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Measuring inductance

In measuring the inductance of the primary on a pair of SE output transformers of 5K:6 Ohms, I'm coming up with 3.3 H and a DCR of 138 Ohms. I am using an inexpensive hand-held $40 LCR meter.

My first thought was that this is obviously a very low inductance value. I would expect it to be in the 20~30 H range. But then I thought that maybe the LCR meter measures at a lower than standard frequency typical for this measurement. Could anyone shed some light on this?

Could the transformers really be this low in inductance? I checked the meter against a manufacturer specced 2H power choke, and the reading was close at 2.3H, so I don't think that the meter is defective.
 
Howdy, Musicalnoise: I am a very experienced magnetics engineer, perhaps I can be of help. Your experience is indeed likely caused by the LCR as jazbo8 proposes. The main cause of the error is likely the amount of iron being driven with the LCR circuit unable to distinguish properly between magnetizing current and power going toward core losses. If you have an oscilloscope, a DC power supply (around 20-30V and at least .5A) and either a current probe or a current sense resistor, you can build a simple test setup that will not be fooled by the core power. Let me know if you do and I will detail better.
Rene
 

tomchr

Member
Paid Member
2009-02-11 12:58 am
Calgary
www.neurochrome.com
It may be that the meter is getting fooled, or not have enough oomph to drive (and measure) a 10+ H inductor, but 3.3 H doesn't sound way off. I've measured OPTs on an HP 4194A impedance analyzer and gotten values in the 10~15 H range for Electra-Print 20 W 5k:8/4 ohm transformers.

138 ohm, 3.3 H would give you a LF pole at 6.7 Hz. So a tad high, but not badly so if the transformer is spec'ed for operation down to, say, 50~60 Hz.

Note that if you were to increase the number of turns in the primary (and secondary), you could get higher primary inductance. But you'd also get higher parasitic capacitance and inductance. It's the HF pole formed by those two parasitic components that end up limiting the HF performance of the transformer. As with everything else in engineering: tradeoffs, tradeoffs...

~Tom
 
Here is the methodology. Unfortunately, I have no ready access to a sketching program so the setup will have to be described textually. Fortunately, it's simple. First the math. Inductance in Henry is equal to (the change in voltage times the time interval) divided by the change in current. L=(dv x dt) / di. The voltage source will be connected across the winding to be measured for inductance, the current measured and the time of the current ramp determined.
You want to connect the sense resistor on the return side, with the scope and PS common to each other. The scope second channel, if used, measures between common and the "top" of the coil under test. There will be a small error caused by the sense resistor but it can be ignored for our purposes. As for the trigger, I have had reasonable results with a banana plug quickly jammed into the PS jack for minimum bounce and simply ignoring the very beginning of the current ramp. Some trial and error on the scope setup may be required. I have also made test circuits which had elaborate and very accurate switching circuits but for a large valued L I don't think it's worth it.
OK, now for the details. The biggest challenge will be to make sure the scope is triggered at the same time that the voltage is applied to the test winding. It is helpful to display both the voltage and the current, to make sure there isn't something funky going on but not necessary. You are after the current ramp that will result, with a starting value of zero and an ending value equal to the applied voltage divided by the DC resistance of the coil. Your sense resistor should not exceed 5% or so in value relative to the DC resistance of the coil. Pick a most linear portion of the ramp you see, some portion typically between 20 and 60% of the current amplitude, determine the time involved (keep in mind that the higher the ramp current, the more the ramp is modified from linear to logarithmic due to the resistive component). Use the power supply voltage for the dv term. Example: The time between the lower and upper points of the selected ramp portion is 10mS. The change in current for that interval (highest current point minus lowest current point) is 10mA. The applied voltage is 20VDC. The inductance is 20V times .01Second, that product divided by .01A = 20Hy. You want to keep the peak measured current for the selected ramp portion well below the steady state level which in your case will be around 100mA for a 20V supply. This will yield maximum accuracy. Now, for a very important caution: I am assuming a transformer made for PP operation, which implies little or no air gap. This means that the core may likely saturate, or at least be pushed hard in one direction of the BH curve, and remanence will keep it from fully returning. So, to make sure we are not being mislead, energize the coil in one direction, then reverse the connections before taking the actual measurements. If a retake is in order (I would advice several retakes until convinced of uniform results), always measure with the transformer leads in the opposite polarity as the last measure.
Good luck!