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Re: Cascading Transfo
Subject: Re: Cascading Transfo
Date: Mon, 28 Apr 1997 16:29:55 -0400
From: "Thomas McGahee" <tom_mcgahee-at-sigmais-dot-com>
To: "Tesla List" <tesla-at-pupman-dot-com>
>
Robert,
I am responding to your responses to my response to your original post
not
out of any desire to keep this thread going (like you, I have many other
more important things to attend to, but I'm too lazy right now to do any
of
THEM). I *do*, however, have a concern that someone else out there who
doesn't understand what is going on when you cascade transformers might
ruin a perfectly good transformer.
First, let me say that you are perfectly correct in saying that
transformers can be cascaded. Heck, our whole system of electrical
distribution is based on transformers being connected to transformers
being
connected to even more transformers. So obviously, that is not what I am
concerned about.
The basic thing that I wanted to point out is that there is a LIMIT to
how
much voltage you can put across ANY transformer winding before excessive
currents begin to flow. In your original post you explicitly mentioned
applying 1,000 volts to a 120 volt winding.
The *no load* condition that I mentioned is actually the most BENIGN of
all
conditions.
In a perfect transformer with a humongous iron core the primary voltage
can
indeed be made quite high. Transformers with small iron cores will
saturate
more rapidly than ones with large iron cores. Assuming a real-world
transformer, it will have a point where excessive primary current will
be
drawn even *if* there is absolutely no current being drawn from the
secondary. Needless to say, if there is ANY secondary current, that will
be
reflected back at the primary as an even larger primary current
(assuming
the 120 to 1000 volt transformer as an example).
OK, now let us assume that the saturation occurs when the primary
receives
500 volts. (We don't know where this will actually occur without
experimenting.)
To understand what will happen, I will assume that we are slowly
increasing
the input to the first transformer's primary. Up to the point where the
first transformer's secondary reaches about 500 volts the primary and
secondary currents will be very very small. As the secondary voltage
attempts to rise above 500 volts, the second transformer's primary will
begin to cause saturation of its own CORE. We are talking here about the
number of magnetic flux lines the core can handle. This is a VOLTAGE
THING,
not a current thing. Once you have exceeded the allowable number of flux
lines, any further increase in the second transformer's primary
*voltage*
will cause power loss in the transformer. To put it another (simplified)
way: once the core has been saturated, ALL input *increases* can be
treated
as if they were an *excess* voltage applied across only the DC
resistance
of the winding. (That explains why power increases so drastically once
you
hit saturation!)
This means that now increasing the primary *voltage* will result in
increasing primary current. Remember, that up to now *neither*
transformer
has had any appreciable currents flowing in either their primary or
secondary windings. But once the second transformer begins to waste
power
due to saturation, THAT current will be drawn from the secondary of the
first transformer, which will in turn get it via the obviously increased
primary current of the first transformer. Just as the voltages are
cascading UP from input to output, so the currents are cascading UP from
output to input!
Now we are at one of the points of contention. You contend that the
secondary of the first transformer is only rated at 1 amp, so it's
primary
will only draw 8.33 amps. Don't confuse maximum rating with current
limiting. *I* on the other hand, contend that since this is a wonderful
under-rated MIL-SPEC beast of a transformer, that the first transformer
will valiantly attempt to supply *whatever* current the second
transformer
requests. As we continue to raise the first transformer's primary
voltage,
the second transformer will demand MORE than one amp at some point in
time.
Now exactly WHERE this point is depends very much on the transformer in
question.
Let's take a little step back right here. There are a bunch of
interacting
things going on right now that need to be carefully looked at. When the
second transformer begins to REQUEST more current, the resistances in
the
windings will come into play and the output of the first transformer
will
DROP a bit with each attempted increase in transformer number one's
primary
voltage. It will always rise more than it drops, but more and more of
the
time it will be causing saturation. Please note that the core saturation
occurs only during a *portion* of the AC wave. The net result of all of
this is that the second transformer now spends *some* of its time
saturated. During this saturation time it will draw current from the
secondary of the first transformer.
