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Dead xray transformer


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From:  Bill the arcstarter [SMTP:arcstarter-at-hotmail-dot-com]
Sent:  Friday, March 27, 1998 8:08 AM
To:  tesla-at-pupman-dot-com
Subject:  Re: Dead xray transformer

Coilers,
 I sent this message to hollmike-at-aol-dot-com regarding a dead xray 
transformer.  Perhaps someone else on this list would find it useful!

(I deal with medical xray equipment during my "working" hours!)

 Our X-ray power supplies are rated at 20 to 60 KW.  Input is 220-440 
volt, single or 3-phase.  We don't mfg them, but buy them from outside 
vendors and integrate them into our product.

These units cost (typically) $10,000 - 20,000 new.  Typical weights 
range from 400-800 lbs.

The output is typically not a smooth DC - there are no output filter 
caps.  The output is a pulsed-DC in sync with the mains frequency.  This 
means that x-ray production is also pulsed, which is not a problem for 
our application.  Our camera system grabs the images in sync with the 
radiation bursts, etc.  Of course, the 3-phase units have a smoother 
output.

Voltages are from 40-150KV, at currents up to 400ma.

Rectification is performed by either a vacuum-tube bridge, or a large 
solid state diode bridge, like the one on my page: 
http://www.geocities-dot-com/CapeCanaveral/Hangar/6160/electric.html

Note on that bridge - there are two separate 70KV bridges in series.  
The large transformer has two separate 70KV windings, which feed each 
bridge.  This is done to ease the insulation requirements of trying to 
build a single winding capable of producing 140 KV!

Now - We do use one model which is smaller - only 200 lbs or so.  This 
is actually a switcher - the input power is rectified into a large bank 
of caps at 400-600 volts (depends on the line voltage).  This DC is 
chopped, etc, just like any switcher, except the output is 40-140 KV!

Also - I think these switchers don't have a very large or complicated 
rectifier, as if the input switching is done properly, the output will 
be mostly uni-polar.  But I'm not completely certain.

>was meant to power the X-ray tube and has a control circuit attached 
with all
>the caps etc(I warned him to bleed the caps before he even thought 
about

$50K for an x-ray generator, eh?  That seems high, even including the 
tube!

Sounds like the great expense in that unit was the caps.  Perhaps the 
inspection process required a uniform (non-pulsed) or continuous-duty 
production of x-rays?

 I would think these caps might be useful for TC use.  They are 
certainly rated for a high enough voltage.  But - are they pulse-duty? 
Probably not, but perhaps the price is right!

> I am
>guessing that the control circuitry has failed and not the tranny 
itself. 

Our systems use a separate filament power control system.  That is, the 
power transformer is not current limited.  However, the filament 
temperature inside the tube only liberates a certain number of free 
electrons, hence the tube current is limited.

Perhaps only this filament control is burned out on this NJ unit??  If 
so, the rest might be useable.

>figure some HV switch or similar component failed(maybe a tube 
rectifier if it
>is DC out).  He said the replacement unit cost $50K.  I figure the 
tranny
>should be worth a lot if it is intact or repairable in a manner similar 
to
>neon repair.  I would not be opposed to unwinding it to half the 
voltage
>output if necessary.  

Also - our units are center-grounded, just like a neon.  I'd expect your 
supply to be the same.  Thus - using one hot wire to ground would get 
you down to 30 KV.  IF it takes 240 volts in, then running it on 120 V 
would get you down to 15 KV.  You have arrived!

I'd say it might be useable!  Since this is a 4KVA unit, you might not 
even have to ballast it (much)!

If it is a switcher (look for a large 400-600 volt cap bank hooked 
directly to the input mains) then it might be very hard to get going 
again for TC use.  (Depends on what is burned out)

Our non-switchers consist of a huge spiral-wrap of transformer steel, 
just one single lamination (one strip, about 4 inches wide, about 4 
inches thick, and about 1.5 feet in diameter!  This is immense!  If this 
NJ unit is a non-switcher and you aren't adverse to rewiding it, you can 
certainly build a pig-type supply out of the core, etc.

I think the key is to identify the voltage rating of the cap bank.  This 
might tell you if it is a switcher.

Go for it!  But don't get yourself "kilt"!

-Bill the arcstarter
Starting arcs in Cinci, OH
http://www.geocities-dot-com/CapeCanaveral/Hangar/6160