The transformer can only change AC power, and the change over is still AC power. Because the transformer works according to the principle of electromagnetic induction, it is a process of electricity generating magnetism and magnetism regenerating electricity. The frequency of DC is 0, so DC can not be changed through the transformer to change the voltage value.

The reason is that

The transformer changes over the AC current. When AC current is passed in the primary coil, AC flux is generated in the iron core (or magnetic core) to induce voltage (or current) in the secondary coil.

The transformer consists of an iron core (or magnetic core) and a coil, which has two or more windings, of which the winding connected to the power supply is called the primary coil and the remaining winding is called the secondary coil.

In the generator, whether the coil movement through the magnetic field or magnetic field movement through the fixed coil, can be induced in the coil potential, the two cases, the value of the flux are unchanged, but the number of flux intersecting the chain with the coil has changed, which is the principle of mutual induction. Transformer is a device that uses the electromagnetic mutual induction effect to transform voltage, current and impedance.