It is obvious that as you continue to increase the input voltage to the
first transformer, the second transformer will increase its demand for
primary current (which it gets via the first transformer's secondary).
When
the first transformer can no longer supply the current needed during
that
*part* of the AC cycle where saturation is occuring (the positive and
negative peaks), then the waveforms will become very distorted. Thus the
cascade circuit will tend to self-limit itself around these peaks. As
you
further attempt to increase the main primary voltage, the time during
which
waveform distortion occurs will increase, causing the average power
dissipation to increase.
I agree that the secondary will not be able to supply the requested
current. It will, however, TRY to supply as much as it can. And since it
is
NOT current limited, I contend that it will injure itself in the
attempt.
If there is current damage, it will occur at the secondary of the first
transformer. If there is voltage damage, it will occur at the secondary
of
the second transformer.
Please note that what I am fundamentally saying is that for most
transformers there is an input voltage beyond which the transformer
saturates. I believe it is long before you would reach 1000 volts on a
110
primary. Yes, I can believe that you might be able to run 500 volts in
and
not yet reach saturation.
By the way: whether you are saturating or not, if you attempt to draw
more
than 1/8.33 amps from the final secondary, then you may burn out the
secondary of the first transformer as it attempts to supply the
necessary
primary current to the second transformer.
See also my additional comments interspersed in the following thread.
Fr. Tom McGahee
>
> T>>
> T>> Cascading Mil. Spec. Power Transformers
> T>> ---------------------------------------
> T>>
> T>> To some of you this may be blitheringly obvious. To others
> T>> --- well --- perhaps not, judging by some of the posts I've
> T>> seen here recently.
> T>>
> T>> The essential idea is this: =If= one has a power (or
> plate)
> T>> transformer which was manuf. to military specifications,
> then
> T>> the ratings on the spec. plate of such a transformer can be
> T>> taken as =starting point= or minimum values.
> T>>
>
> T>So far so good. Mil spec stuff can almost always be pushed as regards
> T>the
> T>electrical specs. Pushing to 200% is rather commonplace.
>
> [ ... ]
>
> T>We are missing a basic fact. If you apply 1,000 volts to the primary
> of
> T>a
> T>transformer designed for 120 volts input, you are gonna fry the
> primary
> T>*even if* the secondary of that transformer is not connected. Heck,
> I'm
> T>sure there are guys on this list that can recount what happened to
> them
> T>when they applied even 220 to a 120 primary with the secondary
> unloaded.
> T>Its a little thing called flux density. Too much of it and the core
> T>can't
> T>take it anymore. Even mil spec transformers are not going to take 1000
> T>volts on the 120 volt primary. They will start to magnetically
> saturate
> T>LONG before *that*!! You might be able to push a 120 v primary to
> about
> T>300
>
> In most of the matters discussed on this list --
> I would readily defer to your patently
> greater knowledge and experience.
>
> In the instant case I cannot.
>
> The thousand volts in - to a 120-V mil. spec.
> transformer primary may be a bit optimistic,
> but I can absolutely vouch for 500 volts in
> - to a 120-v. primary. Ran stone cold - and
> gave every indication of doing so forever.
>
Yes, but as soon as you DO hit saturation you are in for a surprise.
>
>
> T>Let's assume that (a) you have a variac driving the primary of the
> first
> T>transformer, (b) the two transformers are connected as you describe,
> and
> T>(c) the second transformer has NO LOAD whatsoever. (Making it totally
> T>useless for doing anything, by the way, but let's do it this way just
> T>for
> T>jollies to see what happens.)
>
> It was never my intention that the setup I depicted
> be run with no load. As you state this has no
> practical value.
>
> If you are proposing this as a cautionary amendment
> to the point I was making, then it is accepted with
> thanks:
>
> Don't run at no-load. Else, use especial
> caution if you do.
No, precisely the opposite: Running NO-LOAD is the safest since you draw
no
current. But once you saturate, you WILL have current flow! And it will
get
large VERY fast once you hit it.
>
> [ ... ]
>
> Your hypothetical experiment makes a =lot= of
> tacit assumptions about the relative impedances
> of the two transformers.
