Transformer on DC

In the introductory chapter, we defined an electrical transformer as an AC machine because it works only on alternating current electricity. Therefore, a transformer cannot change (increase or decrease) the value of the DC voltage. In this chapter, we shall know the reason, why a transform does not work on the direct current (DC).

Consider an electrical transformer as shown in Figure-1, and it is connected to a battery (or a source of DC voltage) V. When, we apply this DC voltage V to the primary winding of the transformer, it will draw a constant current (DC) and therefore produces a constant magnetic flux flowing through the magnetic core.

According to the principle of electromagnetic induction, an EMF can induce in a coil or conductor only when it is subjected to a changing magnetic field, i.e.,

$$\mathrm{\mathit{e}\:=\:\mathit{N}\frac{\mathit{d\phi }}{\mathit{dt}}}$$

Consequently, the applied DC voltage to the primary winding does not induce EMF in the primary winding or secondary winding. Hence, this discussion proves that a transformer does not work on DC supply. In fact, connecting a DC supply to the primary winding of a transformer could be dangerous.

The equivalent primary winding circuit of a transformer connected to the DC voltage is shown in Figure-2. In this case, there is no self-induced EMF in the primary winding to oppose the applied voltage V (according to Lenz’s law), and the current in the primary winding is given by,

$$\mathrm{\mathit{I_{\mathrm{1}}}\:=\:\frac{\mathit{V}}{\mathit{R_{\mathrm{1}}}}}$$

Where, $\mathit{R_{\mathrm{1}}}$ is the resistance of the primary winding. Due to very small value of R₁, the current $\mathit{I_{\mathrm{1}}}$ through the primary winding will be very large. This large current will cause the overheating and burning of the transformer or fuses will blow. Therefore, we must not connect the primary winding of a transformer to the DC supply because it may damage the transformer or may cause an electrical accident.