Reverse-engineering a small XRT

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Rich Feldman
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Reverse-engineering a small XRT

Post by Rich Feldman » Sun Nov 21, 2010 6:49 pm

This is to report some electrical measurements on a dental x-ray transformer (nominally 70 kVp, 7 mA, 60 Hz). They give some perspective to low-duty-cycle imperatives.

My transformer has a 5.25-inch square laminated E-E core, in two riveted sections.
I had to take it apart to rework the mounting brackets.
Took the opportunity to set aside the plastic-shrouded secondary coils (figure 2). Made some electrical measurements with no load, and no high voltage in the way.

At 120 V RMS, the core is nearly saturated - I figure +/- 1.82 T flux swing. As in a MOT, this is to get plenty of volts per turn per pound. Magnetizing current reaches about 6 amps RMS. A real wattmeter (Figure 3) indicated that about 86 W were heating the transformer: 36 W copper loss in the thin, 1.0 ohm primary winding, and 50 W core loss.

To get the flux, I made an inordinately fancy sense winding with 17 turns and the best flux linkage I could manage (Figure 4). Voltage ratio shows that the primary has exactly 174 turns, in 2 layers: from the white lead it's right hand helix up, then left hand helix back down to the black lead. We get 0.69 V RMS per turn from 2.2 in^2 (14.2 cm^2) core area.

Figure 5 is a chart of the data. Magnetizing current was measured two ways: first with an old Amprobe clip-on analog meter, then with a true-RMS Fluke DMM in series. The difference at lower voltages could be due to the clip-on meter reading low, or to a bigger core air gap when I assembled it for the series measurements. (minimizing the airgap is uncritical at such high flux levels). My excuse for the large number of data points: successful intent to see the nonlinear magnetization at the low-voltage extreme, and the effect of primary winding I*R drop at the high-voltage extreme.

For next time: secondary winding details, and estimates of losses with a HV loads. (Passive, non-reactive loads can only increase the power dissipated in the transformer.)
We can see that the stress on transformer materials will be greatly reduced if we avoid operating above, say, 80% of nominal voltage.
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All models are wrong; some models are useful. -- George Box

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Re: Reverse-engineering a small XRT

Post by DaveC » Wed Nov 24, 2010 12:12 am

Rich -

That's real nice work! Should be quite helpful to folks trying to understand what their transformer treasures are.

Brought back a few memories of EE work way back when, too.

There's nothing better than getting the basics done right, and finding it all makes sense.

Thanks for sharing this effort.

Dave Cooper

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