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Re: Cascading Transformers
Subject: Re: Cascading Transformers
Date: Wed, 23 Apr 1997 15:14:36 -0400
From: "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
To: "Tesla List" <tesla-at-pupman-dot-com>
>
> 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.
>
So far so good. Mil spec stuff can almost always be pushed as regards
the
electrical specs. Pushing to 200% is rather commonplace.
> A case in point: Imagine such a transformer rated at 120-V.
> in and 1000-V. center-tapped out.
OK, imagination tuned in and turned on to 100% of normal capacity.
Entertain my imagination...
> (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.
>
URK. URK. URK. Imagination overload! Switching to reality mode!!!
Depends on what you mean by impunity. If by impunity you mean that it
will
actually *work* without blowing the nearest fuse, then you are dreaming.
Must be the fumes in Detroit, or maybe you actually *did* get some of
that
stuff that the guy with the 2 million volt neon (for $10 bucks) was on.
(Count me out. I want to maintain my sanity a little longer, thank-you).
We are missing a basic fact. If you apply 1,000 volts to the primary of
a
transformer designed for 120 volts input, you are gonna fry the primary
*even if* the secondary of that transformer is not connected. Heck, I'm
sure there are guys on this list that can recount what happened to them
when they applied even 220 to a 120 primary with the secondary unloaded.
Its a little thing called flux density. Too much of it and the core
can't
take it anymore. Even mil spec transformers are not going to take 1000
volts on the 120 volt primary. They will start to magnetically saturate
LONG before *that*!! You might be able to push a 120 v primary to about
300
volts or so before the fuses would start popping, but that's about it.
Let's assume that (a) you have a variac driving the primary of the first
transformer, (b) the two transformers are connected as you describe, and
(c) the second transformer has NO LOAD whatsoever. (Making it totally
useless for doing anything, by the way, but let's do it this way just
for
jollies to see what happens.)
What happens is that as you turn up the voltage on the first primary,
the
secondary feeds its voltage into the second primary. When the second
primary gets to 120 volts you can be sure that simply due to magnetic
losses there will be a primary *current* in the second transformer. This
of
course will be supplied by the secondary of the first transformer, and
therefore there will be a primary current in the first transformer of
about
8.33 times the current in the second transformer's primary. Right now
this
will be fairly small and therefore we can ignore it. (But not for long!)
Being the adventuresome kind of guy that you are, you blithely turn up
the
variac. You increase the input to the point where the secondary of the
first transformer is applying somewhere between 200 and 300 volts to the
primary of the second transformer. The secondary of the second
transformer
is still open-circuit. Which is of no concern whatever to the
transformer,
because it has just *saturated* the core. As you non-chalantly increase
the
input via the variac, ALL of the extra input is now converted to EXCESS
flux in the second transformer. This causes its primary to start
conducting
more than reasonable amounts of current. This current is coming from the
secondary of the first transformer. By the simple law of conservation,
it
can be shown that the primary current of the first transformer is now
8.33
times the value of the current in the second transformer's primary.
Things
are starting to happen. You crank the variac up another notch. WHOOPS!!!
That second transformer just SATURATED, remember? The current in its
primary is attempting to go as high as nature will allow it. Primary of
first transformer needs to have current 8.33 times bigger than that
flowing
in its secondary. First transformer heroically attempts to supply
requested
power to second transformer. 8.33 times Reasonable=UNREASONABLE. Fuse
pops.
OK, assume you are not using a fuse. First Transformer smokes. Smoking
is
not good for transformer's health. Transformer dies.
> 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.
Yes, one might. And with good cause, I might add...
The current in the primary of the first transformer will be 8.33 times
the
current in its secondary. *Assuming* the thing is working as you
describe,
the current in the primary of the first transformer will be 8.33 *TIMES*
8.33 the current in the secondary of the second transformer. That's
about
69 times the current of the final secondary. Yes, you say, but we are
running with the secondary unloaded. Even so, you are totally ignoring
the
flux issue! The second transformer is going to start drawing ghastly
amounts of primary current as soon as its core saturates, because then
it
is limited only by its DC resistance! Which is pretty LOW. That strange
humming sound you hear is not the buzz from your latest inhale of
whatever
it is you are smoking. And YOU are not the *only* thing smoking in the
room
at this particular moment in time!
You don't need two transformers to prove what I am saying. Hook up a
light
bulb in series with the 12.6 volt side of a 12.6 volt transformer (or
any
other low voltage transformer). Connect bulb/low volt side to variac and
slowly increase voltage until lamp *begins* to light. Measure voltage
across low volt side of transformer. This is the voltage at which this
particular transformer begins to saturate with a no-load secondary
condition.
Here's another fun demo. Once you get the 110 volt lamp lit dimly, try
shorting what used to be the primary (black wires, which are now the
secondary). Ooooh! Look at how the primary current increased when you
shorted the secondary, causing the lamp to get brighter. Guess what? If
the
lamp was not there you would have blown a fuse. Once the core is
saturated
the primary will draw excessive currents.
> 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.
>
You are only looking at the voltage stress issue here. By the law of
conservation, the current in the primary of the first transformer will
be
8.33 times the current in the second transformer's primary. I don't
really
think that the military stuff is built THAT much over the rated specs! I
especially don't think that they have over-spec'd the size of the CORE
by a
factor of 8.
833% over spec? I doubt it. But hey, the proof is in the doing of it.
Have
you personally lashed together two 1KV transformers and gotten 8.33 KV
for
more than 16.66 milliseconds? Maybe Gary has done something along these
lines... Gary seems to do many strange experiments. And many GREAT
experiments, too! Gary Weaver's latest post on "Best Primary Coil" was
excellent. Much better than his microwaved blackbird thing...
>
> 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.
>
This is true ONLY if you do not saturate the core of either transformer.
Once you saturate either core, all bets are off.
>
>
> ||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
To anyone attempting to try this, I suggest you try to determine where
the
second transformer is going to saturate by using a variac and attaching
the
input in series with a low wattage 110 v light bulb. When the light bulb
begins to light, you have hit the wall. Any increase from this point up
will be totally wasted energy which will be used for smoke generation.
For transformers with higher amp ratings you can use a larger wattage
light
bulb.
Fr. Tom McGahee