Measuring Power Supply Chokes

[KE6DF]

I have a few power supply filter chokes and recently got a new DVM which can measure inductance.

I tried it on the chokes and am getting weird readings.

Two of the Thordarson chokes are rated at 8 hy and read 7.5 and 8.3 on the meter. This seems reasonable.

Another UTC choke (with an OEM part number not in any catalog) is rated at 10 hy stamped on the side of the choke.

It measures 14 hy.

I also have a UTC S-37. In the old UTC catalogs, the S-37 is lists at 20 hy. But the newer catalogs have it at 8 HY.

I was wondering which, so I mearsured it and came up with 1.4 hy. The DC resistance of the choke is close to the catalog number, and there are no shorts to ground.

What gives? Are there likely shorted turns? Or are DVMs really innaccurate measuring power supply chokes.

I wonder how these inducance readings are made. Are they measured at 60 hz or 120 hz or at a higher frequency (I've heard 1K hz in some cases).

It seems that if the signal used to measure the inductance is too high in freq, one could get weird readings because these old filter chokes may have high internal capacitance.

On the other hand, when you use the DVM on a choke in the few milli hy range,  you would want it to use a high freqency signal.

I've never had an inductance meter before, so perhaps I'm expecting more from it than it can deliver.

The DVM is a mid/upper range Amprobe.

[Tom WA3KLR]

The choke's inductance is rated/measured at the rated current.  The inductance is higher than this with no dc flowing through it, such as when you have it on your meter.  The ratio of 1.4 to 1 higher no load is a very reasonable number for a "non-swinging" choke normally referred to as a smoothing choke.  So the 14 Henries for the 10 Henry rated choke is good.

The choke rated at 8 Henries and measures only 1.4 Henries sure sounds like it has shorted turns; no other reason I can think of for the difference.

The inductance of the iron lamination chokes drops continuously as the current increases.  With smoothing chokes this slope is fairly linear even up to the rated current.  Beyond this point, there is a knee in the curve and the inductance decreases sharply as the current increases further.

With swinging chokes, they are engineered to be down on the knee of the inductance curve a bit at the rated current.   Here the ratio between the no current inductance to rated current inductance can be 3:1 to 6:1.  Only a small percentage of the power chokes out there in the real world are swinging chokes.