What frequency is used to test standardized inductor values?

[codex653]

i notice that all the inductors that have labels on them listing the uH or mH is just a single value....but doesn't inductance properties change with frequency? so is there a standard frequency that all inductors are tested at to give the value they have labeled when you buy them from retailers? or maybe, depending on the engineer's design for the inductor, they only test the frequencies that the inductor would be used throughout and then average the inductance???

reason why i am asking is because of a project i have, and when i worked out the math for the inductor value (786 uH) it hit me that i had no idea the "proper" way of measuring the inductor....i have the way to measure inductance roughly by knowing the frequency and measuring the current and voltage, but as far as i understand it, inductance changes with frequency so i didn't really know what frequency to measure it by :/

unless i'm missing something really huge here i don't really know which frequency i should measure these inductors at.

EDIT:
oh and these are homemade inductors btw, so its not like i'm buying them from a supplier

 

[MikeMl]

Depends on what frequency the inductor is to be used at. For example, for smoothing chokes for an AC power supply, it makes no sense to measure at 1 or 10MHz. An RF coil which will be used at VHF should be measured at > 10Mhz, etc.

I had a HP inductance meter that had a provision for measuring at 100Hz to 10MHz. You could also connect an external signal source.

 

[codex653]

oh ok! so do the suppliers then list their inductance values based on their application frequency? i've never bought inductors from a supplier before so i wouldn't know

so really, for my project that will have a 80-100KHz signal through the inductor, i just use my frequency generator and test through the 80-100khz range i will be using it at and measure everything else from there?

 

[RMMM]

Thats correct.

After all, you dont need to care about what inductance the coil will be at frequencies outside of your circuits operating parameters.

 

[Pommie]

It's my understanding that impedance changes with frequency but inductance stays the same.

Mike.

 

[MrAl]

Hello there,

If you are going to use the inductor in a switching power supply it also helps to test it with a square wave. Alternately, you can test it right in the power supply itself once it is constructed simply by loading the power supply a little at a time and watching the current through the inductor and voltage across it both on a scope. Watching the current alone is a good way to test for core saturation.
Even a not so good inductor will often work in a switching power supply up to some load point, so you can start it up and test with various loads to get an idea how good it is going to perform with different load values. If you get to some load that begins to cause saturation you know you cant go any higher than that unless you use a different inductor or add an air gap.

 

[JimB]

It's my understanding that impedance changes with frequency but inductance stays the same.

Quite right.

However, all inductors have capacitance between the windings and are self resonant at some frequency.
So, as we vary the frequency, at frequencies well below resonance, we will measure the inductance, the capacitance is insignificant.
As we approach the resonant frequency, the APPARENT inductance will reduce until at resonance there will be no inductance. We will just see the resistance, the inductive reactance will be cancelled by the capacitive reactance.
Above the resonant frequency, the inductor will effectively behave like a capacitor.

As for the original question, the datasheet for an inductor should state the frequency at which the inductance is measured.

JimB

 

[Nigel Goodwin]

i have the way to measure inductance roughly by knowing the frequency and measuring the current and voltage, but as far as i understand it, inductance changes with frequency so i didn't really know what frequency to measure it by :/

As others have said, inductance doesn't change with frequency, but you should measure at a frequency applicable for the inductor in question - for a mains choke use 50-100Hz or so, for shortwave chokes use 100KHz-30Mhz, for VHF use 50MHz upwards.

 

[codex653]

ahh ok so i HAVE missed something! thank you very much for clarifying that! so with the capacitance thing, i reaalllly need to make sure the frequency never goes above or at resonance cause the inductance properties are either diminished or completely gone correct? so since that is a resonant circuit (well, when it gets to the right frequency anyways) and i need the inductance to be at a certain value, what are some techniques to limit the capacitance between the windings so that the inductor doesn't go near resonance?

 

[Nigel Goodwin]

You really need to state exactly what you're trying to do, vague questions aren't much use.

 

[codex653]

sorry, i didn't really think i would have needed to specify the project use if i was just asking about the windings and frequency stuff my bad...anyways, the inductor is going to be used in a switchmode supply at around 80-100Khz with a 3A average current draw and 5A peak, 786uH inductance...the entire switchmode will be powering my amp at +/- 30vdc...(i'm building 2 smps's)..is that what you wanted??

the question is based off of what JimB said

As we approach the resonant frequency, the APPARENT inductance will reduce until at resonance there will be no inductance. We will just see the resistance, the inductive reactance will be cancelled by the capacitive reactance.
Above the resonant frequency, the inductor will effectively behave like a capacitor.

what are some ways i can reduce the capacitance so it doesn't do what that quote right above this says would happen? keep windings spaced farther apart?

 

[Nigel Goodwin]

You need to do nothing, your inductance value is far too low for any concern at the low frequencies you're using.

Which is why you need to tell us what it's for!.

 

[codex653]

hahahaha yahhhhh oops...well thank you anyways lol