Chapter 12 Electromagnetic Induction
223
M = 0.75 √20
______________
mH × 5 mH
= 0.75 √100
________
mH
= 0.75 × 10 mH
= 7.5 mH
The mutual inductance of these two coils
is 7.5 mH.
Transformers
The most common use for mutual induc-
tance is in transformers. A transformer’s job
is to transfer electrical energy from one circuit
to another by means of induction. It usually
consists of a pair of coils wound around the
same iron core or shell. The coils are wound
on the same core to ensure that the magnetic
coupling between the two coils is as high as
possible. To help you get an understanding
of how this is done, a simplifi ed version of a
transformer is shown in Figure 12-16. Actual
construction is shown on the following pages.
The transformer winding connected to
the energy source is called the primary. The
winding with the induced energy is called
the secondary. A transformer and its symbols
are shown in Figure 12-17. When shown on
printed circuit boards, the letter T is used.
There are numerous ways to construct
transformers depending on the size and output
performance required. There is a confl ict on
some of their names, but the types and uses
will be discussed as follows.
One type of transformer is called a closed-
form transformer, Figure 12-18. In a closed-
form transformer, the core is a closed ring
with the primary wrapped around one side
Figure 12-16. The basic idea for transformer
design is a pair of wire coils wound around the
same core.
Coil 1 Coil 2
Core
Figure 12-17. A transformer and two schematic
symbols.
Transformer Iron Core
Transformer
Figure 12-18. This type of transformer is called
closed form.
Secondary
winding Primary
winding
Laminated
core
Closed-Form Transformer