Inductance is the effect caused by the magnetic field when electric current passes through a conductor. During inductance , time varying magnetic fields develop in the conductor due to changes in current flow. The unit of Inductance is Henry and the symbol used is L to honour the Physicist Heinrich Lenz. The components inducing inductance are called as Inductors which are mainly coils that are capable of concentrating magnetic field.
Inductor is a passive component that can store energy present in the magnetic field. Due to the time varying magnetic field in the Inductor, a varying voltage will be generated in the inductor. The magnetic field that develops around the inductor resists the changes in the current and results in Inductance. The current flowing through the conductor creates a magnetic flux proportional to the current. A change in this current then creates a corresponding change in the magnetic flux which in turn creates the EMF (Electro Motive Force) that opposes the change in current. So the changes in the EMF are directly related to the Inductance. For example 1 Henry Inductor creates 1 volt when the current changes at the rate of 1 Ampere per second. Inductance can be increased by increasing the number of coil loops and using high permeability cores like Iron. A good inductor has inductance only and lacks both resistance and capacitance and dissipates no energy. But the resistance of the wire, Parasitic capacitance by the electric filed between the coil turns etc will reduce the inductance.
Inductors are extensively used in analog circuits in conjunction with capacitors to tune or filter certain frequencies. Toroid inductors are used around cables to prevent radio frequency interferences. Radio tuners widely use inductors to tune frequencies for reception as well as broadcasting.
Power supply uses inductors in conjunction with filter capacitors to remove residual hum and interferences in the mains supply. In SMPS power supply, inductor is used as an energy storage device and the inductor energizes for a particular fraction of the switching frequency of the regulator and remains de energized during the other part of the cycle. This energy transfer of the inductor determines the input and output voltage ratio.
An inductor usually consists of a core made up of Ferromagnetic material (due to high magnetic permeability) and wrapped around the core, there are many windings of copper wire. The number of windings as well as the gauge of the wire (represented in SWG– Standard Wire Gauge). The permeability of the core also determines the inductance. Ferromagnetic materials having permeability to magnetic field higher than that of air increases the inductance.
Air core Inductor
In Air core inductors (as one used in FM circuits) there is no core and the inductor has only the coil. The gauge of the wire (in FM it may be from 22-26 SWG), number of coil turns and diameter of the coil are the factors responsible for tuning the particular frequencies. In AM frequency 32 – 36 SWG wire is used with Ferrite core. Air core coils are usually wound around plastic, paper, ceramic or other non ferromagnetic materials. Inductance of air core inductors is very low compared to ferromagnetic inductors but is free from Core loss (energy loss in core). To avoid frequency variations due to vibrations, Foam (sponge) is placed inside the air core inductors.
The RF inductors are usually wind closely in a single layer parallel to each other with spacing between the turns. This is for preventing Parasitic capacitance (Capacitance between the turns) and Proximity effect( the eddy current induced by closely located coils) because both will change the behaviour of coil .
Laminated core Inductor
Laminated core inductors used in low frequency detectors use laminated cores to prevent Eddy currents. This is similar to the core of the transformer. The core has a number of steel sheets called Laminations with a coating of insulation. These are arranged parallel to the field and the insulation prevents eddy currents between the laminations.
Ferrite core Inductor
Ferrite core inductors are used for higher frequencies and are made up of Ferrite core and coil. The ferrite core is a ceramic ferromagnetic non conductive material, so eddy current will not flow through it. The ferrite material is represented as XX Fe2O4 and the XX represents the metal used with the ferrite.
Toroid inductors have a straight rod shaped core with windings. In Toroid core, the magnetic field from one end of the core passes through the air and reenters the core through the other end. To reduce the loss of magnetic field, the closed loop Toroid core is used as inductors so that minimum magnetic flux will escape from the core.
Variable Inductors have a sliding contact that can be moved over the coil. So depending on the number of coil turns involved, the inductance can be varied. By moving the core also it is possible to change the inductance (tuning of AM radio by adjusting the core position).
Fixed inductor uses multiple coils of a conducting material to store energy using a magnetic field. It belongs to the electromagnet family. The voltage produced is proportional to the rate of change in current flowing through it. The higher the number of coils and input current, the higher is the inductance.
Fixed value inductors are now available with colour coded body. Most common type is the LGA series colour coded lead type (Resistor like) one which is readily available. It has excellent ferrite core material and has high Q and self resonant frequencies. These inductors are widely used in oscillator circuits. The value (in uH) of the inductor can be easily obtained using the colour code chart.
The Fixed Inductors are made up of coils of high conducting material such as copper or aluminum which is wound around a core of ferrous material. The electric current is passing through the coil, creates a magnetic field in the inductor. When the current passing through the inductor changes, the magnetic field also changes which causes an electromotive force. This e.m.f is against the current that induces it and this produces a voltage in the direction opposite to the change in current. This voltage is called inductance. The unit of measurement is the Henry (H). The ferrite core induces more capacitance.