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April 10, 2012


John Dunn

Just to mention:

This device was used on transformer windings of roughly 1 to 2 mHy of magnetization inductance in parallel with reflected capacitances from high voltage secondary windings, capacitances that approached 0.1 µF. The inductance meter I had at the time was thrown hopelessly off by that combination.

These dual-resonance equations are the same as in the "Dip Oscillator and Dual Resonance" item:


William Ketel

This system must be intended for smaller transformers, it seems. At first I was thinking that it would be for the larger power transformers in the multiple KVA class.
The winding being checked must be shielded because both ends are connected to relatively high impedance points and both ends have AC signals present. So it is quite susceptible to noise. That must be included in the considerations for the circuit's use. In order to have one end grounded the circuit would become a bit more complex.
It is also possible to do the same test using an external oscillator and oscilloscope and capacitor substitution box. That would be much less convenient but it would not need the shielding enclosure.

david pacholok

Clever idea indeed!
In my experience with 60 Hz xfmrs the inductance changes with excitation level due to the nature of the slope of the B-H curve in Silicon Steel, and even with frequency due to lamination eddy current losses among other things. So if your goal is to look for variences in L from a production lot - great!
But if you need to get closer to the operating L when the transformer is energized a method we like is solving the simple differnential equn:

V = -LdI/dT + IR

With a DC power supply, fast switch like a MOSFET, a DC current probe and a DSO you can choose your core H excitation level at which you wish to measure inductance by direct examination of I vs. T slope. You can actually watch permability change with this method, then flip the polarity to see what is hapening near the B/H orgin, and look at core retentivity as well.
The paralell capacitance is rapidly charged at the moment of switch turn-on and is effectively not there for the test, given a low Z DC supply.
Safety Note: Use a clamp (MOV, Zener) to safely dissipate energy in L when switch opens!

John Dunn

You're quite right, David. This worked on ferrite core transformers and coils and with the grid dipper, on slug tuned and air wound coils. When I tired to use this on 60 Hz transformers, the winding inductance made large values shifts with only small changes of excitation.

As to capacitance, the biggest challenge, the problem that actually brought this whole idea about, was the reflected capacitance from several thousands of turns on the secondary windings of high voltage transformers plus capacitance effects of voltage multipliers that went as high as x20.

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