In a general way inductance can be characterized as that property of a circuit element (or component) by which energy is capable of being stored in a magnetic flux field.
A significant and distinguishing feature of inductance, however, is that it makes itself felt in a circuit only when there is a varying current. Thus, although a circuit component may have inductance by virtue of its geometrical and magnetic properties, its presence in the circuit is not exhibited unless there is a time rate of change of current.
The current voltage relationship involving the inductance component is given as:
Above equation describes a situation in which the voltage across the element (or component) is proportional to the time rate of change of current through it. The constant of proportionality L is the self-inductance or simply the inductance of the component and is measured in henrys (abbreviated H).
ADVERTISEMENTS:
The voltage v in Eq. (1.10) is a voltage drop in the direction of current and can be considered to oppose the increase in current. Fig. 1.11 depicts the schematic representation of inductance and its associated reference direction for current and voltage polarity. Any circuit component that exhibits the property of inductance is called the inductor.
An inductor is a circuit component which opposes the change of current flowing through it and induces a voltage when the current flowing through it varies in magnitude and/or direction.
ADVERTISEMENTS:
In case of an inductor current does not change instantaneously. It offers high impedance to ac but very low impedance to dc i.e., it blocks ac signal but passes dc signal.
A piece of wire, or a conductor of any type, has inductance i.e., a property of opposing the change of current through it. By coiling the wire the inductance is increased as the square of the number of turns. The inductance is represented by English capital letter L and measured in henrys.
Specially made components consisting of coiled copper wire are called the inductors. Inductors are of two types’ viz., air-core (wound on non-ferrous materials) and iron-core (wound on ferrite cores). Inductors range in value from the tiny (few turn air-core coils of 0.1 µH used in high frequency systems) to iron-core choke coils of 50 H or more for low frequency applications. The symbols for air-core and iron-core inductors are given in Figs. 1.12(a) and 1.12(b) respectively.
Variable Inductors:
ADVERTISEMENTS:
Some applications call for variable rather than fixed inductors. Tuning circuits, phase shifting, and switching of bands in amplifiers sometimes requires a variable inductance. Such inductors can be made in different ways. Fig. 1.12 illustrates how inductance is varied in several commercial elements.
The inductor shown in Fig. 1.12 (c) can be varied by switching from one tap on the coil to another. In Fig. 1.12 (d) a movable core is used. As more of the core is inserted into the coil, the inductance increases. By appropriately varying the spacing of the coil windings a relatively linear variation of inductance with core insertion can be obtained.