Measuring Inductance

[JackB]

I have been building a DC Motor controller, and to test it I want to create a dummy load
that has the same inductance as the motor it will be controller, at least in the ballpark.

So researching into how to measure inductance, I've found a few methods, and what looks like
a simple way is to vary a sine wave frequency and use a resistor and find the frequency which
the resistor and inductor match. It explained very clearly in this video.

http://www.youtube.com/watch?v=Pv6zsQiBzXo

So now I just need a variable sine wave frequency generator, ok it turns out there are cheap little
boards that can do it, just need a micro to feed it the right tune word. Used one of my MSP430 boards
and created the user interface to adjust the frequency. Nice, sweet to see it work right on the scope.

So now I use it to measure the voltage, but my crap dvm's can't really do ac voltage like this,
so instead I just hook the scope to see the AC waves around the inductor and the resistor,
and adjust them so the AC signals have the same amplitude, i.e. they have the same AC voltage.

This all looks good, and I can adjust the frequency and see how they diverge and converge,
and think I am a genius :wink: , but when I do the calculations, the inductance I get is nothing close to what it should be.
I test a known 33uh inductor and compare it with a known 100uh inductor, and they require difference frequencies
to match voltage, so that part seems to work. Somehow my math is wrong, or I am wondering if
I am just using the wrong units in the formulas, or something else.
Not sure how I can figure this out without some help, so I'd appreciate any I can get.

 

[amberwolf]

You could also use a number of free programs on your computer (and some of the cheap little toy music keyboards) to generate a sinewave of just about any frequency in the audible range.

With the programs you can either specify the frequency, or draw the waveform, etc., or download a sinewave and pitch shift it up or down to where you want it.

With the keyboards you hold a note close to where you want, and use the pitch wheel to shift that up or down.

If you need it at a particular voltage/current you can probably build a little powerstage if you don't have any audio amplifiers laying around to do it with.

 

[JackB]

It takes fairly high frequencies using the method described.
Another method is using a fixed frequency, such as a 60hz power,
and measure the different voltages accurately,
but that requires a very accurate voltage measurement, cheap DVMs won't do AFAIK,
but I might try it if I can't figure out what I'm doing wrong with this setup.

 

[John in CR]

Get one of these. http://www.ebay.com/itm/High-Quality-New-High-Precision-Inductance-Capacitance-L-C-Meter-LC100-A-/121395234557?pt=LH_DefaultDomain_0&hash=item1c43b846fd