Basics of the inductor: Its classification, construction, etc.

Basics of the inductor | Fixed and variable inductor | Air core inductor | Core type inductor | Construction | Advantages | Application |

Basics of the inductor:

Basically, an inductor is a coil or says an electromagnetic device that will oppose any change in A.C current. When the change in the current or electromagnetic field then the E.M.F will induce in the circuit. This induced E.M.F will directly proportional to the rate of the change of current.

∴ e ∝ di / dt

∴ e = − L . di ∕ dt


e = Magnitude of an induced E.M.F

L = constant of proportionality known as the inductance of the coil.

I = Current flowing through the coil.

The circuit symbol of an inductor as shown in the figure.

Symbol of the inductor.
Figure: Symbol of inductor

Classification of inductor:

All inductors are classified on the basis of their inductance value and type of core used. The detailed classification of the inductor is as shown in the figure.

Basics classification of the inductor
Figure: Classification of inductor.

Fixed Inductor:

Those inductors which have a fixed value of inductance called fixed inductors. The fixed value of inductors have following types:

  • Air core inductor
  • Core type inductor

1. Air–core inductor:

The Air-core inductor consists of a lot of turns of wire on ordinary cardboard called as former. Since there has air present inside the former, hence this type of coil known as an air-core inductor.

  • Stability of inductance.
  • The ratio of inductive reactance to effective loss resistance at the operating frequency is high.
  • Low-temperature coefficient.
  • Reasonable size and cost.
  • Low self-capacitance.

The basics construction of an air-core inductor is as shown in the figure. In the construction of air-core inductors, a core is made up of ceramics, plastic, or cardboard type insulating material.

air cored inductor
Figure: Air cored inductor

The conductive wire is wound on this core hence there is air inside the coil. Due to this all magnetic lines produced by the charging current, do not link with every turn of the coil.

These magnetic lines last in the surrounding space therefore the inductance value is reduced. The air-coil has no core loss at high frequency.

Losses in the air-core coils:
  • Heat dissipated in the winding itself.
  • At high-frequency eddy current loss happened in the copper.
  • Dielectric losses in the former and mounting.
  • This inductor used for intermediate or radio frequency respectively (I.F or R.F) applications in the tuning circuits.
  • In the Interstage coupling.
  • In the I.F coil.
  • Its inductance will unaffected by the current it carries.
  • Less harmonic and distortion in air core inductor
  • Air core inductors are free from the ‘iron losses’ which affect ferromagnetic cores.
  • It has better Q-factor, greater efficiency and greater power handling
  • It has designed to perform at frequency as higher than 1GHz.
  • Without high permeability of core, to achieve a given inductance value it requires more turns
  • So for more turns, there is lower self-resonance and higher copper loss.
  • It has greater stay field radiation and pickup.

2. Iron core inductor:

A coil in which solid or laminated iron or other magnetic material form part or all of the magnetic circuit linking its winding and it also is known as an iron-core choke.

The iron core inductor has allowed high inductance value but this will be limited in high-frequency application due to hysteresis and eddy current losses.

Iron-core increases the magnetic induction of a coil of wire. Because of iron has high permeability, it allows more magnetic lines of flux to concentrate, therefore increasing induction.


The basics of iron core inductor consist of a copper coil that it is wound over a solid or laminated iron core. The construction of the iron core inductor as shown in the figure.

Iron core inductor
Figure: Iron core inductor.

The iron core laminated used to avoid eddy current losses. The laminated iron core consist of thin iron laminations pressed in the rectified A.C supply amplifier stages and in other D.C applications.

The core materials most commonly used for smoothing chokes are silicon iron laminations and grain-oriented silicon iron. In the amplifier, application radiometer is frequently used or where the anode current is very small, mumetal is used.

Copper winding is usually used for enameled solid copper wire with interlayer insulation and impregnated after winding.

If the flux density in the core due to D.C greatly exceeds that due to the A.C as commonly occurs, it is usual to reduce the unwanted D.C magnetization by inserting an air gap in the core.

Up to a certain point, this results in a useful increase in inductance. Beyond that point, inductance decreases.

  • The iron-core inductor has a high-quality factor.
  • In the iron core inductor only laminated core is used to avoid eddy current loss.
  • This inductor has a high permeability.
  • It has high inductance value than the air core inductor.
  • In this inductor, all kind of losses increase with frequency.
  • It has more eddy current loss.
  • Iron core inductors used in the filter circuit.
  • They will also use in the A.F application.

Ferrite core inductor:


The ferrite core inductor symbol is as shown in the figure. Ferrite is used as a ferrous magnetic material. In this type of inductor, the copper wire is wound on the laminated ferrite core.

Symbol of ferrite core inductor
Figure: symbol of ferrite core inductor
ferrite core inductor
Figure: Ferrite core inductor

It has used at a high and medium frequency because it has high permeability with low loss, so it is more effective than an iron core inductor. This inductor is usually employed in pit cores that core consisting of an outer cylinder with closed ends.

The winding placed in the annular space. The air-gap will induce in the central core and by choosing a suitable length for this air gap, the properties of the pot may change to suit a wide range of design requirements.

  • This inductor will provide suitable inductance.
  • The quality factor can arrange to fall in the required frequency band.
  • It almost provides complete screening.
  • The parameters such as temperature coefficient and hysteresis loss can be controlled by air-gap adjustment.
  • This inductor use at high and medium frequency applications.
  • Ferrite rod antenna has used in the medium wave receivers.

Basics comparison of Air-core inductor and Ferrite core inductor:

SR. No.Air core inductorFerrite core inductor
1Q-factor is lowQ-factor is high
2Size is largeSize is small
3This inductor used in frequency applications up to 1MHz.Used in high-frequency application up to 100Mhz
4Cost is lowCost is high

Basics comparison of Iron-core inductor and ferrite core inductor:

Sr. No.Iron core inductorFerrite core inductor
1.The iron core inductor consists of a coil which is wound over a solid or laminated iron-core.Ferrite used as a ferrous magnetic material. In this type of inductor wire wound on a ferrite core.
2.The iron core is laminated to avoid eddy current loss.The windings are used to placed on annular space.
3.Iron-core inductor is used in filter circuits, AF applications.It will use at high and medium frequencies.

Specification of inductor

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