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Re: Dead xray transformer (fwd)



---------- Forwarded message ----------
Date: Sat, 28 Mar 1998 09:21:53 -0800
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Dead xray transformer



Following message much edited to save download and reading time
 
> ----------
> 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
> 
> 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!

I might add that this is a fairly common strategy for HV supplies. The
other reason it is done (for three phase sources) is that one transformer
is connected in Y and the other in Delta, so the ripple out of the
rectifiers is out of phase, and when combined it produces even less ripple.

> $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?

Perhaps it was a Flash X-ray pulsed unit? These use a small Marx bank
charged to 100 kV stage voltage to produce very short, intense pulses of
300 kV - 1 MV xrays.

> 
>  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!

> 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.

This is how my Xray unit works. It senses the current in the HV secondary
(between the two HV windings, near ground potential) and then modulates the
power going to the filament to adjust the current to the nominal 100 mA. Of
course, there is all sorts of other stuff to start the exposure with a good
guess as to the filament current, and so on.