X-ray transformer modifications

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Jim Stead
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X-ray transformer modifications

Post by Jim Stead »

I previously had started a cursory thread on getting this unit - viewtopic.php?f=33&t=10293
Walking through the modifications may be beneficial to someone else someday, so I have started a new thread here.

The assembly appears quite clean and the oil looks real good.
Transformer2.jpg
After taking a look around inside, and taking a couple of measurements, it looks like this is the correct schematic:
Tank Schematic.jpg
As I had guessed, it is a split supply with a center-tapped ground. My plan was to join both output coils in series, but the small gauge of the wire to ground and to the M connection has me concerned. I haven't been able to wrap my head around how they can be so much smaller than the other side of the coils, which feed the diodes.

I added the section at the bottom right of the schematic to show where the "M" wiring goes. This is the current meter in the control panel. It's also the source of my greatest confusion. The M wire running to the control panel is 18 gauge wire in a cable of other 18 gauge wires. There isn't any special insulation. Isn't this the H.V. side of the transformer? I get that the draw by the meter is very small, but to complete the H.V. circuit to ground for the rectifier it seems this wire could have some big power on it. Anyone recognize what I am overlooking?
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Richard Hull
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Re: X-ray transformer modifications

Post by Richard Hull »

Wire size is a function of current only. This is the ground to the entire system's HV. It may handle a kilowatt. There is just no need for a large wire here A 24 gauge wire would be more than large enough. That kilowatt is all voltage and no current. It is the connections that must be ultra secure in this circuit!!!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jim Stead
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Re: X-ray transformer modifications

Post by Jim Stead »

That's good news Richard, thanks. I hear you loud and clear about the connection quality! My plan is to remove the filament transformers at the top to make room for ballast resistors, then join the main coils in series, and set the + side of the rectifier to ground. Although I could use the x-ray cable's shield as a ground, I do have an extra cable and figured it's best to use that. The shields will still be grounded as well, but I like the idea of the full sized return.

The following is the modified schematic I plan to put in place. To be honest, I've never created a negative-only power supply, so the grounding of the + leg seems unusual to me. If this is not proper, I would love to hear about it. Since I am removing the connection for the MA meter, I will work on adding a circuit to the output for operating the meter.

EDIT: I added the meter circuit, and changed the resistors. The Overload breaker is part of the original control unit for this transformer, it will interrupt the primary power if the transformer is overloaded.
Tank Schematic - plans.jpg
Last edited by Jim Stead on Tue Sep 29, 2015 5:17 pm, edited 3 times in total.
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Rich Feldman
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Re: X-ray transformer modifications

Post by Rich Feldman »

Jim, the M feedthrough from oil tank is at low voltage, but only if the front panel meter circuit is complete or the M terminal is externally grounded.

Conceptually, it might help if you take the original tank schematic and draw a temporary internal connection from that M node to ground.
Now both HV coils are wired identically.
They make a 125 or 150 kVp secondary winding, with center tap connected to earth ground.
There is a full-wave bridge between secondary winding and the normal HV load, so the secondary winding current will be AC milliamps.
That includes both coil-to-ground wire segments, which are of course at GND potential.

To monitor the HV current, we interrupt one of those coil-to-ground connections and insert an external shunt resistor or meter movement.
In this case, it's AC current so we also need an external rectifier (rated for full mA, and the low voltage across shunt resistor or meter movement).
Verstehen Sie?
If the external meter circuit fails open, without OVP, you are likely to get an external arc closing the HV circuit.

Wait a minute, I just saw your proposed modification.
IMHO you should keep the normally grounded ends of both HV coils near GND potential, and not go for double voltage.
The solid and liquid insulation in tank, connectors, and thick cables aren't designed for that.
Look at which end of each coil is far from the core, and which end isn't.

Original design voltage for cathode terminal, from GND, should be plenty for a fusor.
Just keep your fingers out of anode receptacle connector, or remove the anode rectifier strips.
Then I think the two HV coils will conduct in alternate half-cycles. If you connect both coils' inside ends to the M terminal, I think the external mA meter circuit will be accurate without modification.
Last edited by Rich Feldman on Mon Sep 28, 2015 11:22 pm, edited 1 time in total.
All models are wrong; some models are useful. -- George Box
Jim Stead
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Re: X-ray transformer modifications

Post by Jim Stead »

Thanks Rich. The new 125kv to Ground is different, as you noted. But, I'm not planning to push the voltage that high. Putting the coils in series is intended to allow a higher duty cycle, while operating at a lower voltage and current than originally designed.

