Electrical Power Systems Technology

Electrical Power Distribution Systems 221

PP = 15,550 watts

The losses are those that ordinarily occur in a transformer. In trans-

former theory, an ideal device is usually assumed, and losses are not con-

sidered. Thus, since PP = PS, and P = V × I, then

VP × IP = VS × IS

where:

IP = the primary current, and

IS = the secondary current.

Therefore, if the voltage across the secondary is stepped up to twice

the voltage across the primary, then the secondary current will be stepped

down to one-half the primary current. The current relationship of a trans-

former is thus expressed as:

IP NS

— = —

IS NP

Note that whereas the voltage-turns ratio is a direct relationship, the

current ratio is an inverse relationship.

Sample Problem:

Given: an ideal transformer circuit has the following values:

VP =600V

VS =2400V

IS =80A

Find: the primary current drawn by the step-up transformer

Solution:

IP NS

— = —

IS NP

IP 2400 V

—— = ——

80 A 600 V

IP = 3,200 A

Multiple Secondary Transformers

It is also possible to construct a transformer that has multiple second-

ary windings, as shown in Figure 8-11. This transformer is connected to a