My first XRT and some questions

[Fred Stock]

Yesterday I received a first, fresh XRT (shown in the image below) still dripping oil from its core. I was told that it is a Siemens 85 kVp at 15 mA dental XRT, running on 230 V AC.



The XRT has for its rather small size quite some iron in the core (weight 9 kg) and it also has a separate coil (in the image at the left). First of all I ploughed through the HV forum posts to see if I could find more info on this type of XRT but unfortunately without success.
It has ten cut wires, which is a few more than I expected and this puzzles me.

I hope that more experienced and more knowledgeable forum members will be so kind to answer my questions:
Most important question: will the XRT be suitable for a Fusor high voltage power supply (HVPS), or did I acquire a hi-tech doorstopper?

What purpose has the smaller coil outside the core (left of the image). Is it for heating the X-ray tube? Can it be missed or is it necessary for operating the XRT?

For my other questions I have marked the ten wires according to a color and wire thickness system. Also I did some resistance measurements between wires and core.

Input side (right side of the image):
The YG lead (yellow/green) has a resistance of 1 Ω across the core, which I suppose is therefore to be connected to ground. This makes sense considering the colour code yellow/green.

The resistance between yi1 and bi1 (yellow and blue, both combined in a yellow sleeve) measured 55.7Ω. When connected to mains (230V) this would draw just over 4 Amps. equivalent to 950W and therefore it could be the mains wire.

The second yellow and blue set of wires (yi2 and bi2) measured 1.3 Ω. It seems unlikely that these wires should be connected to mains. What are these?

Also on the input side is a single yellow wire (yi0) which appears to have an infinite resistance when measured to the other leads in the input bundle, but it measures a resistance of 80k Ω with the blue wire on top of the right big coil (Bo).

Output side (top and right of the image)
The smaller coil on the outside of the big core is connected with one yellow wire (yo1) to the big yellow wire (Yo1) that comes from the big coil, or with other words the two coils are connected in series. The resistance between these combined wires (Yo1 and yo1) and the core mass measures also 80 kΩ.
The resistance between the two wires of the smaller coil (yo1 and yo2) is 0.5 Ω.

I’m very grateful for suggestions which wires should be used for connecting the XRT into a HVPS circuit.

[Rapp Instruments]

Hi Fred,
at the end You got this transformer, I was also bitting for it, congratulations.
I think the left, smaller coil is the filament transformer for the cathode. yo2 and Y01+yo1 was connected to the cathode of the x-ray tube, Bo was connected to the anode. The tube was operated earth symmertical. Maybe the connection between the big coils is or should be connected to the ground. For a fusor with normaly grounded vacuum housing You can only use half the voltage, 42,5 kV, but still quite enough.

Thomas

[Chris Bradley]

A question is whether it is mains, or is intended to operate at a higher frequency?

[Fred Stock]

Thank you, Chris, for your answer. It is indeed the question if this is mains or higher frequency operated. I have the impression that the XRT is rather modern and that makes higher frequency operated more plausible. This is something I need to find out.

Thank you, Thomas, for your reply. I'm sorry that I have cut you off in the process of obtaining this XRT. I know your website and I'm sure that you had some nice experiments in mind with it!
Your explanations seem correct to me. I do expect that it is a symmetrical XRT because I measured the same resistance of 80 kΩ over both the big coils: one between Yo1 (the left big coil in the image) and the core, which means that this coil is connected to the core. The other between yio and Bo (the right big coil), which probably means that an mA-meter was connected to yio. As far as I know this is the classical design of an XRT.
It still remains the question if I can remove the smaller outside coil or just leave it and disconnect it from Yo1.

I could not see the actual connection of one of the coils to the core. My first evaluations and measurements had to be done quickly in the open air because I was banned from the house and it was freezing cold. The XRT had a strong smell of mineral oil and that gave it a very low WAF (wife acceptance factor).
Assuming that the XRT is mains and that the 85 kVp and 15 mA are correct, than it is my intention to construct a HVPS for minus 40 kV DC.

[Fred Stock]

Thank you, Chris, for your answer. It is indeed the question if this is mains or higher frequency operated. I have the impression that the XRT is rather modern and that makes higher frequency operated more plausible. This is something I need to find out.

Thank you, Thomas, for your reply. I'm sorry that I have cut you off in the process of obtaining this XRT. I know your website and I'm sure that you had some nice experiments in mind with it!
Your explanations seem correct to me. I do expect that it is a symmetrical XRT because I measured the same resistance of 80 kΩ over both the big coils: one between Yo1 (the left big coil in the image) and the core, which means that this coil is connected to the core. The other between yio and Bo (the right big coil), which probably means that an mA-meter was connected to yio. As far as I know this is the classical design of an XRT.
It still remains the question if I can remove the smaller outside coil or just leave it and disconnect it from Yo1.

