Re: Higher Mains Frequency

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jolyon Vater Cox by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, June 18, 2003 3:03 PM
Subject: RE: Higher Mains Frequency


 > Original poster: "Steve Conner by way of Terry Fritz
<teslalist-at-qwest-dot-net>" <steve-at-scopeboy-dot-com>
 >
 >
 >
 > -----Original Message-----
 > From: Tesla list [mailto:tesla-at-pupman-dot-com]
 > Sent: 18 June 2003 01:39
 > To: Tesla-at-pupman-dot-com
 > Subject: RE: Higher Mains Frequency
 >
 >
 >  >The concept indeed looks interesting, but why change an already
perfectly
 >  >working system!!!  60Hz is more than readily available - why add the
 >  >complexities of a multi-kilowatt inverter system.
 >
 > Because it's fun darnit! And educational too. People probably said the
same
 > about the SSTC when it was first invented.
 >
 > Anyway, personally I have two thoughts on this.
 >
 > 1) By driving a transformer at a higher frequency you can get more voltage
 > before it saturates. Ferinstance an MOT at 400Hz could give 12kV and (more
 > importantly) one of those little 12v:240V transformers would give 1600V.
 > Also they have chambered plastic bobbins that give good primary/secondary
 > isolation.
 >
 > One could imagine an 'MMT' made of about a dozen of these little
 > transformers in series, maybe in oil, with the 12V primaries all in
parallel
 > driven off a half-bridge inverter that runs off the rectified 120V line.
An
 > HVDC supply would be equally easy to make by just putting a small bridge
 > rectifier and cap on each transformer and stacking them in series.

Problem- just what is the maximum voltage the little plastic bobbin  will
withstand? I would imagine that allowing a safety margin of least twice the
voltage expected would be a wise precaution e.g if the max rating of
inter-winding/ winding-to-core insulation under test conditions is 5kV, then
the maximum voltage it should be  exposed to  in use should be no  more than
2.5kV.

The difficulty with stringing together all the low-voltage windings and
mains windings respectively to make a high-voltage transformer is that the
more transformers there are in the string, the more the voltage on the
high-voltage output leads  approaches the maximum  allowable voltage of the
interwinding/ winding-to-core insulation
if this is 5kV the max output (allowing for a safety margin of 2) will only
be 2.5kV.

The voltage rating of the interwinding/ winding-to-core insulation could be
doubled by wiring all the transformers in pairs with their cores "floating"
(i.e. mounted on an insulator) and their low-voltage windings tied together,
the first transformer (A) working as a "step-down" and the second (B) as a
"step-up"
at the cost of (slightly) greater circuit complexity and twice the weight!
The primaries to "A" transformers would of course have to be connected in
PARALLEL across the mains supply
and all the secondaries of the "B" transformers would have to be connected
in series.

  Also it must be remembered that all those little transformers are voltage-
not current- limited (they are like pole-pigs in this respect if no other!)
and will therefore require some form of external current limiting.

Jolyon