at what frequency are inductors measured

[Charles Wasserf]

When a parts supplier advertises say a 6mH inductor at what frequency does it reach this value. With 'f' in the formula for inductive reactance the inductance is changing all the time resulting in the desired XO performance. So what am I missing here. I haven't seen anybody specify or advertise for example 6mH @ 500Hz. I need a low DCR 9mH inductor as specified in the design I am building but when asking the local transformer guy to wind me one, he had the same query ! 9mH at what freq. Can somebody enlighten me.

[SY]

There's a difference between inductance and impedance. The impedance of an inductor varies with frequency (after all, that's its job). Inductance is pretty constant- you won't see any significant difference between measurements at 60, 600, and 6000 Hz.

[infinia]

Hi
Q> at what frequency is inductance measured at?
A> all of them.

Ideally inductance does not change with frequency... untill its gets near the self resonance (SR) point at high frequencies.
I think the confusion exists because Reactance (Xl) does.

http://www.murata.com/emc/knowhow/pd...-1/21to22e.pdf


In speaker crossovers the SR of normal solenoid designs using wire is very high compared to stop band concerns. IMO Copper tape designs are suspect for certain applications.

[speakerdoctor]

Inductance, it is proportional to the impedance of interest at the crossover frequency YOU select. So, you need to measure your driver's impedance at the crossover frequency you want. Then, the inductance can easily be calculated with the known impedance at that frequency and a constant which varies with the type (degree, 1st order, 2nd order, etc) of XO YOU choose to have. Dickason (Loudspeaker CookBook) lists 36 different formulas for L for 1st thru 4th order - and that's just for 2-way speaker. The number of formulas jumps to 52 for 3-way speakers. Of course, those counts include woofer mid and tweeter coils.

For winding an inductor with a low DCR, find the largest magnet wire you can. Some commercial ones go down to 14 ga. wire and if you look hard enough, even lower (and more expensive). The DCR will ONLY be a function of wire diameter and length.

Look up 'inductor' at Wikipedia. There is a formula ther for wire length. Also here.http://www.66pacific.com/calculators/coil_calc.aspx

[infinia]

I think a 9mH air coil will have quite a bit of copper in it at higher costs and Rdc. So in practice, to lower Rdc and copper, a gapped iron core with thin laminations for high saturation would be advised here. A transformer house would know the tradeoffs better.

[jneutron]

Originally posted by Charles Wasserf
When a parts supplier advertises say a 6mH inductor at what frequency does it reach this value. With 'f' in the formula for inductive reactance the inductance is changing all the time resulting in the desired XO performance. So what am I missing here. I haven't seen anybody specify or advertise for example 6mH @ 500Hz. I need a low DCR 9mH inductor as specified in the design I am building but when asking the local transformer guy to wind me one, he had the same query ! 9mH at what freq. Can somebody enlighten me.

The standard measurement frequencies are 100 hz and 120 hz.

Some equipment can do both, some only 1.

If they have a good meter, the frequencies will be:

100, 120, 1Khz, 10Khz, and 100Khz.

You need to know your supplier's capabilities, and you need to know the frequency that your inductor has to go to.

Don't over-specify the frequency, as it may not provide sensical numbers, the vendor will be going crazy trying for unrealistic ones.

ps..I have a meter that you input the frequency, anything from 20 hz to 2 Mhz, but it becomes very difficult to get correct values as you go up in frequency. The last magnet I tested, the inductance went negative at about 53Khz...the meter plus setup could not interpret resonance correctly.

Cheers, John

[Charles Wasserf]

Thank you gentlemen

[Charles Wasserf]

I need a large 9mH. Now, wound as an air-core which is stable, all is OK, but to keep costs and DCR low requires an iron-core. Here lies my concern. The permeability of iron will vary as the voltage varies, and the inductance will change with the varying permeability. In other words what may well be 9mH at low back ground levels will be somewhat different at full blown big band party levels. So what is the correct design method, or are the parameters set up for an average level.

[soongsc]

Inductor impedance curves may vary even though they measure the same value on a meter. This was very annoying when I found that out because this means the characteristics will be different from the original design. Most problems are with cored inductors.

[Shaun]

Permeability varies with voltage? No, rather it is current dependent. More correctly, current density. But this is again dependent on the material used (its relative permeability). I have successfully used gapped ferrite cores for audio applications (highest was 2.2mH). The laminated core with air gap as mentioned before is also a good solution.

I believe that you are are concerned about core saturation. With the correct design (core material, size and air gap) iron or ferrite core inductors won't saturate under designed-for loads.

Measuring at the operating frequency is wise, I think. But measuring at a particular frequency won't give you the whole picture. I once used a salvaged laminated core inductor. I measured the inductance using Speaker Workshop. The resultant impedance curve had wiggles in it. That alerted me to the idea that the core was not operating in its linear region (even at that low current level). This inductor would have measured differently at different frequencies. A good inductor would measure the same over the full range of specified operating frequencies. However, at low frequencies series resistance can begin to swamp the measurement if you are measuring at a spot frequency. SpeakerWorkshop and other programs use frequency sweep and/or MLS (a pseudo-digital signal) signals to plot the inductor's impedance curve.

BTW, in an inductor Impedance comprises of both inductive reactance (Xl) and series resistence (R). Z= Xl + R.

You can use a power amplifer to drive real-load magnitude signals through the inductor and plot the impedance curve for various frequencies (use a current-limiting resistor). This will give you an idea of the goodness of the inductor.

HTH