The existing 125kv, 300ma rating of this transformer is peak, and probably for a second at best. The ampacity is also way more than I need. My thought was to join the coils and then feed it a lower power. In the end, I hope to see a 100% duty cycle of 40-50kv @ <150ma. It may not end up being capable of that much, I'll have to watch the internal temperatures and develop a scale to know what I can output over time.
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Re: X-ray transformer modifications

Post by Rex Allers »

I was waiting for someone with more real experience to pipe up, but it hasn't happened so I'll add what I'm thinking.

The original design is basically two ~63 KV supplies relative to ground. One provides + HV and the other -HV. The xray tube is placed between them so it sees ~125 KV. The two ends of the tube are both hot relative to ground.

You propose tying the center of the two HV transformer sections together to put them in series and tacking one of the former HV outputs to ground. As Rich F mentioned, I think the two HV coils were wound expecting the inner turns to be near ground, with (probably, I think) the near-ground primaries next to them on the low-V end. Your connection will have these inner windings bouncing ~63 KV relative to ground. I think that isn't a good gamble to take.

In my view, just don't use the positive side. You could remove the two diodes that connect to the +HV out, or leave them and just not use that output. The two diodes on the right make a full wave rectifier with the two HV windings, and the center of the two HV windings remains at ground. You don't really want more than 60+ KV out, for a fusor (?), anyway, do you.

The two HV coils will feed opposite AC phases so you will be filtering output at half the transformer input frequency which is good. I assume you do intend to add some kind of capacitive filtering on the output.

The only change I would suggest is that the metering of current in the original design is only done between one side of one HV coil and ground. This was OK since the current of the xray tube flowed equally between both coils. If we use only the -HV output, better metering would be to tie the two low ends of the HV coils together and pass them both through the metering. I assume you could leave it alone, too, and assume the real current is approx double what is measured. Of course you need some kind of gap or voltage limiting diode to ensure the coils center connection can't get far from ground voltage if the metering opens up.

These thought are how I would approach it. No one else has thrown up red flags about where you are heading now. I hope you consider what I have said or others who may have more engineering or real-world experience might comment.
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Richard Hull
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Re: X-ray transformer modifications

Post by Richard Hull »

I feel the original circuit is the best. It is correct that the secondary coil windings close to the iron core for each secondary needs to be at or near ground. Leave it at half voltage as it was designed that way. The plus/minus change should be simple and involve the rectifiers and their connections only. In this fashion you are warranted to do fusion with ease and safety.

Your idea for metering and ballasting is sound, never hook only a meter to the meter connection, as I show in my old diagrams. Use a low ohm wire wound to ground and place your meter across that resistor and read the voltage across the resistor which is an analog of the current.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: X-ray transformer modifications

Post by Rex Allers »

Richard said, "The plus/minus change should be simple and involve the rectifiers and their connections only."

In my post, I was trying to point out that the original design was already a DUAL full wave rectifier, BOTH plus and minus relative to ground. My point was ignore the plus output (or save it as a bonus for some other application, like in the original) and use the existing minus output that is already designed relative to a sensible ground for our fusor purposes.

No change in the rectifiers needed, they are already (half of them) just right.

Don't change the circuit in any major way, just accept a [mere] over 60 KV maximum output.
Rex Allers
Jim Stead
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Re: X-ray transformer modifications

Post by Jim Stead »

I guess what I am not understanding is where this over 60kv output is coming from. It was my understanding that X-ray transformers are rated at a high number for the few seconds they are in operation. If I were to run the transformer for an hour straight at that output level, my expectation was to overheat or damage the transformer. How much should I derate the transformer for continuous use? I had assumed I was only going to get 20-25kv out of each coil. That's why I wanted to join them. Maybe I have dertated to far. I was hoping to get 40-45k out of this in continuous use.
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Richard Hull
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Re: X-ray transformer modifications

Post by Richard Hull »

You really need to have powered this up and measured the volts out. No need to take it all the way up. Just variac the thing and take it to 40 volts on the primary. A stock 30kv DC TV anode probe should do for this or a crude homemade HV meter would give you a relative idea of what the all up voltage output will be. My guess is this is a 40-60kv unit originally.