I could not see the actual connection of one of the coils to the core. My first evaluations and measurements had to be done quickly in the open air because I was banned from the house and it was freezing cold. The XRT had a strong smell of mineral oil and that gave it a very low WAF (wife acceptance factor).
Assuming that the XRT is mains and that the 85 kVp and 15 mA are correct, than it is my intention to construct a HVPS for minus 40 kV DC.

[John Taylor]

If the core is 9kg and the transformer is rated at over 1200 watts (85kV x 15mA), I would venture to say it is NOT meant to be run off 50-60Hz line frequency. I would be willing to bet that the core is ferrite and that this transformer operates off tens to hundreds of kHz.

I would most likely connect it to a variable frequency generator and vary the frequency until I found out about where it was resonant. If you don't have the means to do this, then this transformer may not be the best choice for you. When doing this, you put a small voltage level alternating current on a winding and measure the voltage coming out of the other windings. This information, along with other stuff, like resistance of the windings and size of the conductors going to those windings will tell you a lot about which winding is which. I would think that there would be a noticeable difference in the size of the primary (lower voltage & higher currrent means larger wire). After you find out, you can apply a known AC voltage of variable frequency to the primary and as you vary the frequency, measure the output voltage. As you increase the frequency, the output voltage should increase for a while and then after a certain frequency, voltage will probably start to decrease with increasing frequency. This will tell you where the resonant frequency is roughly. This resonant frequency will change when you add components on the output or secondary winding. Most transformers are probably not designed to operate at resonant frequency, so some frequency below this is where I would try to operate it. There are others on this site that are much more knowledgeable of this than I am. One other thing that may be misleading is that some x-ray transformers have a very low voltage winding for the filament of the x-ray tube.

I would be surprised also if this transformer were designed to be operated directly from 120 or 240 VAC mains voltage at any frequency. A lot of high voltage power supplies for x-rays that I have seen have a driver circuit that provides something like 400-1000 volts that is applied to the primary. A lot also don't put out the rated voltage from the transformer alone, but have a voltage multiplier to step the voltage up further from the transformer. Many have the high voltage transformer and the multiplier in an enclosure that is filled with oil to provide arc protection as well as heat removal.

The very best thing in my opinion would be to have the manufacturer's specifications for this transformer, but often that is impossible.

Of course, with the right driver circuitry, if it is intended for high frequency, the filter caps and/or voltage multiplier capacitors could be much smaller (and cheaper) values to give the same results as you would achieve with line (mains) frequency transformers and capacitors. Besides being much smaller and more efficient, higher frequency power supplies do not contain as much stored energy and are less dangerous than low frequency, large capacitance versions.

[John Futter]

John et al

the clues are all there, you know the voltage and current it can produce with probably a little margin.
first using a low voltage and a multimeter or oscilloscope work out what the turns ratio is --this will work at all frequencies
measure the primary inductance and resistance - if you can at several different frequencies

this is now enough info to work out what frequency is required to power it up with a reasonably close educated guess.

If you cannot work it out post the measurements and i'll do it for you

[Chris Bradley]

It was a hunch that this could possibly be a steel transformer designed to operate at audio frequencies.

Built on a steel framework looks a tad too bulky for a ferrite, but is clearly too light for a mains.

Might be worth getting a frequency generator and an old audio amp out for some investigations.

Just a suggestion.

[Fred Stock]

Thank you all for the valuable advises. I shall set up a number of tests as recommended. I have a function generator and a scope, so at least this should bring me some results.
The core is -as far as I can see- probably all iron as can be seen in the image:



The radial core blades definitely will need vacuum applied onto the transformer housing to get them filled with transformer oil again!

[Richard Hull]

The laminated core looks like mains operation, but very short duty cycle at rated output. The industry realized that with modern films and screen backings and now digital imaging, that there was no need for significant transformers in any x-ray machine as exposure times would rarely exceed 1 second in the medical biz. (especially dentistry)

The lowest ohm coil should be a filament winding and it might be center taped to the core via a milliamp meter. You have a lot to guess at there.

The change over to HF stuff was significant with ferrite cores. This guy is laminated and probably can be used with the mains, making it rather bullet proof without the need for HF electronics. A giant leap backwards, based on ever shortening exposures in dentistry and a .0001% duty cycle.

Whether it is suitable for fusor use is another question...That I can't answer. Fusors have 100% duty cycle at full current for many minutes at best and an hour or two at worst. Off the mains, this usually means big, big iron in the core.

Naturally, if you try and use it, it will have to go back under oil for cooling and insulation.

I currently have a 0-80kv 50ma rated DC supply for an old 70's XRF machine using a separate mains fed 0-240 volt variac that I haven't had the temerity to wheel into my fusor system yet. Needless to say, this is a high duty cycle item.

Without the large variac, just the supply is a cubical shaped, steel cased affair that is about 2 X 2 X 2 feet and weighs in at 760 lbs! It has giant "eye hooks" for chain hoisting. That is a suitable mains system for a really good fusor if working off the mains with rather continuous operation. Just sayin'

Richard Hull