This is depressing. I thought I had a decent brand/model picked out....
Not so fast ...
I remember reading some posts on a guitar amp forum about valve output audio transformers. They love to measure them because the output inductance on the amp forms a low pass filter in the audio chain, and that affects the way it sounds. Conclusion was the level a transformer is driven at alters its electrical properties.
I was taught that as the AC voltage on an unloaded transformer primary (or inductor) increases for a given frequency, the core becomes more saturated with magnetic flux, and this affects the permeability of the core (assuming it's not air). As the permeability goes down, so does the inductance as the two are directly related. This means less reactance (Ohms) and the transformer will pull more current as you increase the driving voltage on the primary, even if the secondary is unloaded.
Spent last night searching the web for some charts on transformer\inductor driving voltage and inductance but couldn't find anything. This topic isn't discussed much, but it's important for LCR meters as you'll see. So I ran my own tests today.
Results and data attached.
I'm driving the original transformer (unloaded) with a variac, all the way from 300mV to 250V AC RMS. Recording the voltage and current using two DMMs. Reactance (XL) calculated using Ohms law as we know the current through the Xfmr and the voltage across it. Inductance then calculated using L = XL/2(pi)F.
The inductance starts off low and increases, peaks then decreases as the core starts to saturate. Did the same test again for an MOT I had lying around, as I know these saturate easily.
I don't know what's causing the low inductance at low voltages, but the figures don't lie.
I also ran 600mV AC RMS from the variac as this is used by the ET432 LCR meter (see post above), and got 0.68H. That's not too far off the 0.528H I got in AUTO on the LCR meter. A 22% difference, which sounds poor but when you look at the data I collected the inductance hits 7H @ 100V, so about 10x that.
What does all this mean? You need to drive an inductor or transformer at the voltage it will be used at to determine its actual inductance.
This brings LCR meters back into focus. They only output about a volt, so they're not going to tell us a whole lot unless the target circuit runs at that voltage as well. I'd hold off on that 1K bench LCR meter for now.
Incidentally, capacitors have a similar issue with voltage as well, but depends on the dielectric used. As you increase the bias voltage, the capacitance goes down, so you need to test them at working voltage.