Electrical Power Systems Technology

Electrical Power Distribution Systems 219

Step-Up and Step-Down Transformers

Transformers are functionally classified as step-up or step-down types.

These types are illustrated in Figure 8-10. The step-up transformer in Figure

8-10 has fewer turns of wire on the primary than on the secondary. If the

primary winding has 50 turns of wire and the secondary has 500 turns,

a turns ratio of 1:10 is developed. Therefore, if 12-volts AC is applied to

the primary from the source, 10 times that voltage, or 120 volts ac, will be

transferred to the secondary load (assuming no losses).

The example in Figure 8-10B is a step-down transformer. The step-

down transformer has more turns of wire on the primary than on the sec-

ondary. The primary winding of the example has 200 turns, while the sec-

ondary winding has 100 turns—or a 2:1 ratio. If 120-volts AC are applied

to the primary from the source, then one-half that amount, or 60-volts AC,

will be transferred to the secondary load.

Transformer Voltage and Current Relationships

In the preceding examples, a direct relationship is shown between

the primary and secondary turns and the voltages across each winding.

This relationship may be expressed as:

VP NP

— = —

VS NS

Figure 8-10. Step-up and step-down transformers

Figure 8-10. Step-up and step-down transformers