Transformers

Author: J.B. Hoag

A transformer consists of two coils of wire near each other. An alternating current in one of the coils, the " primary " coil, sets up an alternating magnetic field which cuts the other " secondary " coil, producing an e.m.f. between its terminals. When the secondary is connected to a load resistance, an alternating current will flow.

The coils may have air cores, or they may be wound upon a laminated iron core, as in Fig. 5 D.

Fig. 5 D. A simple transformer circuit

In the case of iron core transformers, the secondary voltage is very nearly equal to the primary voltage multiplied by the ratio of the number of secondary to the number of primary turns. In a step-up transformer, there are more turns on the secondary than on the primary; and the output voltage is correspondingly greater. The reverse is true of a step-down transformer. The power (watts) delivered to the load is about 95 per cent of that taken from the supply line. Since watts = volts · amperes, the secondary-to-primary current is in the inverse ratio to the secondary-to-primary voltage. If the secondary voltage is twice that of the primary, the secondary current will be one-half (or less) that of the primary.

The impedance of the load in the secondary (Z_S = E_s/I_s), divided by the impedance presented by the primary to the supply line (Z_p = Ep/Ip), is nearly equal to the square of the ratio of turns of the secondary (N_s) and primary (N_p). Thus, the impedance ratio is given by

We see, then, that a transformer can be used to transform voltages, currents, and impedances.

There is a proposition general throughout electrical circuits, that maximum power is transferred from a generator to a load when their impedances are equal to each other. If the load's impedance is not equal to that of the source, a transformer can be connected between them to bring about the desired impedance match.