rp

The current flowing through primary of the transformer is;

Ip = Vp/ Rp

The current through the primary winding will be very high if DC voltage is applied to the primary of the transformer. The followings are the reasons of high current flowing in the primary.

The back EMF(Ep) induced in the primary is zero therefore there is no limiting back EMF in the primary of the transformer. The second reason is that the coil of the primary does not have impedance rather than it has resistance only.The resistance of the primary winding is very low. Therefore, the large current flow through the primary and the winding gets overheated resulting into insulation failure and that leads to short circuit.

Therefore, the DC supply should not be fed to transformer for its operation.