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21
Transformers
21.1 INTRODUCTION
Chapter 12 discussed the self-inductance of a coil. We shall now examine
the mutual inductance that exists between coils of the same or different
dimensions. Mutual inductance is a phenomenon basic to the
operation of the transformer, an electrical device used today in almost
every field of electrical engineering. This device plays an integral part
in power distribution systems and can be found in many electronic circuits
and measuring instruments. In this chapter, we will discuss three
of the basic applications of a transformer: to build up or step down the
voltage or current, to act as an impedance matching device, and to isolate
(no physical connection) one portion of a circuit from another. In
addition, we will introduce the dot convention and will consider the
transformer equivalent circuit. The chapter will conclude with a word
about writing mesh equations for a network with mutual inductance.
21.2 MUTUAL INDUCTANCE
A transformer is constructed of two coils placed so that the changing
flux developed by one will link the other, as shown in Fig. 21.1. This
v g
+
–
Primary (L p , N p )
i p
+
e p
–
f p
Transformer
f m(mutual)
FIG. 21.1
Defining the components of a transformer.
Changing flux
pattern
+
e s
–
Secondary (L s , N s )