Archived - Yet another HV power supply project

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Rich Feldman
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Archived - Yet another HV power supply project

Post by Rich Feldman » Thu Oct 23, 2014 8:53 pm

Yesterday I unpacked my new plaything, found on ebay (indirectly, following a tip on this forum).
Should be cathode and anode tanks, and HV cables, from a Bennett 125 or 150 kVp x-ray generator.
DSCN8813.JPG
As packed
Here is the simpler of the two tanks, before and after my first-ever use of a big old HV plug.
DSCN8814.JPG
DSCN8815.JPG
Little time was lost before starting to take it apart,in fading daylight.
Saw thick, rectangular magnet wire to primary winding. I think the design frequency is 100 kHz.
DSCN8817.JPG
There are about nine wires from secondary windings to the lower-deck board.
I bet they represent several isolated windings with individual doublers or quadruplers.
I'd welcome any reference to a schematic for the tanks.
(not present in user manual and board schematics that I have found).
DSCN8818.JPG
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Re: Yet another HV power supply project

Post by John Futter » Fri Oct 24, 2014 2:21 am

Rich
I think 100kHz would be very optimistic
my guess 35-75kHz to limit strain on Trr of rectifiers

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Re: Yet another HV power supply project

Post by Rich Feldman » Fri Oct 24, 2014 7:06 pm

Yeah, it seems high to me too, esp. for an early-1990's product. For reduced voltage testing, where am I going to find a power amplifier that's good at 100 kHz?

Here's where the frequency estimate came from.

X-ray repair dealers say this tank part number is used in HFQ series generators. http://www.xraypartsdepot.com/e30040-c.html

The Bennett company history page http://www.bennettxray.net/index.php?page=about-us says:
In 1989, Bennett X-Ray introduced a technological breakthrough, the 100 kHz high frequency generator
...
now producing the Bennett Renaissance Series of Chiropractic Systems. The Bennett Renaissance product line includes both conventional radiography and direct digital systems, both of which feature the 100 kHz technology. For more on this exciting technology, see the "100kHz High Frequency Story".


Maybe it turned out that that 100 kHz was not the most economical frequency, engineering-wise, but the company was locked in by the marketing campaign and a growing body of legacy equipment in the field. I am committed to learn about resonance in HV transformer-multiplier systems. Maybe this one can be run much slower, with voltage limited by core saturation.

That same dealer sells Bennett power inverters. http://www.xraypartsdepot.com/bennett_parts.html
From my limited understanding, the 100 kHz systems would use FET versions because IGBTs are too slow.
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Re: Yet another HV power supply project

Post by Rich Feldman » Fri Oct 24, 2014 9:27 pm

Just found more info in a partial manual for equipment using these tanks.
Bennett HFQ 300, 450, and 600 generators are named for their mAs ratings at 125 kVp.
They are available with Stored Energy option that reduces electrical supply requirement from 220V-100A to 115V-15A.

Theory of Operation says the "Current Amp" units send 100 kHz current to high voltage tanks.

Here is a connection diagram for the smallest configuration.
E30040_HFQ-300.PNG
The -450 and -600 add third and fourth BR-C-Driver modules in parallel, but use the same tanks.
The SE options replace the BR-C's with +/- 211 V battery buses and battery chargers.
The schematic section covers all boards except the tanks and E10136/E10137 drivers.
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Re: Yet another HV power supply project

Post by John Futter » Sat Oct 25, 2014 6:26 am

Rich
my only comment would be that full wave 50kHz =100kHz halfwave

the idea of specmanship was alive and well in the 80-90's

however they my have chosen to strain the rectifier stack to prove a point


A bit of inductance measurement on the primary at 50 and 100 Khz with a function generator might shed some light

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Re: Yet another HV power supply project

Post by Rich Feldman » Sat Oct 25, 2014 7:40 pm

Good point there, John, about factors of 2 in market-speak. Like single phase X-ray systems that advance from 60 to 120 pulses per second when bridge rectifiers are used. Also I will be sure to try a LCR measuring instrument at work that goes down to 70 kHz.

I wish it weren't too late to change the Subject of my original post
to something more indicative of high-frequency X-ray transformers, and learning by reverse engineering.

Here are some things I have learned since yesterday.

Tank capacity is about 2.2 US gallons.

No voltage multiplication is involved.
The anode tank has nine independent secondary windings, each charging a 0.05 uF 7000 V capacitor through a bridge rectifier with 4 discrete diodes.
The capacitors are tied in a series string, with HV output socket on positive end and MA sense connection on negative end.
Secondary winding resistance ranges from 34 ohms on the innermost layer to 41 ohms on the outermost layer.
If the nominal system voltage (125 kVp) is divided equally between cathode and anode, then there's practically no margin in the capacitor voltage rating.

The cathode tank primary winding has DC resistance of 1.3 milliohms and 60 Hz AC impedance of 3.3 milliohms (linear up to 10 A). I think that works out to inductance of 8 microhenries. But then if driven with 400 V, the magnetizing current would change at 50 A/us, which is too high to believe.

Ferrite core round sections have diameter of 0.795" (2.02 cm). Core effective length is about 12 inches. If pushed to Bmax of 4000 gauss, I figure it could take about 50 V/turn at 100 kHz or 25 V/turn at 50 kHz. We ought to see signs of saturation at 10 Oe, which would need 250 ampere-turns in the primary.


