Electrical Power Systems Technology

220 Electrical Power Systems Technology

where:
VP = the voltage across the primary winding,
VS = the voltage across the secondary winding,
NP = the number of turns in the primary winding, and
NS = the number of turns in the secondary winding.

Sample Problem:
Given: a transformer circuit has the following values:
VP = 240 volts
NP = 1000 turns
VS = 120 volts
Find: the number of secondary turns of wire required to accomplish
this step-down of voltage.
Solution:

VP NP
— = —
VS NS

240 V 1000
—— = ——
120 V NS

NS = 500 turns

The transformer is a power-control device; therefore, the following re-
lationship can be expressed:

PP = PS + losses
where:
PP = the primary power, and
PS = the secondary power.

Sample Problem:
Given: the power output (secondary power) of a transformer is 15
kW, and its losses are as follows:
iron loss -200 W
copper loss -350 W
Find: the power input (primary power) required. Solution:

PP = PS + losses
= 15,000 W + (200 W + 350 W)