RE: On the Trail of a MegaVolt (fwd)

[<sroys@xxxxxxxxxxxxxxxxx>]

---------- Forwarded message ----------
Date: Thu, 11 Nov 2004 13:08:58 -0600
From: Carl Litton <Carl_Litton@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: RE: On the Trail of a MegaVolt (fwd)

Thanks, Godfrey.  I appreciate the input.

I know that some are built as you describe.  I have a few of them.  It
is a common misunderstanding, however that all are built in this way.
We are fortunate enough to have an X-ray Systems Engineer as part of our
group so I think it might be useful to all to put this rumor to rest
once and for all if possible here.

There are actually **at least** 2 other ways in which such PSU's are
designed in which there is no center tap and we have a few of these
also.  Ignoring the capacitive discharge type with a very small HV
transformer which is not relevant to this discussion, another type is
pictured on our site along with the actual schematic wiring diagram of
the HV transformed taken directly from the original manufacturer's O&M
manual showing the use of 2 separate transformers which are not core
tapped in any way and 2 separate bridge rectifiers with **both** leads
of the output of each transformer going to a separate bridge from which
the positive of one and the negative of the other are used for the
output.  See this here:

http://dawntreader.net/hvgroup/XrayPSUschematic.jpg


The M1 and M2 leads are actually the other polarity side of each bridge
and are interconnected by way of a large varistor.  As overvoltage
protection, 1 of the meter leads is connected back to mains ground to
give a path through which the full kilovoltage is shunted in case of a
fault that trips the varistor.

Removing the bridge rectifiers, filament transformers, and M1, M2 leads,
leaves 2 (two) 62,500 volt transformers with primaries in 240 Volt
parallel and phased oppositely and not tapped to the frame in any way..
The secondaries then may be put in either series for the full rated
voltage or in parallel for 62,500 Volts at twice the current.

It may very well be that the megavolt is impossible due to insulation
issues but it will not be because the transformers are center tapped
making series connections impossible.


Thanks again

Carl Litton



-----Original Message-----
From: High Voltage list [mailto:hvlist@xxxxxxxxxx] 
Sent: Wednesday, November 10, 2004 8:15 PM
To: hvlist
Subject: RE: On the Trail of a MegaVolt (fwd)

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Wed, 10 Nov 2004 19:49:28 -0600
From: Godfrey Loudner <ggreen@xxxxxxxx>
To: 'High Voltage list' <hvlist@xxxxxxxxxx>
Subject: RE: On the Trail of a MegaVolt (fwd)

Hello Carl

In x-ray transformers, the bottom center of the high voltage coil
section is grounded to the core. A lead from one side of the center is
grounded to the tank, while a lead from other side of the center is
connected to a MA terminal on the tank. A milliammeter is connected
across the ground terminal and the MA terminal. With the core resting in
a frame welded to the tank cover, the center becomes ground the core.
With this arrangement, there is no need for substantial insulation
between the bottom center and the core or primary. Even if one
rearranged the leads so that the high voltage section was floating above
the core, there would be internal arcing. So these x-ray transformers
cannot be stacked for higher voltage, and 1000,000 volts is out of the
question. 

Mots can be stacked. There are pictures on the internet somewhere of
such stacks in action, but I forget where.

Godfrey Loudner      

This preamble brings me to my question that I want to submit for review
and comment by all the great science available on this list.  Has anyone
actually used any of the so-called 'stacking techniques' for MOT's that
are all over the HiV pages on the net but without any real photos of
such in actual operation?  These techniques purport to allow the
'stacking' of notoriously friable MOT's such that 4 to 8 times their
2100Volt rated output can be continuously produced without subjecting
the windings to more than twice their rating.

One such advises the direct connection of the secondaries of 2 MOT's to
produce "120 volts which 2100Volts above ground" at the primary of the
second MOT???  I can't get my mind around this to believe it.  Then the
technique advises that the primaries of 4 more MOT's may now be
connected to this output in 'anti-parallel' manner.  What in the world
is 'anti-parallel?'  Then the secondaries of the 4-pack may be series
for a usable 8400 volts.  Further, it states a 'mirror image' stack can
be built and added in series to give twice that voltage.

This would, of course, be wonderful if it were true.  Since most (~ 60%
of them) of these megavolt transformers are not center tapped, an MVT
stack is certainly conceivable following the same concept.

Please share your thoughts.

Carl Litton, Director
Raleigh-Bartlett Science Club