Choke Coil
CA choke coil is an electrical appliance used for controlling current in an circuit. If we use a resistance R for the same purpose, a lot of energy would be wasted in the form of heat etc.
The choke coil consists of a number of urns of thick copper wire wound closely over a /laminated soft iron core. The inductive reactance offered by the coil XL = w L is large. Therefore magnitude of a.c is reduced since
Iu = Eu / XL
In an ideal choke coil, ohmic resistance = 0 therefore there is no energy dissipation. Through an ideal inductor current lags behind the applied voltage by a phase angle of 90.
∴ Average power consumed by the choke coil over a complete cycle is
P = E0 I cos ∅ = E I cos 90 = 0
However in actual practice a choke coil of inductance L is having a small resistance r it may be treated as a series combination of L and r. therefore average power consumed over a complete cycle in the practical choke coil is
P = E I cos ∅
P = E I r / r2 + w2 L2
This comes out to be much smaller than the power loss (i2 R) if a resistance R is used for reducing a.c.
For reducing low frequency alternating current choke coils with laminated soft iron cores are used.
This is because I = Eu / XL = Eu / wL = Eu / 2 π vL
When v is low L is made high using soft iron core.
These are called the chokes represented in fig. 1
For reducing high frequency alternating currents air cored chokes are used.
I = E0 / XL = Eu / wL = Eu / 2π v L
When v is high L need not be made high air cored chokes are used.
These are called the chokes represented in fig .2
We have seen the use of choke coil in series with a fluorescent tube working on A.C mains.
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