Transformer Characteristic

Power Transfer in Transformers

In Figure we can now look at  the circuit from the example, redrawn with the calculated voltage and current values included. The power in the primary winding of a transformer transfers to the secondary. This is illustrated in the example below.

Transfer Of Power in Transformer Circuit
Transfer Of Power in Transformer Circuit
Transfer Of Power Formula

Remember that the winding with the higher voltage has the lower current and the winding with the lower voltage has the higher current. In the above example the voltage ratio is 20:1 while the current ratio is 1:20. The secondary winding would need to be wound with wire of a larger cross sectional area than that of the primary winding in order to carry the larger current.

Reflecting on Formulae

Transfer Of Power Formula

Power Rating of Transformers

The power rating of a transformer may be calculated by multiplying the secondary AC voltage by the full load secondary AC current.

Transfer Of Power Formula


A rating quoted in VA will apply to small transformers. The rating of larger transformers will be quoted in kVA or MVA

Transformer Power Rating Formula

Transformer Losses

Iron Losses in a Transformer

Eddy currents are induced into the transformer core. These unwanted eddy currents, cause heating of the core. They represent a loss in the transformer and are referred to as iron losses. The iron core is made up of steel lamination, to reduce eddy currents to a minimum. Silicon steel is generally used as its magnetism is easily reversed.

Copper Losses in a Transformer

Copper losses are due to, current flowing through the resistance of the windings, and are often referred to as the I2R losses ( P = I2 x R ). The greater the current flow through the windings, the greater the copper losses.

Transformer Efficiency

A transformer has no moving parts and is a highly efficient device. Normal transformer efficiency is around 98%. In general, the larger the transformer the higher its efficiency. Efficiency is defined as the ratio of power output to power input.