Regardless of the milliamp rating on this, it is certainly beefy enough to give 15-20ma+ forever and a day.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jim Stead
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Re: X-ray transformer modifications

Post by Jim Stead »

Okay, that's what I'll do, I'll stick with the split supply. I'll get the resistors added, close/vacuum it, and do some testing.

On another note, I am wondering about the purpose of a resistor that was placed on the transformer contactor in the control unit.
X-ray Contactor.jpg
X-ray Contactor.jpg (7.02 KiB) Viewed 8104 times
This contactor uses 30 amp contacts, which are undersized for the current needed. Because of that, they used two contacts rather than one, but they added a limiting resistor on one of them. Why would they do that?
As it is, the contacts being protected by the resistor look to be in good shape. The contacts where the majority of the load is going are pitted and are burning away. I thought the contactor would have had a better chance if the load was equally distributed on the two contacts, rather than being leaned towards one of them.
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Re: X-ray transformer modifications

Post by Jerry Biehler »

Maybe that contact has been adjusted to close first to lessen the power surge as the transformer saturates.
Jim Stead
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Re: X-ray transformer modifications

Post by Jim Stead »

I can't believe how quickly time passes!

I completed the mods we discussed previously, and added a voltage divider while the tank was open. Here's the final schematic and a photo of the finished modifications.
Tank Schematic - Final.jpg
Transformer3.jpg
My next problem is that the transformer was out of oil for 5 months. I don't have the equipment to properly refill the tank under vacuum, so I was wondering what's the next best thing.
I can twist it rapidly CW and CCW as I lower it into the oil, hoping to dislodge some bubbles and force oil into any nooks it can find. I can also move it around each day until I am ready for it (3-4 weeks at least). I was thinking I could put a hot plate under the tank and slowly increase the oil temperature prior to applying a light vacuum. But, I can't apply much of a vacuum because the thin tank wall obviously won't stand it. If that is a viable idea, what temperature can I safely it heat to?

Any other tricks I might try?
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Richard Hull
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Re: X-ray transformer modifications

Post by Richard Hull »

I think you will be Ok if you agitate through a large arc as you lower into the oil. If, after you lower it all into the tank and seal it up, kind of bounce it up and down hard by as little as 1 or 2 inches, this will further help. After all of this, wait a full week and do it again. Only after this time period should you take it up very slowly with a variac to what you think is half voltage and if no arcs, let it sit again for a week and bounce it a bit more and finally take it up under a demo fusor load, playing with it at extended lower high voltages and glow currents for protracted periods. I think it will be OK.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Rich Feldman
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Re: X-ray transformer modifications

Post by Rich Feldman »

Not having tanked any XRT's myself,
I think it might be straightforward to process your tank inside a larger tank that can safely handle full vacuum.

Hundreds of vendors sell vacuum chambers made from aluminum stock pots with thick flat plastic lids, used for things like degassing mixed resin.
Here's a 5-gallon size from what looks like a reputable vendor (of course you can save money & make from scratch). http://www.provacusa.com/5-gal-Vacuum-C ... _p_13.html
chamber2.PNG
If 5 gallons isn't deep enough, thick aluminum and stainless pots come much bigger & maybe with taller aspect ratios. Here is a restaurant supply catalog page with pots up to 160 quarts. https://www.hotelrestaurantsupply.com/m ... 0-80-.html

Of course they are not designed for vacuum service, and the flat bottoms come with strength-of-materials issues. Flat-ended cylinders are discussed in David Kunkle's thread about designing his 20" diameter vacuum chamber.

p.s. check out this video of a brand new pole transformer getting its oil. I don't have sound on, so can't tell how much vacuum (if any) is used. Even without full vacuum, I think it helps to put coil-and-core assy into a dry tank, then let the oil level rise slowly. https://youtu.be/sqMGgVZXseA?t=222 Rectangular pad-mounted utility transformers have walls reinforced to withstand full vacuum whenever the oil is installed or changed.

