1
1
EE – 4420
Electric Machine Analysis
Fall 2005
Instructor: Ernest Mendrela
TRANSFORMERS
1.
MAGNETIC CIRCUIT EXCITED BY ALTERNATING CURRENT
According to the Faraday’s experiment the voltage
e
induced in a loop linking a
changing magnetic field (see Fig.1) is proportional to the time rate of change of flux
Φ
:
e
d
dt
=
Φ
(1)
The polarity of the induced voltage can be determined by the Lenz’s law that says:
“The induced voltage is always in such a direction as to tend to oppose the change in flux
linkage that produces it”
Φ
e
Φ
i
i
i
Fig.1 Explanation to equation (1)
This is shown in Fig.1. For multitern coil the induced voltage is:
e
N
d
dt
d
dt
=
=
Φ
λ
(2)
where:
N – is the number of coil turns and
λ
– is the flux linkage in Weber turns.
Suppose we have a coil wound on one leg of a closed iron core as shown in Fig.2. To
draw an equivalent circuit of such a device called inductor, and then to analyze its
behavior under variable supply condition let us consider first an ideal inductor.

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