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Cascading Transformers
Subject: Cascading Transformers
Date: Mon, 21 Apr 1997 20:03:57 GMT
From: robert.michaels-at-online.sme-dot-org (Robert Michaels)
Organization: Society of Manufacturing Engineers
To: tesla-at-pupman-dot-com
Cascading Mil. Spec. Power Transformers
---------------------------------------
To some of you this may be blitheringly obvious. To others
--- well --- perhaps not, judging by some of the posts I've
seen here recently.
The essential idea is this: =If= one has a power (or plate)
transformer which was manuf. to military specifications, then
the ratings on the spec. plate of such a transformer can be
taken as =starting point= or minimum values.
A case in point: Imagine such a transformer rated at 120-V.
in and 1000-V. center-tapped out.
(The center tap is irrelevant here but nearly all
power transformers are so-designed. [ Hence the
secondary voltage may be give as 500-0-500 ] )
Two such transformers may be connected back-to-back to obtain
approx. 8.33-kV with relative impunity.
This connection is illustrated below. The secondary of the
first transformer is used to power the primary of the second.
Given the transformation ratio of 8.333 (1000-V/120-V =
8.3333),
and -- given that the primary of the second transformer is being
supplied with with 1000-V. from the first, Then approx. 8-kV.
(less the losses incurred) may be expected from the secondary
of the second transformer -- 1000-V x 8.333 = 8.33-kV.
- - - - - - - - - - -
One might wonder how it's possible to get away with such a
thing.
The answer lies in using =only= mil. spec. transformers. They
are typically underspec'd. by a =very= wide margin.
Mil. spec. transformers are ferociously expensive when bought
new. Most fortunately, they are the stock in trade for most
surplus and electrical junk dealers. Also most fortunately,
the vacuum tube equipment for which they were designed is going
out of style at a high rate of speed. So, such transformers
go for very little money and seem to be a glut on the market.
- - - - - - - - - - -
Note 1: The above presupposes two identical transformers.
Such
need not be the case. If the first transformer were
rated at 500-V, then the final output would still be
a respectable and =much= more conservative 4.17-Kv.
and well within the actual specs. of many a 1000-V.
mil.
transformer.
Most any mil. spec. transformer worthy of the
appellation and rated for 1000-V. output would
be insulated for at least 5000-V.
Alternatively -- the center-tap of the first trans-
former may be used to power the primary of the sec-
ond for the same conservative results.
Note 2: Only the second of the two transformers is
over-stressed
so only it needs to be mil. spec. The first
transformer
could just as easily be commercially rated, or in fact
merely something pulled from an old tv.
Note 3: This license to play fast and loose with transformer
specs. applies mostly to voltage ratings. If you kick
up the voltage of a 1000-V. transformer to 8000-V. you
must take care to reduce the current draw in near
proportion. So, if that 1000-V. transformer is rated
at 500-mA., then you've got 60-mA. available
per spec. or perhaps 100-mA. if you want to gamble a
little (a little =more=).
In using two disparate transformers, it's the power
rating of the lowest-rated transformer which
determines
how much current can be drawn from the pair.
||c---------------- ||c------------------->
||c |___
||c
-----------C||c C||c output
120-V. C||c--c.t. C||c--- c.t. approx.
in C||c C||c 8.33-kV.
-----------C||c ___C||c
||c | ||c
||c---------------- ||c------------------>
First Transformer Second Transformer
120-V. primary 120 V. primary
1000-v. secondary 1000-V. secondary
c.t. = secondary center-tap.
Damn the specs., full Tesla-ing ahead,
Robert Michaels - Detroit, USA