HT x-ray transformer cable insulation short

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Thomas Sheeleigh
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HT x-ray transformer cable insulation short

Post by Thomas Sheeleigh » Sun May 17, 2020 1:32 am

I purchased this large Hitachi medical x-ray transformer second hand about 1.5 years ago but have only just gotten around to experimenting with it. I have posted about this transformer a while ago inquiring about the terminals on the top of the transformer's case as they are not something I have seen before (and the label on the transformer only states model # and serial number), and I haven't been able to find much information online (made in 1985). I gathered some voltage measurements that I am hoping may hint towards the purpose of these terminals. below I have posted some pictures and the data I gathered.
20200516_191731.jpg
These are the terminals mentioned (on transformer body)
The black components attached between "N", "NE" and earth (one of the body panel bolts) are labelled as 2NR14K391 and appear to be Panasonic varistors. There is also a broken component which appears to be a capacitor but the markings are not wholly visible. T1 and T2 seen on the bottom right are the terminations of the primary coil.
transformer test.JPG
Setup for measuring output voltage
transformer test.JPG (24.2 KiB) Viewed 431 times
This is the setup I used to measure the voltage values seen below. In addition I took measurements on the C, Cs1 and Cs2 terminals on the transformer body (using "N" terminal as reference). These were taken with a primary voltage of 1.87VAC
V(c-n)=2.52VAC
V(cs1-n)=2.54VAC
V(cs2-n)=2.54VAC

As you can see, these values are all the same so I would guess that they are in parallel with each other. Their purpose however, remains a mystery to me.

tfx measurements.JPG
Voltage measurements made on secondary cables
These are the measurement values I found. Note that because of the resistor network, all values should be multiplied by 100 (16.7=1670V). All the conductors appear to be at the same potential, however the diagram I found (last image) sates that the two bare cables ("E" node) are not high voltage carriers, im not sure what their purpose is.
candlestick and recepticle.jpg
Transformer side cable "candlestick" and receptacle
This is a photo of the cable receptacle on the transformer. These cables appear to be a standard in high-tension medical transformers. I'm not sure why there are 3 conductors as the output is rectified DC. The values I measured are all equal which suggests that all 3 conductors (bare cables are in parallel) are at the same potential.
cable.jpg
Cable end
This is a picture of the cable end. You can see 4 conductors here, the 2 bare cables are tied to one of the 3 pins seen on the cable in the previous picture. The other two cables (yellow and gray insulation) are connected to the remaining 2 pins.

In addition, I am having a problem with the transformer cable's conductors shorting themselves and burning/lighting the insulation on fire! I am not sure as to the exact cause of this but I have a theory. I found a cross-sectional diagram of the cable (High-Tension xray cable type L3) and it appears as though there is a thin layer of semi-conductive tape around the conductors. This "shorting" happens when the output exceeds about 7kV so I suspect that the high-voltage is arcing from the conductor to this layer of tape. However, I really do not know for certain so any ideas would be appreciated. I attached a photo of the diagram referenced.
Capture.JPG
HT cable cross-sectional diagram
Finally, here is a video showing the "shorting" behavior. I only allowed the voltage to go high enough to show this behavior but it gets more violent as the voltage is increased. The black insulation layer seen on the cable (the further back grey stuff is just duck tape) is the High-Voltage insulation layer labelled in the diagram. The cables are rated for 75Kv, and although I did cut it in half to expose the conductors (it was originally 1 cable) I would think this behavior is unusual.

https://www.youtube.com/watch?v=IdQt4CpKDSI

Any information/ideas as to the purpose of the "C", "Cs1" and "Cs2" terminals as well as the reason for the cable "shorting" would be greatly appreciated. As it stands I am not comfortable with using this piece of equipment in my house if the cable is at risk of catching on fire. As the arc appears to be on the cable and not through the air, I dont think that even oil will prevent this.

John Futter
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Re: HT x-ray transformer cable insulation short

Post by John Futter » Sun May 17, 2020 8:16 am

There are three terminals so the x=ray tube filament can be lit the filament is typically 3 -7 volts not very much difference out of 80kV
so you have two filament pins and other one cathode (indirectly heated cathode or one grid pin grid typically is 0-300 volts less or more negative than direct heated cathode.

if you are not conversant with the terms i have used above--- do a lot of googling on them so you understand

John Futter
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Re: HT x-ray transformer cable insulation short

Post by John Futter » Sun May 17, 2020 8:27 am

Semiconducting tape is connected to cathode via high value resistor so as to fix the electric field in a more smooth way around the central wires ( to help stop corona).
Correct termination of the wires is required so as to not burn up the cable.
What is in the picture is exactly wrong and will burn the cable up at very low voltages as you have found out.

see here for what a simple google search came up with
your cable seems to a version 2181-1

A wide range of unshielded cable types for
voltages up to 150kVDC. Many cable types
are furnished with semiconductive layers.
These layers are smoothing the electrical
field, lowering the field strength and
provide low partial discharge levels.
Silicone, PE and EPR dielectrics are
standard – miscellaneous dielectrics on
request.
UL recognized appliance wiring material –
 AWM style 3239 and other UL styles.

Thomas Sheeleigh
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Re: HT x-ray transformer cable insulation short

Post by Thomas Sheeleigh » Sun May 17, 2020 8:29 pm

It seems like moving forward I should properly terminate the exposed cable ends. I found another post that contained information regarding this topic here.

viewtopic.php?f=11&t=4481&p=29040&hilit ... ion#p29040

When terminating the cable, is it preferred to keep the full cable intact where the splicing occurs (as apposed to exposing different layers of the insulation at different lengths from the splice as I have done)?

This still leaves me with the problem of what to do about the conductors. John, you had mentioned that the filament conductor is typically a few volts difference from reference while the cathode conductor has a few hundreds volts differential from reference. Please excuse my ignorance as this is my first dive into DC high voltage transmission. Would connecting the conductors together (making one termination) be possible? I would think that it would be OK given that both the filament and cathode lines carry rectified DC signals.

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Richard Hull
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Re: HT x-ray transformer cable insulation short

Post by Richard Hull » Sun May 17, 2020 9:19 pm

We might assume that the filament leads are the HV leads. Do not short them!! the ground in this system is assumed to be positive HV DC with the negative hot connected to the cathode/filament leads. This was a common practice in such supplies.

I would suggest a huge battery of ohmic readings. (not given)

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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