How does a Transformer Work?

Introduction

When an electric current flows through a copper wire it generates a magnetic field.

When the wire is wound around a core of magnetic material, flux is induced in the magnetic material in proportion to the size of the voltage applied and the number of turns. If a second wire is wound around the same core a voltage will be induced across the ends of this secondary coil.

Magnetic core Transformers work well with alternating current due to the nature of their magnetisation. The 'Hysterisis Loop' of a magnetic core is a plot of flux density versus magnetisation force and shows that it is possible to magnatise a core until its saturation, if the current is then reversed the energy is efficiently used to reach the opposite saturation point. (pushing a core beyond saturation only increases losses).

Knowledge of core satuartion and hysterisis allows a designer to select a suitable core for varying applications, selection is dependent on frequency, maximum flux required and allowable losses.

Flux

Flux is measured in Tesla (T) and the Symbol for Flux Density is B.

Since B is directly proportional to the voltage, it can be seen that by varying the turns on the secondary coil the voltage output of a transformer can be selected.

The transfer of power from the primary to secondary coils involves losses. These losses are made up of copper losses in the primary and secondary coils due to their resistance and impedance, in addition to Core losses related to the permeability and eddie currents. Well designed transformers operate above 90% efficiency. Since the losses are converted mainly to heat (some sound and vibration) the transformer design has a thermal limit.