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Inductors in Series

The coil, or inductor, has a property which forces us to treat it differently from resistors and capacitors: its magnetic field. Where a resistor generates no such field and a capacitor generates an electric field that remains internal to the capacitor, the coil's magnetic field extends beyond itself, and can easily overlap the turns of wire in an adjacent coil.

Because of this, we must deal with two separate concepts when combining inductors in a circuit. These are known as self-inductance, which is the inherent inductance of the coil under consideration; and mutual inductance, which is the inductive effect of magnetic interaction between two coils.

Mutual inductance behaves just like self-inductance in many ways, and is defined in the same way. If a change in current of 1 ampere/second in one coil causes a counter EMF of one volt to be generated in the other coil, they have a mutual inductance of 1 henry.

Two inductors connected in series.

When we connect two inductors in series, as shown to the right, we have the question of whether or not their magnetic fields interact. If not, then their inductances simply add:

LT = L1 + L2

However, if they are physically placed so that they do exhibit a mutual inductance, this isn't sufficient. We must include a mutual inductance where each coil's magnetic field affects the other coil. Furthermore, we must take into account whether the magnetic fields of the two coils are aiding each other or opposing each other, since each self-inductance can be either increased or decreased by the value of the mutual inductance, designated M. Therefore, we must select one of two equations:

LT = (L1 + M) + (L2 + M)
LT = (L1 - M) + (L2 - M)


LT = L1 + L2 ± 2M

If you try to connect three or more coils in series, you must take into account the mutual inductance between each pair of coils. That's three different mutual inductances for three coils, and six mutual inductances for four coils.

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