Choke Coil

If the goal is simply to REDUCE the amount of AC current flowing through some part of a circuit (for example, to safely limit the current through a fluorescent tube or a fan), an obvious approach would be to insert a plain resistor in series. But this wastes a substantial amount of electrical energy as heat through ordinary Joule ($I^2R$) heating -- energy that is simply lost, not put to any useful purpose. A choke coil offers a far more efficient alternative: it reduces the current just as effectively, but with only a small fraction of the energy loss.

A choke coil is, physically, an inductor -- constructed from thick, insulated copper wire wound in a large number of closely-spaced turns around a soft-iron core that is LAMINATED (built up from many thin, mutually insulated sheets rather than a single solid block of iron). Its large inductance L gives it a correspondingly large inductive reactance $X_L=\omega L$, which reduces the AC current through the circuit exactly as intended. Crucially, though, the AVERAGE power it dissipates, $P_{av}=e_{rms}\,i_{rms}\cos\phi$ with power factor $\cos\phi=\dfrac{R}{\sqrt{R^2+X_L^2}}$, stays very small -- because a well-made choke coil is designed to have a LARGE L (and hence large $X_L$) together with a very SMALL ohmic resistance R (achieved using thick copper winding, which has low resistance), the ratio $\cos\phi$ works out to be very close to zero, so almost no power is genuinely dissipated even though the coil is effectively reducing the current. …