Not to contradict RH, who speaks from experience.
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Jim Stead
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Re: X-ray transformer modifications

Post by Jim Stead »

You are thinking the way I was originally. The X-ray tank is 15.5" in diameter and 2 feet tall, so I was going to need a pretty good sized chamber. I thought I might find a large enough section of used pipe that I could make into a chamber with a steel plate welded to the bottom, and another clamped to the top on thick rubber gasket material. If I lived in oil country, that pipe would've been in my garage by now. But up here in the Northeast, it's super premium stuff. I thought about beefing up a 55 gallon drum for the purpose, and I looked around for a worn out air compressor, or unserviceable propane tank, but nothing jumped out at me. In the end I realized I simply do not have enough extra time to prepare such a chamber, unless it was absolutely necessary.

I was very happy to hear from Richard that I could get away with a simpler method. I imagine this may ultimately limit the overall life span of the transformer, but I can live with that. Sad as it may be for such a beauty. As it is, I've saved it from the scrapyard, so we're already ahead. I expect/hope it will serve many experiments in the near future. In time, I can always pull it back out of service and refurbish it properly, if that seems like the smart move.
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Re: X-ray transformer modifications

Post by Richard Hull »

In general, the longer a non-vacuumed transformer sits in its oil, the better. Far better is using it with voltage on it for the high tension will work out bubbles over time and, or, reduce their size. I have seen this way back in the Tesla days. We modified a bunch of high voltage tranaformers and ran them open topped and after a first run at tension, we saw a gang of bubbles come up and later as we used the transformer, the number of bubbles diminished to near zero.

The inter-winding paper has already absorbed oil and will retain it for years. Just resoaking it will re-wick oil all throughout. Vacuuming is hyper important with first dip, bone dry paper right from manufacture.

Just approach max voltage slowly and leave a small vent valve open on top of the tank for a while if you seal the thing up tighly. If the max voltage is 70kv don't go near it for a long time. Work for a while at half max voltage.

No guarantees here, just past experience.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: X-ray transformer modifications

Post by Jim Stead »

I bounced the transformer around some more last night and powered it up slowly with 120v to get close to the halfway mark. It's completely quiet until I get to around 10kv, where the transformer begins its natural hum. At 124v it reached 29.3kv. The hum was smooth and consistent the whole way up, not broken up or varying in any way. No signs or sounds of any arcing. Now I'll let it sit some more.

The schematic for the X-ray setup shows a max input of 300 volts to the transformer, provided by the autotransformer in the control panel. Doing the math from 124v suggests a max output around 70kv as Richard already figured. I'll naturally be at less than half that for a long time while I learn to run a fusor properly.
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Richard Hull
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Re: X-ray transformer modifications

Post by Richard Hull »

Good man Jim!!! You are very wise and the transformer will thank you for your conservatism as the oil insidiously, over time and use, re-insinuates itself into and amongst the windings and paper. It sounds like you have an ideal transformer for fusor work. All the best in your future efforts.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Re: X-ray transformer modifications

Post by Roberto Ferrari »

Hi Jim

I plan to follow your approach to use just a half of an X-ray transformer.

The last setup you published wiil work as half rectified PS.

How advanced are you in testing it?

I will appreciate any suggestions.

Regards

Roberto
Jim Stead
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Re: X-ray transformer modifications

Post by Jim Stead »

Hi Roberto,

Unfortunately, I have not used the transformer under a load yet.
I continue to "bounce" it occasionally, and have brought power up a few more times.
I'm happy with the way it came out, and I expect that it will work well.
If I was to do this again, the only thing I would do differently is to keep in under oil when I am not actively working on it.

Jim
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Re: X-ray transformer modifications

Post by Roberto Ferrari »

Hi Jim

You mention that because you had problems with trapped air or just because that way to handle it left you with a unsafe feeling?

Roberto
Jim Stead
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Re: X-ray transformer modifications

Post by Jim Stead »

I haven't had any problems with trapped air yet, but I'm also not operating it under a load or at the higher voltages it's capable of producing.

There wasn't any benefit to leaving it out of oil for as long as I did. I could have easily lowered back into the oil when I wasn't working on it.
That's what I'm recommending. I think it's a mistake to leave the coils exposed to air any longer than they need to be, since that could allow air, and moisture, to work their way deeper into the windings.
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