A transformer consists of an iron core (or magnetic core) and a coil, which has two or more windings. The winding connected to the power source is called the primary coil, and the remaining windings are called the secondary coils. It can transform AC voltage, current, and impedance. A simple iron core transformer is composed of an iron core made of soft magnetic material and two coils with different turns placed on the iron core. The function of the iron core is to strengthen the magnetic coupling between the two coils. In order to reduce eddy currents and hysteresis losses inside the iron, the iron core is made of coated silicon steel sheets stacked together; There is no electrical connection between the two coils, and the coils are wound with insulated copper wire (or aluminum wire).

One coil connected to an AC power source is called a primary coil (or primary coil), while the other coil connected to an electrical appliance is called a secondary coil (or secondary coil). The actual transformer is very complex, and inevitably there are copper losses (coil resistance heating), iron losses (iron core heating), and magnetic leakage (magnetic induction lines closed by air). The ideal conditions for a transformer to be established are: ignoring the leakage flux, ignoring the resistance of the primary and secondary coils, ignoring the loss of the iron core, and ignoring the no-load current (the current in the original coil when the secondary coil opens). For example, when a power transformer is operating at full load (with the rated output power of the secondary coil), it is close to the ideal transformer situation.