INDUCTORS SELF RESONANCE EFFECT
By Frank McClatrchie
INDUCTORS SELF RESONANCE EFFECT
When purchasing coils and inductors or in the process of selecting them for a project you will come across a specification called Self Resonance that may seem a bit obscure, however that specification should be examined carefully to make sure that the inductors self resonance effect does not ruin your project. Self Resonance of any inductive component can be measured however the manufacturers of the coils and inductors will normally publish this information for you to study
Inductive reactance increases with operating frequency in proportion to the frequency of operation, but as the inductor approaches self-resonance, an additional increase in inductive reactance takes place that must be compensated for. The increase can be extreme when approaching full resonance and then it reverses to become a capacitive reactance at frequencies beyond self-resonance, so that particular inductor cannot be used as an inductance at all beyond the full self-resonance frequency. When selecting an inductor for your project, be mindful of the maximum frequency that can be used with that inductor in your project.
The increase in inductive reactance is related to how close the project operating frequency comes to the self resonant frequency. The formula for calculating this increase in reactance or Inductance Multiplication Factor is as follows:
INDUCTANCE MULTIPLICATION FACTOR = NOMINAL INDUCTANCE DIVIDED BY ONE, MINUS THE RATIO OF THE OPERATING FREQUENCY TO THE SELF RESONANT FREQUENCY.
In effect, dividing the NOMINAL INDUCTANCE by ONE minus that ratio will cause the inductance multiplication factor to approach infinity as the operating frequency approaches the self resonant frequency. The table below will indicate the degree of effect as the operating frequency approaches the resonant frequency wherein Fo = OPERATING FREQUENCY AND Fr = SELF RESONANT FREQUENCY OF THE INDUCTOR.
Fr INDUCTANCE MULTIPLICATION
Fo FACTOR
.01 1.011
.03 1.031
.10 1.111
.30 1.429
.50 2.000
1.00 INFINITE (operating inductor at a higher frequency results in a capacitive reactance)
It is usually unwise to operate a series resonant inductor at a frequency closer than 30% of the self resonant frequency. In a shunt resonant circuit, the capacity between the windings of the coil adds to the other parallel capacitance and becomes part of the resonant circuit and so does not add to the effective inductive reactance any more than the parallel capacitor it resonates with.
So when you are designing a circuit with a series resonant inductor be sure to check the self resonant frequency of any inductors you plan on using and make sure their self resonant frequency will not trip you up.
