Inductive Reactance of a Coil
Important Formula
What do the formula symbols mean?
Inductive reactance
$$ \class{brown}{X_{\text L}} $$ Unit $$ \mathrm{\Omega} $$In the case of a DC circuit, the voltage frequency is \( f = 0 \). In this case, the induction coil has no reactance. Without an internal resistance, the coil would produce a short circuit when a DC voltage is applied to it.
Frequency
$$ f $$ Unit $$ \mathrm{Hz} = \frac{ 1 }{ \mathrm{s} } $$The alternating current flowing through the coil also changes its direction at this frequency.
With the angular frequency \(\omega ~=~ 2\pi \, f\) the inductive reactance can be written as: $$ X_{\text L} ~=~ \omega \, L $$
Inductance
$$ \class{brown}{L} $$ Unit $$ \mathrm{H} = \frac{ \mathrm{Vs} }{ \mathrm{A} } $$If we apply an AC voltage \( U_{\text L}(t) \) to a coil of inductance \( L \) , then an AC current \( I_{\text L}(t) \) flows through the coil. The alternating voltage changes polarity with the frequency \(f\). With this frequency, the alternating current also changes its direction.
A coil to which an alternating voltage is applied has a complex non-ohmic resistance which is called inductive reactance. This resistance is usually abbreviated as \( X_{\text L} \).
You can easily calculate the inductive reactance. You need the AC frequency \( f \) and the inductance \( L \) of the coil:
\(\pi\) is here a mathematical constant with the value \( \pi = 3.14 \). The unit of the inductive reactance is Ohm:
By the way, the factor \( 2 \, \pi \, f \) in Eq. 1
is often combined to angular frequency \( \omega \):
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If you use a very large AC frequency, then the inductive reactance will also be very large and the coil will barely let the current through.
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If, on the other hand, the AC voltage frequency is very small or even zero, that is if a DC voltage is applied, then the inductive reactance also becomes zero. The coil allows an arbitrarily high current to pass, which corresponds to a short circuit.
As you can see from Eq. 1
or 2
, you can also use the inductance \( L \) to adjust the reactance of the coil.