Question
Explain the use of chock coil.
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Answer
Chock Coil: It is a device which is used in a.c. circuits for varrying the value of current in the circuit which involves little loss of energy. It consists of a high inductance coil which is made of thick insulated copper wire wound closely in a large number of turns over a laminated core made of soft iron.
Since thick copper wire is taken, its resistance R is almost zero, but due to large number of turns of the wire and high permeability of the iron-core, its inductance (L) is very high. The choke coil, therefore, offers a large inductive reactance $X_{L}=\omega L=2\pi fL$ and contributes to the impedance $Z=\sqrt{R^{2}+(\omega L)^{2}}$ of the circuit and current is reduced in the circuit. There is an arrangement to insert the core into the coil to any desired length. More is the length of the core in the coil greater will be inductance (L) of the coil, hence greater will be the impedance. In this way the choke coil can be used to vary the current in A.C. circuits.
The choke coil is put in series across an electrical appliance (say an electric bulb) of resistance R and is connected to an a.c. source. This forms an L-R circuit.
The average power dissipated per cycle in the circuit is given by
$P_{av}=V_{rms}I_{rms}cos \phi$ ...(1)
Hence power factor
$cos \phi=\frac{R}{Z}=\frac{R}{\sqrt{R^{2}+(\omega L)^{2}}}$ ...(2)
Ohmic resistance of the choke coil itself is nearly zero and its inductance L is very high, therefore $R<\omega L$ Hence $cos \phi\approx 0.$ Thus, the average power dissipated in choke coil will be nearly zero i.e. negligible.
As $Z=\sqrt{R^{2}+\omega^{2}L^{2}}$ is very large, so current is reduced without appreciable loss of energy i.e. without wastage of power. Thus working principle of choke coil is the principle of wattless current.
In practice the ohmic resistance of the 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 using laminated iron-core.
Since thick copper wire is taken, its resistance R is almost zero, but due to large number of turns of the wire and high permeability of the iron-core, its inductance (L) is very high. The choke coil, therefore, offers a large inductive reactance $X_{L}=\omega L=2\pi fL$ and contributes to the impedance $Z=\sqrt{R^{2}+(\omega L)^{2}}$ of the circuit and current is reduced in the circuit. There is an arrangement to insert the core into the coil to any desired length. More is the length of the core in the coil greater will be inductance (L) of the coil, hence greater will be the impedance. In this way the choke coil can be used to vary the current in A.C. circuits.
The choke coil is put in series across an electrical appliance (say an electric bulb) of resistance R and is connected to an a.c. source. This forms an L-R circuit.
The average power dissipated per cycle in the circuit is given by
$P_{av}=V_{rms}I_{rms}cos \phi$ ...(1)
Hence power factor
$cos \phi=\frac{R}{Z}=\frac{R}{\sqrt{R^{2}+(\omega L)^{2}}}$ ...(2)
Ohmic resistance of the choke coil itself is nearly zero and its inductance L is very high, therefore $R<\omega L$ Hence $cos \phi\approx 0.$ Thus, the average power dissipated in choke coil will be nearly zero i.e. negligible.
As $Z=\sqrt{R^{2}+\omega^{2}L^{2}}$ is very large, so current is reduced without appreciable loss of energy i.e. without wastage of power. Thus working principle of choke coil is the principle of wattless current.
In practice the ohmic resistance of the 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 using laminated iron-core.
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