Series Circuits - Inductance

Inductance is the characteristic of an electrical circuit that makes itself evident by opposing the starting, stopping, or changing of current flow. You could almost call it the "mass or inertia of the electric circuit." Even a perfectly straight length of wire has some inductance, but it really begins to show when you wrap wire into a coil where the current produces a strong magnetic field. This "inertia" of current flow or the tendency to resist changes in flow is measured in units called "henries." The amount of actual power dissipation with current flow is called inductive reactance and is measured in units of ohms. The following formula is used to calculate the inductive reactance:


f = frequency in hertz (60 cycles in the US AC circuits)
L = inductance in henries
p = 3.1416

Voltage across and current through an inductor are described by Ohm's Law for inductors:

Find the current flow in the circuit with the pure inductor shown at right:

First finding the inductive reactance as:

then applying Ohm's Law:

The sine wave current in an AC circuit with pure inductance and negligible resistance will lag the voltage sine wave by 90 degrees. The figure here shows the current and voltage waveforms for a pure inductance circuit. This can also be expressed in what is called "phasor" form which is the use of vectors math. This is done to keep track of the fact that the inductive reactance value in ohms will be out of phase with any resistance value in a circuit so the inductive reactance ohms and resistance ohms cannot be directly added together.

The counterclockwise position of the current from the voltage is a convention to show the current lagging the voltage.

The total reactance of several inductances in series can be obtained by the following rule.

For series circuits:

For a series circuit with 3 inductive reactances of 10, 20, and 30 ohms, the total inductive reactance is calculated as follows:

If the circuit voltage is 120 volts, then ohms law is used to find the total current flow in the circuit.

2 amps of current flows through each inductance in the series circuit.