Choke Coil

13.10 Choke Coil

If a resistance is used to reduce the current passing through an AC circuit, there will be a loss of electrical energy in the form of heat, $i^2Rt$, due to Joule heating. A choke coil helps to minimise this effect.

Definition and Construction

A choke coil is an inductor used to reduce the AC passing through a circuit without much loss of energy.

It is made of thick insulated copper wire wound closely in a large number of turns over a soft-iron laminated core.

Diagram in words: a rectangular soft-iron core built up from thin sheets (laminations) stacked and insulated from one another, with many closely packed turns of thick insulated copper wire wound around it; the two ends of the winding are the terminals connected in series with the load.

How It Works

• The coil offers a large reactance $X_L = \omega L$ to the flow of AC, and hence the current is reduced.
• Thick copper wire keeps the ohmic resistance $R$ of the winding very small, so very little energy is wasted as $i^2R$ heat.
• The soft-iron core makes $L$ large; the laminations reduce eddy-current loss.

Power Dissipated

The average power dissipated in the choke is

$$P = e_{rms}\,i_{rms}\cos\phi$$

where the power factor is

$$\cos\phi = \dfrac{R}{\sqrt{R^2+\omega^2L^2}}$$

For a choke coil $L$ is very large and $R$ is very small, so $\cos\phi$ is nearly zero and the power loss is very small.

The only loss of energy is the hysteresis loss in the iron core, which can be reduced by using a soft-iron core.

Standard Results

• Purpose: to reduce AC current with negligible energy loss, unlike a resistance which dissipates $i^2Rt$.
• $X_L = \omega L = 2\pi fL$; $P = e_{rms}i_{rms}\cos\phi$ with $\cos\phi = \dfrac{R}{\sqrt{R^2+\omega^2L^2}} \approx 0$.
• A choke coil is of no use in a DC circuit, since for DC $f = 0$ and $X_L = 0$.
• Losses present: a small $i^2R$ loss in the winding, eddy-current loss (reduced by lamination) and hysteresis loss (reduced by using soft iron).