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Capacitors are capable of storing a charge in an electrostatic field. Inductors are capable of storing a charge in an electromagnetic field.

The ability to induce a voltage across itself with a change in current is known as self-inductance or simply inductance. Inductance also opposes a change in current.

Inductors have no opposition to steady DC.

L is the symbol for inductance. The basic unit of inductance is H henry named after the American physicist Joseph Henry.

Inductance in electrical circuits is similar to inertia in mechanical operations. It requires more energy to start or stop current in an inductor than it does to keep it flowing.


An inductor is a coil of wire. A coil of wire is an electromagnetic when current is passed through it.

Inductors are also called chokes, impedance coils, and reactors.

The core of an inductor may be a magnetic material such as iron or an insulated material. The term air-core is used for any inductors that do not have a magnetic core.

Inductance is greater with more coils, larger cross-sectional area, and shorter coil length.


Any conductor has some inductance because it produces a magnetic field around it. When the current changes the magnetic field changes. When the magnetic field changes an electromotive force is induced in the conductor. The polarity of this induced force is in the opposite to the applied voltage of the conductor. The effect is that inductance opposes a change in current magnitude.

Lenz’s law states this. The induced emf in any circuit is always in a direction to oppose the effect that produced it.

When AC (Alternating Current) is passed through an inductor there is a continuous change in current. The effect of the opposition to current is then continuous.

When DC (Direct Current) is passed through an inductor the opposition to current is only present when there is a change, such as starting, stopping, or a change in current flow.

The Self-Induced Voltage Equation

1 H (Henry) of inductance is seen when a change in current of 1A per second causes an induced voltage of 1V.



  1. Greater number of turns increases inductance
  2. A larger diameter coil has greater inductance
  3. Inductance decreases as the coil length increases
  4. A high permeability core increases inductance


The Inductance Equation

L = (uN2A/l)


Mutual inductance is when two coils are located  so that the magnetic flux from one coil links with the turns of another coil. The coils are referred to as coupled.

The transformer for AC circuits is a common example of mutual inductance.

Factors Affecting Mutual Inductance

Tight coupling refers to a high degree of mutual inductance such as a transformer with two coils wound around the same magnetic core.

Loose coupling is when two coils are far apart or at right angles to each other.

Air Core

Coils with hollow or non-magnetic cores are called Air-core coils. They have low values of inductance and are generally used for high-frequency applications.

Iron Core

Iron-core inductors use iron or an alloy for a core. Large values of inductance are possible. Hysteresis and eddy-current losses limit iron-core to low frequencies such as power line and audio. Laminated sheet material is often used to reduce eddy currents. Soft iron material such as silicon steel may be used to reduce hysteresis losses.

Powdered-Iron Core

Powdered-iron is mixed with a nonconductive binder reduce eddy current losses.  Higher current flow is possible before the inductor saturates.

Ferrite Core

Ferrites are good magnetic conductors but poor electrical conductors. This reduces eddy current losses.

Toroidal Core

Because of the shape most of the flux flows within the core resulting in very little flux leakage loss.

Movable (Variable) Core

These are variable inductors which can be turned.

Printed Circuit Board Core

A spiral of copper on a printed circuit board may be used as a coil. Only small inductance values are possible which limits its usefulness to high frequency applications. 


Inductors in Series

When inductance are not coupled (far enough apart to not influence each other) and connected in series the total inductance is the sum of the individual inductances.

LT = L1 + L2 + L3 + … + LN

When two mutually coupled coils are connected in series the total inductance is affected by their fields either series-aiding or series-opposing each other.

LT = L1 + L2 +/- 2LM

Inductors in Parallel

When inductors are not coupled and connected in parallel the total inductance is found in a similar manner to total resistance of resistors in parallel.

LT = 1 / ( 1/L1 + 1/L2 + … + 1/LN)

Mutually coupled inductors in parallel:

Aiding fields: 1/LT = 1 / (L1 + LM) + 1 / (L2 + LM

Opposing fields: 1/LT = 1 / (L1 - LM) + 1 / (L2 - LM


Opening the Circuit

When a circuit with an inductor is opened the magnetic field collapses and voltage is induced. The voltage dissipates over time due to I2R loss.


All conductors in a circuit possess some inductance. At high frequencies stray inductance can become significant.

To reduce stray inductance lead lengths should be kept short. Carbon resistors are preferred over wire-wound resistors. However, some wire-wound resistors are made non-inductive by winding adjacent so that the magnetic fields cancel each other.


Inductor Losses

Inductor losses are hysteresis and eddy currents.

Flux-leakage is another type of loss. This is magnetic flux outside the path for which it will do useful work.

Skin effect is another cause for loss. Most of the current flows along the outside of the conductor or skin. Hollow wire can be used to minimize the skin effect.

Troubleshooting Inductor Faults

Inductors can change value (including open) and shorts can develop between windings.

Shorts cannot normally be detected with ohmmeters because the change in resistance is so small. A ringing test can be used which creates a magnetic field and then checks the number of rings as the field collapses.