A transformer is a device that transfers electrical energy from one electric circuit to another, without changing the frequency, by the principles of electromagnetic induction. The energy transfer usually takes place with a change of voltage.

It either increases (steps up) or decreases (steps down) AC voltage.

Step-Up Transformer : The primary winding of a step-up transformer has fewer turns than the secondary winding, with the resultant secondary voltage being higher than the primary.
Step-Down Transformer : The primary winding of a step-down transformer has more turns than the secondary winding, so the secondary voltage is lower than the primary.

Induced Voltage

When an input voltage is applied to the primary winding, alternating current starts to flow in the primary winding. As the current flows, a changing magnetic field is set up in the transformer core. As this magnetic field cuts across the secondary winding, alternating voltage is produced in the secondary winding. In short, a voltage is being induced on the secondary winding.

A transformer does not generate electrical power. It transfers electrical power from one AC circuit to another.

Transformer Ratings

Transformers are frequently rated in kilovoltamperes (kVA).
Very large transformers are often rated in megavoltamperes (MVA)
Very small transformers can be rated in voltamperes (VA).

The rating tells you the maximum current that a transformer can deliver to a load without overheating.

The rating can be calculated if you know the voltage and the current.

The current can be calculated if you know the rating and the voltage.

The rating of a transformer is the same for both the primary and the secondary.

Power is equal to Voltage multiplied by Current.

This is expressed by the formula: P = V x I