Choke Coil

A choke coil is an inductance coil which is used to control the current in an ac circuit.

Construction. A choke consists of a coil of several turns of insulated thick copper wire of low resistance but large inductance, wound over a laminated core. The core is layered and is made up of thin sheets of stalloy to reduce hysteresis losses. The laminations are coated with shellac to insulate and bound together firmly so as to minimize loss of energy due to eddy currents.

Principle.


The average power dissipated in the choke coil is given by
        P = 1/2 E₀I₀ cos Φ
If the resistance of the choke coil is R and the inductance of the choke coil is L, then the power factor cos Φ is given by
        Cos Φ = R / √ (R² + ω²L²)

The inductance L of the choke coil is quite large on account of its large number of turns and the high permeability of iron core, while its resistance R is very small. Hence cos Φ is nearly zero. Therefore, the power absorbed by the coil is extremely small. Thus the choke coil reduces the strength of the current without appreciable wastage of energy. The only waste of energy is due to the hysteresis loss in the iron core.  The loss due to eddy currents is minimized by making the core laminated.

Preference of choke coil over an ohmic resistance for diminishing the current. The current in an A.C. circuit can also be diminished by using an ordinary ohmic resistance (rheostat) in the circuit. But such a method of controlling A.C. is not economical as much of the electrical energy (I2Rt) supplied by the source is wasted as heat. Hence the choke coil is to be preferred over the ohmic resistance.
The energy used in establishing the magnetic field in the choke coil is restored when the magnetic field collapses. Hence to regulate ac, it is more economical to use a choke than a resistance.
Choking coils are very much used in electronic circuits, mercury lamps and sodium vapour lamps.