The Voltech Handbook of Transformer Testing

1. Transformer Basics

1.1 Basic Transformer Theory

Figure 1 represents the essential elements for a transformer - a magnetic core, with a primary and secondary coil wound on the limbs of the magnetic core.

Figure 1. A Basic Transformer

An alternating voltage (V_p) applied to the PRIMARY creates an alternating current (I_p) through the primary.

This current produces an alternating magnetic flux in the magnetic core.

This alternating magnetic flux induces a voltage in each turn of the primary and in each turn of the SECONDARY.

As the flux is a constant e.g. the same in both primary and secondary: -
V_p = const N_p
V_s = const N_s

This equation shows that a transformer can be used to step up or step down an ac voltage by controlling the ratio of primary to secondary turns. (Voltage transformer action).

It can also be shown that: -

Primary VoltAmperes = Secondary VoltAmperes

V_pI_p = V_sI_s

This equation shows that a transformer can be used to step up or step down an ac current by controlling the ratio of primary to secondary turns. (Current transformer action)

It will be noted that there is no electrical connection between the primary and secondary windings.

A transformer therefore provides a means of isolating one electrical circuit from another.

These features - voltage/current transformation and isolation, cannot be obtained efficiently by any other means, with the result that transformers are used in almost every piece of electrical and electronic equipment in the world.