Geoscience Reference
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L
1
L
2
(A)
(B)
FIGURE 11.69
(A) Schematic symbol for two coils (air core) with mutual inductance; (B) two coils (iron
core) with mutual inductance.
Key Point:
The amount of mutual inductance depends on the relative position of the two coils. If the
coils are separated a considerable distance, the amount of flux common to both coils is small and
the mutual inductance is low. Conversely, if the coils are close together so nearly all the low of one
coil links the turns of the other, mutual inductance is high. The mutual inductance can be increased
greatly by mounting the coils on a common iron core.
11.7.12.3 Calculation of Total Inductance
(In the study of advanced electrical theory, it is necessary to know the effect of mutual inductance
when solving for total inductance in both series and parallel circuits. For our purposes here, how-
ever, we will not attempt to make these calculations; instead, we discuss the basic total inductance
calculations with which the maintenance operator should be familiar.)
If inductors in series are located far enough apart, or well shielded to make the effects of mutual
inductance negligible, the total inductance is calculated in the same manner as for resistances in
series; we merely add them:
L
t
=
L
1
+
L
2
+
L
3
+ …
(11.58)
■
EXAMPLE 11.47
Problem:
If a series circuit contains three inductors with values of 40 μh, 50 μh, and 20 μh, what is
the total inductance?
Solution:
L
t
= 40 μh + 50 μh + 20 μh = 110 μh
In a parallel circuit containing inductors (without mutual inductance), the total inductance is
calculated in the same manner as for resistances in parallel:
1111
=+++
(11.59)
LLLL
t
1
2
3
■
EXAMPLE 11.48
Problem:
A circuit contains three totally shielded inductors in parallel. The values of the three
inductances are 4 mh, 5 mh, and 10 mh. What is the total inductance?
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