Name: Alternating Current
Uploaded: 2019-07-19
Duration: 4 min 17 s
Description: An alternating current is an electric current which periodically reverses direction and changes its magnitude continuously with time. Learn more.

Question

Explain the principle, construction and working of a choke coil in controlling current in an

AC

circuit. Why is it preferred to resistance in

AC

circuits?

Accepted Answer

In a direct-current circuit, the current is reduced by means of a rheostat. This results in a loss of electrical energy $I^{2} R$ per second as heat. The current in an alternating-current circuit may, however, be reduced by means of a device which involves little loss of energy. This device is called ‘choke-coil’.

Construction : It is a high-inductance coil made of thick, insulated copper wire wound closely in a large number of turns over a soft-iron laminated core . Since, the wire is of copper and is thick, its resistance (R) is almost zero, but due to the large number of turns and high permeability of the iron-core, its inductance (L) is quite high. The coil, therefore, offers a large reactance
$(ωL)$ and contributes to the impedance $\sqrt{R^{2} + ω^{2} L^{2}}$ of the circuit. Thus, it reduces the alternating current appreciably There is an arrangement to insert the core into the coil to any desired depth. More the length of the core inside the coil, greater is the inductance (L) of the coil and so greater the impedance. Hence, this coil can be used to vary the current in the alternating-current circuit.

The current in an alternating-current circuit can also be reduced by inserting a resistance R through a rheostat, but then an energy ${I_{rms}}^{2} R$ will be lost as heat in each cycle of current, where $I_{rms}$ is the current in the circuit. On the other hand, the loss of energy in a choke-coil is almost negligible.

The average power dissipated in the choke coil (L-R circuit) is given by

$P = V_{rms} \times I_{rms} \times cosϕ$ .....(i)

where the power factor is given by

$cosϕ = \frac{R}{\sqrt{R^{2} + ω^{2} L^{2}}}$

Since, the resistance R of the choke-coil is nearly zero and its inductance L is very high, so

$cosϕ = 0 (approx.).$

Thus, according to Eq. (i), the average power dissipated in choke coil will be nearly zero. This is why a choke coil is provided in fluorescent tubes operated on AC mains.
The resistance R of a choke coil is not exactly zero. That is, why, in practice, some electrical energy is lost as heat. In addition to it, energy is also lost due to hysteresis-loss in the iron-core of the choke coil. The loss of energy due to eddy currents is reduced by laminating the iron-core.
The choke coil can be used only in AC circuits, not in DC circuits, because for direct-current (to $ω = 0$ ) the inductive reactance $ωL$ of the coil is zero, only the resistance of the coil remains effective which too is almost zero.

Explain the principle, construction and working of a choke coil in controlling current in an $AC$ circuit. Why is it preferred to resistance in $AC$ circuits?

Important Questions on Alternating Current

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