Inductor – Working, Types,Measuring


What is Inductor?

There is an emf induced in it if a time-varying current flows through a coil. The emf induced across the coil is directly proportional to the rate of current change over time. Because of the property causing emf, it is possible to refer to all types of electrical coil as an inductor. An inductor is an energy storage system in the form of a magnetic field that stores energy.

What is Inductance?

The induced emf across a coil is directly proportional to the rate of current change through it, as we have already said. The  proportionality constant in that relationship is called inductance.


Theory of Inductor

A current generates a corresponding magnetic field through a conductor. This field’s intensity depends on the value of the current that passes through the conductor. The magnetic field direction is found using right hand grip rule. The flux pattern would be the amount of concentrated circle perpendicular to current detection for this magnetic field.


Now if we wound the conductor as a coil or solenoid, it can be concluded that for each turn of the coil there will be localized circular flux lines as shown. But it is not practically possible, as if for every turn there are concentrated circular flux lines, they overlap each other. Since flux lines can not intersect, however, as shown, the flux lines for an individual turn will distort to form complete flux loops around the entire coil. The current carrying coil flux pattern is similar to a bar magnet flux pattern as shown.


Now if the current is modified through the coil, it will also change the magnetic flux it generates at the same rate. As the flux already surrounds the coil, the coil is obviously connected by this changing flux. Now, according to  Faraday’s law of electromagnetic induction, there would be an induced emf in it if flux connections with a coil were changed. Again, in accordance with the Lenz’s law, this induced emf is opposed to any reason for its development. As a consequence, the induced emf is in opposite to the voltage applied across the coil.

Types of Induction

There are two types of Induction mutual induction and self induction.

Self Induction

When the time varying current flows in a coil, the time varying flux is created and this varying flux is linked to the coil itself, resulting in emf being induced in the coil itself. This type of Induction is called self induction. Because of its own current, the inductance of a coil or inductor is called self-inductance.

Mutual Induction

As we have already said, as time changes the current flows in a coil, it produces time varying flux. This time the flux may be linked to another nearby coil. Because of this flux link, the second coil will have an induced emf. This type of induction is called mutual induction. Mutual induction can therefore be described as emf induction in one coil due to time varying current in any other nearby coil. The inductance of a coil is called mutual inductance due to current in another nearby coil.

Symbols of inductor


Measuring an Inductor

An inductor’s working behavior poses an interesting question–how do we measure an inductor’s behavior in terms that can be easily measured?

By the magnetic field they produce, we might try to measure inductors. We run into trouble as soon as we do that. The magnetic field that an inductor produces depends on the current passing through it, so even a small inductor will create a large magnetic field.

Rather, we could use the same method we used for capacitors, and we could describe a circuit’s inductance as the voltage change caused when the current changes at a certain level.

V = L(dI/dt)

Where voltage is V, inductance is L, current is I, and time period is t.

L’ is expressed in Henrys, named after the American scientist Joseph Henry who invented electromagnetic induction.

This equation contains the formula for calculating the inductance of a wire coil:

L =(µn2a)/l

Where L is inductance in Henrys, µ is the permeability constant, i.e. a function of how quickly the magnetic field can be formed in a given medium, n is the number of turns, a is the area of the coil and l is the length of the coil.

Also, the Henry is a very large unit, and basically inductors are measured in microHenrys, uH, which is a millionth of a Henry, or milliHenrys, mH, which is a thousandth of a Henry. Occasionally you may even consider very low calculated inductances in nanoHenrys,which is a thousandth of a uH.

Different Types of Inductors

Now the µ in the above formula has some interesting implications. This implies that the magnetic field inside the inductor can be controlled. The magnetic field created even by a solenoid, sometimes falls short of the requirements, as mentioned above. That’s why you find inductors formed around a core material in almost every case.

Cores are materials that support a magnetic field to be created. These usually consist of iron and its compounds such as ferrite (which is an iron oxide). Using a core, you can get a larger magnetic field than without it.


As the name suggests, there is no core to this type of inductor,air is the core material!  Since air has a relatively low permeability, there is quite low inductance of air core inductors–rarely above 5uH. Because they have a low inductance, for an applied voltage, the rate of current rise is quite fast, making them able to handle high frequencies. Most of them are used in RF circuits.



Iron is perhaps the magnetic material that is most recognizable,which making it an ideal choice for inductors. They take the form of inductors of the iron-core. Typically they are used for filtering the low frequency line as they can be very beefy and have large inductances. They are used in audio equipment as well.



Ferrite is a powder containing iron oxides. The powder is blended and shaped with an epoxy resin to form cores around which wires can be wound. Ferrite core inductors are easily the most identifiable because of their dull grey-black colour. These also are very fragile and break easily. They are the most widely used types of inductors, as the permeability in the mix can be finely controlled by controlling the ferrite-epoxy ratio.


Inductors in Series and Parallel

Series and parallel associated inductors behave the exact opposite way to capacitors. For example, you can simply summarize the values of the individual inductances to determine the inductance of a group of inductors in series.

L = L1 + L2 + … + Ln Where the maximum inductance is L and the individual inductances are L1, L2 … Ln. Suppose you have two inductors, one of which is 10uH and the other 15uH, then you get a total inductance of 25uH by putting them in series.

Parallel inductors act in the same way as parallel resistors, the inductance is given by: 1/L= 1/L1 + 1/L2 +… + 1/Ln Where L is the total inductance and L1, L2… Ln are the individual inductances.

In this way, you will end up with an inductance of 5uH if you connect two 10uH inductors in parallel.

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