Inductor

Introduction

An inductor, also known as the coil, choke, or reactor. It is a two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. An insulated wire wound into a coil around a core forms an inductor.

The time-varying magnetic field induces an electromotive force (e.m.f.) (voltage) in the conductor when the current flowing through the inductor changes, and it is described by Faraday's law of induction. As per Lenz's law, the induced voltage has a polarity (direction) which opposes the change in current that created it. So inductors oppose any changes in current through them.

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Inductance is the characteristics of an inductor which is the ratio of the voltage to the rate of change of current. The International System (SI) unit of inductance is the henry (H), named for 19th century American scientist Joseph Henry. it is equivalent to weber/ampere, in the measurement of magnetic circuits. It’s values typically range from 1 µH (10−6 H) to 20 H. Magnetic cores of many inductors have been made of iron or ferrite inside the coil, which serves to increase the magnetic field and thus the inductance.

Lenz’s Law

According to Lenz's law, the polarity (direction) of the induced voltage is given, which states that the induced voltage will be such as to oppose the change in current, e.g. if the current through an inductor is increasing, the induced voltage will be negative at the exit point and positive at the current's entrance point, tending to oppose the additional current. To overcome this potential "hill" The energy from the external circuit is necessary and is being stored in the magnetic field of the inductor. The induced voltage will be negative at the current's entrance point and the induced voltage will be positive at the current’s exit point, tending to maintain the current If the current is decreasing. The energy from the magnetic field is being returned to the circuit in this case.

Energy Stored 

The intuitive explanation of why a potential difference is induced on a change of current in an inductor goes as follows:

There is a change in the strength of the magnetic field when there is a change in current through an inductor. As an example, the magnetic field increases, if the current is increased. However, this does not come without a price. 

The potential energy is contained by the magnetic field, and increasing the field strength requires more energy to be stored in the field. Through the inductor, this energy comes from the electric current. Increment in the magnetic potential energy of the field is provided by a corresponding drop in the electric potential energy of the charges flowing through the windings. Across the windings, this appears as a voltage drop, as long as the current increases. Once the current holds constant and no longer increases, the energy in the magnetic field is constant and no additional energy must be supplied, so the voltage drop across the windings disappears.

Likewise, the magnetic field strength decreases, if the current through the inductor decreases, and the energy in the magnetic field decreases. Magnetic field energy is returned to the circuit in the form of an increase in the electrical potential energy of the moving charges, causing a voltage rise across the windings.

An emf is induced when a current passes through an inductor in it. The back emf opposes the flow of current through the inductor. So to establish a current in the inductor, work has to be done against this emf by the source of voltage.

Example of a time interval dt.

During this period, work done dW is given by:

dW = Pdt 

dW = – Eidt

dW = iL di / dt x dt 

dW = Lidi

The above expression must be integrated to find the total work done.

W = 0∫I

Lidi = ½ LI2

Hence, energy stored in an the inductor is given by the equation:

W = ½ LI2

Applications

In analog circuits and signal processing Inductors are used extensively. The analog signals and circuit processing applications have a great range, which is in conjunction with capacitors filter remove ripple which is a multiple of the mains frequency or we can say that the switching for switched-mode frequency power supplies from the DC current output, to the small inductance of the ferrite bead or torus installed around a cable to prevent frequency of radio interference from being transmitted down the wire. As the energy storage device in many other switched-mode power supplies Inductors are used to produce DC current. The inductor supplies energy to the circuit to keep current flowing during the "off" switching periods and enables topographies where the output voltage is higher than the input voltage.

Capacitor is connected to a tuned circuit consisting of an inductor, and acts as a resonator for oscillating current. In radiofrequency equipment tuned circuits are widely used, such as radio transmitters and receivers, as narrow bandpass filters to select a single frequency from a composite signal, and in electronic oscillators to generate sinusoidal signals.