>
> You may be right. You may not. It depends on
> the specs. of particular transformers.
>
Absolutely correct. In general, the larger the core, the better off you
are
as regards possible saturation. Note that in some MIL SPEC transformers
SPACE or WEIGHT was the prime consideration, and so the margin on the
electrical specs may not be as great.
> - - - - - - - -
>
> One of the fallacies in your hypothetical experiment
> is the assumption that the secondary of the first
> transformer can deliver enough current
> to saturate the core of the second transformer. It
> might. It might not.
>
Doesn't matter if it *CAN* or not, it will TRY, and it is the trying
that
will kill it!
> I wonder if you are maintaining cognizance of the
> fact that the high-voltage secondary is a high-impedance
> low current source? My strong sense is that you are
> not.
>
Oh, I took it into consideration. But 1 amp at 1000 volts did not strike
me
as being indicative of a really high impedance. But then again, high and
low are relative terms.
> If one is using 1-kW. transformers, then only one
> amp will be available from the secondary, whereas the
> primary will normally draw 1000/120 = 8.333 amps.
>
Is this a current-limited transformer or a regular transformer? If it is
a
regular transformer, then the 1 amp rating is the SAFE rated deliverable
current. Short out the leads and I can assure you that there will be
more
than 1 amp there! Remember that my basic contention is that when you
saturate the primary (that is a voltage thing) then it will begin to
draw
excessive currents. You increase the primary voltage from say 500 volts
to
550 volts and the primary current goes from being a few milliamps to
several hundred milliamps. This is NOT a linear increase!!!
> In other words, the transformer would not normally
> saturate until =at least= 8.333 amps were flowing in
> the primary, yet only 1 amp. is available from the
> secondary of the first transformer.
>
Nope, saturation is a voltage thing, not a current thing. It often
*looks*
like its current related, because it is *accompanied* by increased
currents. But the trigger, the mechanism that comes into play that
starts
the old snowball rolling is voltage related. In a "perfect" transformer
that was not saturated, the primary would not draw any current if there
was
no secondary current being drawn. But there would still be primary and
secondary voltages. It's one of those weird things lots of people have
trouble visualizing. It's like the old "which came first, the chicken or
the egg" thingies. Or, to look at it another way, pressure causes
current,
even though some textbooks word things in such a way as to make it sound
like current can create voltage. You can have voltage without current,
but
you can't have current without voltage. Things move BECAUSE there is a
pressure difference. It's not a matter of "when" (they both occur at
once,
usually). It is a matter of "why" (pressure causes movement, not the
other
way around... at least as far as causality goes) Don't think about it
too
much or you'll go bonkers. But there really IS something to this
causality
stuff, and it can be important once in a while to reflect on what causes
what.
> - - - - - - - - -
>
> [ ... ]
> T>You don't need two transformers to prove what I am saying. Hook up a
> T>light
> T>bulb in series with the 12.6 volt side of a 12.6 volt transformer (or
> T>any
> T>other low voltage transformer). Connect bulb/low volt side to variac
> and
> T>slowly increase voltage until lamp *begins* to light. Measure voltage
> T>across low volt side of transformer. This is the voltage at which this
> T>particular transformer begins to saturate with a no-load secondary
> T>condition.
>
> T>Here's another fun demo. Once you get the 110 volt lamp lit dimly, try
> T>shorting what used to be the primary (black wires, which are now the
> T>secondary). Ooooh! Look at how the primary current increased when you
> T>shorted the secondary, causing the lamp to get brighter. Guess what?
> If
> T>the
> T>lamp was not there you would have blown a fuse. Once the core is
> T>saturated
> T>the primary will draw excessive currents.
>
> Forgive me but I do not see the relevance.
>
> I cannot accept your assertion that the moment the
> light bulb begins to glow is necessarily the same
> instant the core is saturated.
>
OK, then why would the bulb light at all, seeing as how I explicitly
stated
that the transformer's secondary was open-circuit? Or maybe you are
getting
confused by the fact that saturation only occurs for a PORTION of each
cycle... I had not mentioned that in the previous post. Sorry.
