what is a choke coil?
A choke coil is an essential and required electrical appliance used for controlling the electric current in an AC circuit without the loss of electrical energy in the form of heat.
Principle of Choke coil
The working of a choke Coil is based on the principle that the alternating current flows in an inductor with the phase lagging behind the emf by a phase angle of pi/2
Construction
A choke coil is basically an induction coil consisting of a large number of turns of copper wire well insulated from each other, wound over the core made of soft iron. The losses of electric energy arising due to eddy currents are minimized by using a laminated soft iron core. The insulated copper wire is wound on a soft iron core in case of low frequency choke coils, while in case of high frequency choke coils, the air is used as the core material over which the windings of copper wires are made
working
There are various electric devices that are operated on lower values of current like fluorescent tubes used in households. In order to limit the alternating current passing through such devices, there are two options – (1) we may connect a resistance of suitable value, (say R) in series with the device. In this option, a significant amount of electric energy (I²R) will go waste each second in the form of heat energy which is not acceptable, thus this option is not recommended and so not in practice as shown in Fig. 2
Now, in the second option, the choke coil is connected in series with the low current operating electric device like the fluorescent tube etc. Since, the alternating emf leads the current by a phase angle of pi/2, the average power consumed by the coke coil will be zero as the power factor is zero.
Pav = EV IV Cos(pi/2)=0
Thus, the inductive reactance of the choke coil decreases the current to the desired value without the loss of electric energy as shown in Fig. 3
In actual practice, an induction coil possesses a small ohmic resistance. It means that a practical induction coil (a choke coil) may be treated as a series combination of an inductance L and a small resistance R. Therefore, a practical inductance will consume a small average power owing to the presence of this small ohmic resistance. The modified power factor will now be
Wattless current
The component of alternating current that does not contribute to the power is called wattless or idle current. In general, the expression of the average power of an ac circuit is given to be
Uses of a choke coil
A choke coil in needed to control the alternating current in an electric circuit in place of a resistance. When we use a pure resistance to control the current, a significant amount of electrical energy is transformed into heat energy amounting to the wastage of energy. A choke coil is successfully used to control the alternating current in the circuit without wastage in the form of heat energy.
Power factor
Summary
- The Inductive reactance increases with the increase in the frequency of the AC source.
- The Capacitive reactance decreases with the increase in the frequency of the AC source.
- The value of phase angle between the emf and the alternating current for LR circuit lies between 0 and pi/2.
- The value of phase angle between the emf and the alternating current for CR circuit lies between -(pi/2) and 0.
- A Choke coil is an inductor used to control the flow of current in an ac circuit without consuming electric energy.
Frequently asked questions
Ques. 1 What is the resistance offered by an induction coil to dc ? Ans: For dc, the frequency f =0, Therefore , XL= 2 ( pi ) f =0 , Thus, the resistance offered by induction coil to dc is zero
Ques. 2 How much power is consumed in a purely inductive circuit? Ans : The power consumed is zero in an inductive circuit
Ques. 3 State the principle on which induction coil is constructed. Ans : It is constructed on the principle of mutual induction.
Ques. 4 Name two useful devices based on the phenomena of mutual induction Ans : Transformer and induction coil.
Ques. 5 What is the function of a chalk coil in a fluorescent tube? Ans : The coil decreases the current in the circuit without wasting electrical energy in the form of heat
Ques. 6 A choke coil in series with a lamp is connected to a dc line. The lamp is seen to shine brightly. Insertion of an iron core in the chalk causes to change the lamp’s brightness. Predict the corresponding observation if the connection is to an AC line.
Ans : The choke coil has no reactant, when a constant dc is flowing. Therefore, even when iron core is inserted, the brightness of the lamp will not be affected. In case, the lamp is connected to AC line, the insertion of iron core will increase the inductance of the chalk coil, and hence its reactance due to this lesser current will flow, and brightness of the lamp will be reduced.
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