Working Procedure of Choke Coil:
A choke coil is an inductance coil of very small resistance used for controlling current in an a.c. circuit. If a resistance is used to control current, there is wastage of power due to Joule heating effect in the resistance.
The inductive reactance offered by the coil is given by XL = ωL
In the case of an ideal inductor the current lags behind the emf by a phase angle π/2.
∴ The average power consumed by the choke coil over a complete cycle is
Pav = Erms Irms cos π/2 = 0
However in practice, a choke coil of inductance L possesses a small resistance r. Hence it may be treated as a series combination of an inductor and small resistance r. In that case the average power consumed by the choke coil over a complete cycle is
Pav = Erms Irms cos φ
Pav = Erms Irms [r/( r 2 +ω2 L2)] … … (1)
where, [r/( r 2 +ω2 L2)] is the power factor. From equation (1) the value of average power dissipated works out to be much smaller than the power loss I2R in a resistance R.
Fig: a A.F Choke Fig: b R.F. Choke
Chokes used in low frequency a.c. circuit have an iron core so that the inductance may be high. These chokes are known as audio – frequency (A.F) chokes. For radio frequencies, air chokes are used since a low inductance is sufficient. These are called radio frequency (R. F) or high frequency (H.F) chokes and are used in wireless receiver circuits (Fig: a and Fig: b).
Choke coils can be commonly seen in fluorescent tubes which work on alternating currents.