A transformer cannot change the voltage level of direct current to transmit dc energy. When the transformer changes the ac voltage, the current flowing through the transformer will generate an induced magnetic field in the transformer that the magnetic field is alternating. This induce the same alternating voltage on the transformer’s secondary coil. Although direct current induces a magnetic field, it is a constant field and cannot induce direct current voltage on the secondary side of the transformer. In addition, due to the constant magnetic field that cannot generate induced electromotive force on the primary side of the transformer.
According to the principle of electromagnetic induction, alternating current generates alternating magnetic field, and the change of magnetic field generates induced current in secondary coil. If the electromagnetic field generated by tributary electricity is constant, secondary coil cannot generate induced current. The principle of electromagnetic induction tells us that changing magnetic fields produce electric fields. Alternating current can produce a changing magnetic field, and a changing magnetic field can produce an electric field, while a direct current’s magnetic field cannot change without changing.