Transformers
TRANSFORMER THEORY
Efficiency
Efficiency of a transformer is the ratio of the power output to the power input, as illustrated by
Equation (13-3).
(13-3)
Efficiency
Power Output
Power Input
PS
PP
x 100
where
PS = power of secondary
PP = power of primary
Theory of Operation
A transformer works on the principle that energy can be transferred by magnetic induction from
one set of coils to another set by means of a varying magnetic flux. The magnetic flux is
produced by an AC source.
The coil of a transformer that is energized from an AC source is called the primary winding
(coil), and the coil that delivers this AC to the load is called the secondary winding (coil) (Figure
1).
In Figure 1, the primary and secondary coils are shown on separate legs of the magnetic circuit
so that we can easily understand how the transformer works. Actually, half of the primary and
secondary coils are wound on each of the two legs, with sufficient insulation between the two
coils and the core to properly insulate the windings from one another and the core. A
transformer wound, such as in Figure 1, will operate at a greatly reduced efficiency due to the
magnetic leakage. Magnetic leakage is the part of the magnetic flux that passes through either
one of the coils, but not through both. The larger the distance between the primary and
secondary windings, the longer the magnetic circuit and the greater the leakage.
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