What is the resistance of the inductive
coil takes 5 À current across 240V, 50Hz supply at
0.8 power factors?
Answers
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- Home / Inductors / Inductive Reactance
- Inductive Reactance
Inductive Reactance of a coil depends on the frequency of the applied voltage as reactance is directly proportional to frequency
So far we have looked at the behaviour of inductors connected to DC supplies and hopefully by now we know that when a DC voltage is applied across an inductor, the growth of the current through it is not instant but is determined by the inductors self-induced or back emf value.
Also we saw that the inductors current continues to rise until it reaches its maximum steady state condition after five time constants. The maximum current flowing through an inductive coil is limited only by the resistive part of the coils windings in Ohms, and as we know from Ohms law, this is determined by the ratio of voltage over current, V/R.
When an alternating or AC voltage is applied across an inductor the flow of current through it behaves very differently to that of an applied DC voltage. The effect of a sinusoidal supply produces a phase difference between the voltage and the current waveforms. Now in an AC circuit, the opposition to current flow through the coils windings not only depends upon the inductance of the coil but also the frequency of the AC waveform.
The opposition to current flowing through a coil in an AC circuit is determined by the AC resistance, more commonly known as Impedance (Z), of the circuit. But resistance is always associated with DC circuits so to distinguish DC resistance from AC resistance the term Reactance is generally used.
Just like resistance, the value of reactance is also measured in Ohm’s but is given the symbol X, (uppercase letter “X”), to distinguish it from a purely resistive value.
As the component we are interested in is an inductor, the reactance of an inductor is therefore called “Inductive Reactance”. In other words, an inductors electrical resistance when used in an AC circuit is called Inductive Reactance.
Inductive Reactance which is given the symbol XL, is the property in an AC circuit which opposes the change in the current. In our tutorials about Capacitors in AC Circuits, we saw that in a purely capacitive circuit, the current IC “LEADS” the voltage by 90o. In a purely inductive AC circuit the exact opposite is true, the current IL “LAGS” the applied voltage by 90o, or (π/2 rads).
AC Inductor Circuit
inductor in an ac circuit
In the purely inductive circuit above, the inductor is connected directly across the AC supply voltage. As the supply voltage increases and decreases with the frequency, the self-induced back emf also increases and decreases in the coil with respect to this change.
We know that this self-induced emf is directly proportional to the rate of change of the current through the coil and is at its greatest as the supply voltage crosses over from its positive half cycle to its negative half cycle or vice versa at points, 0o and 180o along the sine wave.
Consequently, the minimum rate of change of the voltage occurs when the AC sine wave crosses over at its maximum or minimum peak voltage level. At these positions in the cycle the maximum or minimum currents are flowing through the inductor circuit and this is shown below.