As a next step, I want to get a handle on the turns counts and the core material BH properties. How about if I wind about 25 turns on the presently empty leg of the core? Then could a) run 60 Hz I-V measurements, hopefully well into saturation without overheating my wire. Or could b) run single-shot tests by applying voltage pulses or a charged capacitor to the primary winding. To design the latter, we want enough energy to saturate the core, and voltage low enough not to create problems on the secondary side.
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Re: Yet another HV power supply project

Post by Rich Feldman » Wed Oct 29, 2014 4:42 am

We use the tools we have. In my case, not yet any HF inverter or even an audio amp of more than a couple watts.

But learned plenty after connecting cathode tank primary to a fancy LC meter at work.
Swept the frequency and measured Z and theta. My high school physics teacher, Mr. Koller, taught that when running measurements by hand, chart the data immediately if possible. Then transcription blunders, and interesting regions, can be re-measured or explored with finer resolution.

Below 100 kHz it behaves like an inductor of 8 uH, gratifyingly close to the value figured from DC and 60 Hz milliohms.
A resonant peak at 164 kHz suggests a parallel LC circuit with C = 0.12 uF, perhaps the collective secondary winding capacitance reflected to the primary. I guess the resonant dip at 453 kHz comes from inductance of my interconnection. Surely this ground has been trod many times before -- what do you experts think?

Later I did another 60 Hz measurement of primary, using variac and 120V:3V transformer and now a clip-on current probe, seeking onset of saturation. No change in apparent impedance, or obvious waveform distortion, up to 55 amps RMS (and terminal voltage of 183 mV). That was a surprise. But it's probably safe to turn it up to 100 A , briefly, next time.
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primary_z_theta.PNG
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Re: Yet another HV power supply project

Post by John Futter » Wed Oct 29, 2014 5:59 am

Rich
Interesting graphs
from this we can see that it is possible to drive it at 100kHz with not much margin.
You certainly want to stay on the inductive side of the curve as most solid state electronics do not like driving capacitive loads and stay stable in the control loop.
We have to remember that this is for transitory x-ray generation with a very low duty cycle so it probably was 100kHz

Who cares if the diodes get hot due to trr losses its only on for a couple of seconds at most.

But for steady state fusor operation a lower freq may be the answer maybe with series resonant drive to peak up the voltage. Remember the warning above about stability. But there are now tools like LTspice to help and with the data you have gathered you can terst stability before sacrificing silicon to the smoke gods

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Re: Yet another HV power supply project

Post by Rich Feldman » Fri Oct 31, 2014 4:58 am

Ta-da! Finally saturated the core of a tanked transformer at 60 Hz, and measured it with the aid of a fluxmeter. In this image, X axis is primary current (100 A/div). Y is flux linkage to a temporary sense winding (0.001 volt seconds per div). Chart can be re-scaled as a BH curve of the core material, after we learn the number of primary turns (7 or 8, I don't yet know for sure).
cath_BH1.JPG
Before that experiment, I measured the primary wire thickness (rectangular, 0.05 x 0.35 inches) and
calculated that 150 A RMS for a full minute would not damage it. That's based on adiabatic heating model. Assume no cooling at all, so the rate of temperature rise is the electric power loss divided by the wire's thermal mass.

Let's illustrate by example. AWG14 building wire can handle 28 A for a minute, 97 A for 5 seconds, 217 A for 1 second, or 2170 A for 10 milliseconds without damage to the insulation.
What those limits have in common is an I^2-t product (current squared x time) of about 47300 ampere-squared-seconds.
One meter of the wire has a resistance of 8.28 milliohms, so total heating, I^2-R-t, is about 392 watt-seconds. The wire weighs 18.6 grams so its temperature will rise about 55 degrees C. That delta-T value makes my I^2-t computations from scratch match tables in the following handy reference:
http://www.cooperindustries.com/content ... tion_2.pdf

I^2-t ratings go up as the fourth power of wire diameter, because thicker wire has less resistance and more thermal mass. The flat wire in my transformer primaries is almost equivalent to AWG6. Maybe actually better at high frequency, if derated for skin effect.
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Re: Yet another HV power supply project

Post by Rich Feldman » Mon Nov 10, 2014 9:41 pm

Following the lead of Bob Reite, I want to try a monochromatic (sinusoidal) power waveform at high frequency.
Starting at much less than design voltage, to defer the transition from air to oil.

Does anyone here have experience with monolithic amplifiers for car audio systems, such as the TDA7850?
4 channels, 25 W per channel into 4 ohms with low distortion.
In parallel, with some balancing resistors, ought to comfortably put out 100 W into 1 ohm.
It's used in DVD players like one lost in my garage, and in $25 single-board amplifiers (heatsink not included).

Some important details are not found in the datasheet.
Can be found by measurement, but maybe someone here knows the answers and won't mind sharing them.
- matching of the 4 channels
- DC offset voltage at output (not an issue if used with series C)
- Response at frequencies above 20 kHz
- Response at DC (for the lab electromagnet project).
- Behavior with inductive loads
block.GIF
player.JPG
board.JPG
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