The relevance of the first experiment is to show the effects of
saturation.
By using a low voltage winding as the primary you don't have to get up
into
hundreds of volts before the effect becomes noticeable. The second demo
was
to show how secondary currents influence primary currents. Yep, that one
wasn't as relevant... unless you are the kind of person that actually
like
to draw some current from your secondary.
Not everyone out there has an AC ammeter. But, we can all get our hands
on
a lowly light bulb. Yes, I know it is non-linear. But anyone can throw
the
demo together and hopefully see first-hand what I am talking about. I
happen to be a person that likes to have little experiments to prove
things. Forgive me. It is one of my basic character flaws. Comes with
being
an educator, I guess. Constantly have to prove to my students the things
we
cover in class. Challenging! :)
By the way, if you use the ammeter without any sort of series load, be
prepared for some fuse-popping currents when you hit saturation! The DC
resistance of the primary is often only a few ohms!
> | Bulbs of differing wattages will |
> | beget different results in this |
> | demonstration! Try it. |
>
True. That is because the AMOUNT of current each requires to begin
visible
light emission is different. OK, so replace the light bulb with a
resistor
of about 1 ohm and measure the voltage drop across the resistor. Or be
real
adventuresome and use an ammeter. (Only if you have a series current
limiting resistor, though... otherwise you may pass excessive currents
through the meter. No joke. Please don't ruin a perfectly good current
meter unnecessarily!)
> [ ... ]
>
> T>You are only looking at the voltage stress issue here. By the law of
> T>conservation, the current in the primary of the first transformer will
> T>be
> T>8.33 times the current in the second transformer's primary. I don't
> T>really
> T>think that the military stuff is built THAT much over the rated specs!
> I
> T>especially don't think that they have over-spec'd the size of the CORE
> T>by a
> T>factor of 8.
>
> With all due respect, sir (sincerely said and meant),
> No, no, and no!
>
Hmmm. Sounds like he means it sincerely, folks.
> The secondary of the first transformer can deliver
> only 1-amp. (assuming for the sake of discussion
> it is a 1-kW transformer).
>
I heartily disagree. Here's the experiment :)
Apply 120 volts to the primary. Load down the secondary to 500 volts.
Assuming the transformer secondary does not fry long before you get to
that
point, you will discover that even though it is rated at 1 amp, it will
attempt to give you all it possibly can, up to the instant it
self-destructs. You will protest that your primary doesn't saturate
until,
say, 700 volts. OK, then load it down to 700 volts. The results will be
much the same. More than 1 amp attempted, and rapidly warming secondary
wiring to show the effort. Of course, if the saturation doesn't occur
until
past 1000 volts then I heartily congratulate you on your good luck in
finding two such wonderful trannies able to operate at 833% of sticker
rating. Yes, I DO believe it is possible. I really do. I have done
enough
weird experiments of my own over the years, and cascading transformers
was
one of them. It's just that personally, I do not want to press my own
luck
quite so far as Robert presses his. I will stick with 200% of rating and
be
happy.
> Therefore, the primary of the second transformer
> can receive only 1-amp. Normally, this primary
> would draw 8.333 amps. from a 120-volt line under
> full load.
>
The primary of the second transformer will not die. You are absolutely
correct on that point. But it WILL try to draw more than one amp once
saturation gets hot and heavy. The secondary of the first transformer
will
not be able to give much more than a couple of amps (its MIL-SPEC and is
HAPPY to give you more than its rated current, Robert. You can push it
in
MANY ways!) It will TRY to give you everything you ask. It will NOT be
able
to comply for long, however! The secondary of the first transformer will
be
the first to die, unless it is current-limited. In giving up its own
life
it will prevent the death of the second transformer. In a cascade
circuit
you can only fry one of the transformers, not both! How convenient! Of
course, if your transformer does not saturate with 1000 volts at the
primary, then you will operate as you advertised at 8.33KV out for the
next
hundred years.
> It's not a "factor of 8" as you write, but a factor
> of =less= than one-eighth (1/8.333 to be precise).
