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19th June 2006, 06:25 PM  #1 
diyAudio Member

Inductance measurement
Hi,
I have tried to measure the inductance of toroid's secondary(12V). The primary (230V) was left unloaded.I used guide from here : http://engr.nmsu.edu/~etti/fall96/el...ct/induct.html The resistor was 20 Ohms.Voltage at amp's output  1V. I tried to measure inductance at 200Hz and 1 kHz. The voltage drop across the resistor was 0.212V and 0.11V , respectively. By using the formulas in the page , i got ~7mH and ~3mH inductance at different frequencies ,which is nonsense. What is wrong in my measurements or calculations ? Best regards, Lukas. 
19th June 2006, 06:47 PM  #2 
diyAudio Member

In my opinion your using to low of frequencies. For inductance some measurements are done at 1000hz but normally I don't go below 10,000hz. The coils Re(Dc) will likely be throwing off your measurements. Since it it a cored inductor that may be interfering but I am unsure.

19th June 2006, 07:04 PM  #3 
diyAudio Member

Sorry for not being specific enough. I am measuring a mains toroid(50Hz, 230V).The secondary winding's resistance is only parts of ohm, which is very low.
I have also tried different frequencies , and the results are inconsistent as well. 
19th June 2006, 09:05 PM  #4 
diyAudio Member
Join Date: Apr 2006
Location: Netherlands

I also had difficulties measuring inductances of transfo's. I tried several methods and frequencies and found it impossible to obtain sensible results. Until I found out that in an iron cored transformer the inductance is indeed frequency dependent (and also voltage dependend, when you push the core close to saturation). So your results might no be such nonsense after all. You must measure inductance at the frequency where you want to know it, and well below the core sat limit. Or at the core sat limit, if you want worst case (it drops considerably there)
The reason why you probably want to know the inductance is to determine the load the trannie will put on your amplifier at low frequencies. In other words, what's of interest is the freq at which the primairy impedance drops below, say, 4 or 8 ohms. I measure this by putting a resistor of 4 ohms or so in series (just as on the link you provided). I measure voltage over the resistor (!) and over the primairy and adjust frequency until they are equal. Don't measure input voltage instead of voltage over the resistor because it will be less than the sum (phase shift!) and your results won't be accurate. Now you know the lowest frequency that you can use the trannie at (core saturation limits aside of course). Using that, you might calculate inductance at that freq (for what it's worth) using Z = 2*pi*f*L Another method is to put a known C parallel to the primairy, drive it with a sine wave from a high source impedance and find the resonance peak. Or feed it a steep narrow pulse and determine resonance freq from the scope screen. But both these methods won't let you chose the frequency at which you measure 
19th June 2006, 09:35 PM  #5 
diyAudio Member
Join Date: Apr 2006
Location: Minnesota

Bazukaz,
The page you sited is for measurement of a linear inductor. Generally, a gapped core or a low U core will be used for an inductor. A transformer uses a high U core and it is nonlinear and has hysteresis. This means the current will not be sinusoidal, so voltmeter readings may be incorrect. To get some idea of what is going on use a scope to measure the voltage across the resistor. I suspect it will be badly distorted. Note that this is not a bad situation, it's just the way transformers behave. Rick 
20th June 2006, 09:13 PM  #6 
diyAudio Member

Hi,
Thanks for help. The purpose of mains toroids are to be used for ESL stepups. I was thinking about a passive 12db/octave crossover , with transformers inductance used as an inductor.However , it seems being not so easy. One more question : does distortion of transformer depend on drive output impedance (amplifier) ? I have read statements that a transformer , driven by zero output impedance amplifier , has zero distortion.I found it on Rod Eliotts web site but could not understand the reason why does it happen so : "When supplied from a voltage source of zero ohms impedance, the real life transformer has no distortion, but again, there is no such thing as zero ohms (actually, it can be done, but yields little real benefit)." The main question is does the addded small value resistor to primary(~0.5 ohms) has some effect on transformer distortion(except for protecting the trafo from saturating so easy). Regards, Lukas. 
20th June 2006, 09:39 PM  #7 
diyAudio Member
Join Date: Apr 2006
Location: Minnesota

Lukas,
A reasonably good approximation for a real transformer is an ideal transformer with an inductance (called the magnetizing inductance, Lm) in parallel with the primary. Lm is the nonlinear component. The current through Lm is typically around 1020% of the full load primary current. Now if you apply a perfect sine wave voltage to the primary, the secondary voltage will also be a perfect sine wave. However, the primary current (including that through Lm) will not be sinusoidal due to the nonlinear behaviour of Lm. I don't know much about crossovers, but I doubt if a highly nonlinear inductor would be useful. Rick 
22nd June 2006, 01:51 PM  #8  
diyAudio Member
Join Date: Jan 2004
Location: away

Re: Inductance measurement
Quote:
L = k times N^{2}, k being a constant. This reduces the problem to that of finding the inductance of the 230 volt primary, and the turns ratio. The turns ratio is the ratio of the voltages..230:12. the primary has 19.1 times more turns than the secondary. If the secondary has an inductance of 1 millihenry, for example, the primary will have 19.1^{2} times that, or 364 millihenries. By connecting the primary to the line, open secondary, determine the current that flows through the primary. That current is set by the inductance. With the measured current, calculate the inductance of the primary. Then, divide by 364. Cheers, John edit: I hate these different sites, wish they'd all use the same html code..brackets, arrows...sheesh.. 

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