DC Circuits
INDUCTANCE
The induced EMF, or counter EMF, is proportional to the time rate of change of the current. The
proportionality constant is called the "inductance" (L). Inductance is a measure of an inductor’s
ability to induce CEMF. It is measured in henries (H). An inductor has an inductance of one
henry if one amp per second change in current produces one volt of CEMF, as shown in
Equation (3-1).
CEMF =
(3-1)
LDI
Dt
where
CEMF =
induced voltage (volts)
L
=
inductance (henries)
=
time rate of change of current (amp/sec)
DI
Dt
The minus sign shows that the CEMF is opposite in polarity to the applied voltage.
Example:
A 4-henry inductor is in series with a variable resistor. The resistance is increased
so that the current drops from 6 amps to 2 amps in 2 seconds. What is the CEMF
induced?
CEMF
LDI
Dt
4
2A
6A
2
4(
2)
CEMF
8 volts
Inductors in series are combined
Figure 4 Inductors in Series
like resistors in series. Equivalent
inductance (Leq) of two inductors
in series (Figure 4) is given by
Equation (3-2).
Leq = L1 + L2 + ... Ln
(3-2)
Rev. 0
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ES-03