>
I think we must be talking about different things here. What I meant to
say
(sorry if I didn't actually say it that way in the first place) is that
when the saturation kicks in, the primary of the second transformer will
request current from the secondary of the first transformer, which will
in
turn request 8.33 times the originally requested current from the
primary
of the first transformer. The request can easily be for more than what
the
transformer is rated at. I'm not saying the second transformer will
*GET*
what it asks, but the first transformer will do all in its power to
attempt
to comply. The effort may be deadly to the transformer. Then again, it
may
just make grumbling noises and get a bit warm. Depends on the level of
saturation. (Yes, the transformers will often make grumbling noises as
the
waveforms are no longer sinusoidal. Instead of humming along they
grumble.)
>
> T>833% over spec? I doubt it. But hey, the proof is in the doing of it.
> T>Have
> T>you personally lashed together two 1KV transformers and gotten 8.33 KV
> T>for
> T>more than 16.66 milliseconds? Maybe Gary has done something along
> these
>
> [ ... ]
>
> As stated, this is based on practical work. I did
> use only 500-v. into the primary of 120-v. mil spec.
> transformers of various types.
>
Amazing that you hadn't saturated yet. My congratulations.
> I did =not= operate open-circuit or no load.
>
Open-circuit is actually the safest mode. Sorry if I was not clearer on
that point.
> I got the voltage boost expected with no obvious
> harm to the transformer.
>
> In one extreme case, the second transformer was
> rated for 10,000-v. output. With 500-v. in I got
> approximately 50,000-volts out, based upon spark-gap
> distance. I had expected an immediate burn-out and
> was startled at my good fortune.
>
Robert, your good luck amazes me!
> I never did any life-testing, so I have no idea if
> my 50,000-volts of good fortune would have lasted
> a week, a month, or a year.
>
I will keep my mouth shut.
> T>lines... Gary seems to do many strange experiments. And many GREAT
> T>experiments, too! Gary Weaver's latest post on "Best Primary Coil" was
> T>excellent. Much better than his microwaved blackbird thing...
>
> He worries me. ( I hope he does not live anywhere
> near Detroit.)
>
Ah, yes, the thought of the TWO of you doing push-to-the-extreme tests
using cascaded transformers to power microwave klystron blackbird
zappers
together in the same garage sends shivers up and down my spine. Brrrrrr!
> [ ... ]
>
>
> T>This is true ONLY if you do not saturate the core of either
> transformer.
> T>Once you saturate either core, all bets are off.
>
> I'm gratefully willing to accept this as a valid
> addition to what I wrote. As stated however, I
> do not believe it is possible to saturate the core
> of a transformer with an 8.333 amp primary by
> forcing into it 1-amp of current.
>
As I mentioned, it is a voltage thing, not a current thing... although
once
the voltage thing does its bit and saturation is reached, the current
thing
comes in with a vengeance! It can be like hitting a brick wall.
> [ ... ]
>
> T>To anyone attempting to try this, I suggest you try to determine where
> T>the
> T>second transformer is going to saturate by using a variac and
> attaching
> T>the
> T>input in series with a low wattage 110 v light bulb. When the light
> bulb
> T>begins to light, you have hit the wall. Any increase from this point
> up
> T>will be totally wasted energy which will be used for smoke generation.
>
> T>For transformers with higher amp ratings you can use a larger wattage
> T>light
> T>bulb.
>
> T>Fr. Tom McGahee
>
> It's always well to proceed with caution. I second
> this thought completely.
>
> Light bulbs are highly non-linear as regards con-
> duction -vs.- wattage -vs.- first appearance of
> light.
>
> So -- to be quite scientific about it, choose your
> light bulb carefully -- or better yet use an ammeter
> in the light bulb circuit.
>
>
> Potentially yours in -
> Detroit, USA
>
> Robert Michaels
Robert, thank you for your original post and your comments to my
comments
on it. I hope that I don't come across as being nit picky. You are
absolutely correct that cascading transformers can often get us some of
that high voltage stuff we are all looking for. Let us also be somewhat
cautious and realize that components that have been pushed to the limit
are
also often on the edge of destruction.
Fr. Tom McGahee