Principle of the transformer

When two wires are near each other, and one of them carries a fluctuating current, a current will be induced in the other wire. This effect is known as electromagnetic induction. All ac transformers work according to the principle of electromagnetic induction. If the first wire carries sine-wave ac of a certain frequency, then the induced current will be sine-wave ac of the same frequency in the second wire.
The closer the two wires are to each other, the greater the induced current will be, for a given current in the first wire. If the wires are wound into coils and placed along a common axis (Fig. 1), the induced current will be greater than if the wires are straight and parallel. Even more coupling, or efficiency of induced-current transfer, is obtained if the two coils are wound one atop the other.
The first coil is called the primary winding, and the second coil is known as the secondary winding. These are often spoken of as simply the primary and secondary.
The induced current creates a voltage across the secondary. In a step-down transformer, the secondary voltage is less than the primary voltage. In a step-up transformer, the secondary voltage is greater than the primary voltage. The primary voltage is abbreviated E_pri, and the secondary voltage is abbreviated E_sec. Unless otherwise stated, effective (rms) voltages are always specified.
The windings of a transformer have inductance because they are coils. The required inductances of the primary and secondary depend on the frequency of operation, and also on the resistive part of the impedance in the circuit. As the frequency increases, the needed inductance decreases. At high-resistive impedances, more inductance is generally needed than at low-resistive impedances

Fig. 1: Magnetic flux between two coils of wire.