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  #1  
Old 03-27-2007, 11:17 PM
chules chules is offline
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Default Finding Inductance with Current

Hello,
I have an inductor which I am trying to measure the inductance of. I don't have an inductance meter, but it seems to me that it would be possible to calculate the inductance by measuring the current through a series circuit of the inductor, a resistor, and a multimeter, connected to a 120V, 60Hz, mains supply. The DC resistance of the inductor and multimeter can be found using the multimeter. Is there a way to calculate the reactance and the inductance of the inductor using these values? If so, how? A mathematical explanation would be greatly appreciated.

Thanks,
Chules
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Old 03-27-2007, 11:36 PM
Dave_ Dave_ is offline
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I dont think you could calculate it from those values, otherwise the person who built his own inductance meter would of done it that way. The site can be found here:
http://users.tpg.com.au/ldbutler/RFInducMeter.htm
So he varies a known capacitor value until there is a circuit resonance, which will enabe the person to work out what value of inductance it is.

The formula for reactance is
X = 2 * Pi * F * L
where F = frequency (Hz)
L = inductance (H)

So looking at that it seems that you must know what frequency and inductance it is for the reactance.

Hopefully that's a point in the right direction
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Old 03-27-2007, 11:55 PM
chules chules is offline
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I was thinking about that, but I still think it might be possible to calculate the inductance with those values. Since changing the inductance/reactance of the inductor would cause a different amount of current to flow, there has to be a relationship between the two, and that relationship can probably be determined mathematically. The reactance could be calculated by other means than the formula you presented, such as by some kind of Ohm's law relations, and then that formula could be used in reverse to calculate the inductance. As far as the frequency, I know the frequency, it's 60Hz line frequency.

Thanks,
Chules

Last edited by chules; 03-27-2007 at 11:59 PM.
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Old 03-28-2007, 12:32 AM
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beenthere beenthere is offline
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For smallish values of L, 60 Hz will not be a good frequency. For a longer life, fooling with live conductors is not advised.

The meter and resistor are ok, but using something like a signal generator for the AC source will let you get more accurate and safer results. When the drop across the inductor and the resistor is equal, then Xl equals the resistor.
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Old 03-28-2007, 12:38 AM
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Here's a DIY inductance meter adaptor for any voltmeter: http://www.armory.com/~rstevew/Public/TestEquip/IndMeterAdapter.htm
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- Michael Faraday
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Old 03-28-2007, 01:51 AM
chules chules is offline
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I can't really build an inductance meter because I don't have the parts for one, and the measurements I do have for the current on the inductors in question were somewhat difficult to get due to my access to the inductors.
As far as using mains current, I think that in this instance it is the best way to go because the inductors in question are Microwave Oven Transformers with shorted secondary coils. The eventual plan is to use them as ballasts in a Tesla Coil, but I need to know the inductance/reactance first. Since these are designed to operate on 60Hz, at mains voltage, and have sufficient inductance to limit current to slightly over 15A without a resistor, I think mains current is the best choice here. (I only have 15A circuit breakers here so I can't just measure current through the inductor itself).
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Old 03-28-2007, 02:24 AM
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I'm sorry, but if you don't know the formulas for Ohm's Law and inductive reactance, then you really should not be attempting a Tesla coil at this time. Thread closed.
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Old 03-28-2007, 05:14 AM
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PM recieved. I apologise for having misunderstood the situation. Thread re-opened.

Z = Sqrt(R^2 + XL^2)

Kirchoff's current law applies, therefore Esource = Sqrt(ER^2 + EL^2)
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