RE: Re: Cascading Transfo
Subject: RE: Re: Cascading Transfo
Date: Sun, 27 Apr 1997 19:41:24 GMT
From: robert.michaels-at-online.sme-dot-org (Robert Michaels)
Organization: Society of Manufacturing Engineers
T>> Cascading Mil. Spec. Power Transformers
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>> The essential idea is this: =If= one has a power (or
T>> transformer which was manuf. to military specifications,
T>> the ratings on the spec. plate of such a transformer can be
T>> taken as =starting point= or minimum values.
T>So far so good. Mil spec stuff can almost always be pushed as regards
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
T>transformer designed for 120 volts input, you are gonna fry the
T>*even if* the secondary of that transformer is not connected. Heck,
T>sure there are guys on this list that can recount what happened to
T>when they applied even 220 to a 120 primary with the secondary
T>Its a little thing called flux density. Too much of it and the core
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
T>LONG before *that*!! You might be able to push a 120 v primary to
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.
T>Let's assume that (a) you have a variac driving the primary of the
T>transformer, (b) the two transformers are connected as you describe,
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>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
If you are proposing this as a cautionary amendment
to the point I was making, then it is accepted with
Don't run at no-load. Else, use especial
caution if you do.
[ ... ]
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.
- - - - - - - -
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.
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
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.
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.
- - - - - - - - -
[ ... ]
T>You don't need two transformers to prove what I am saying. Hook up a
T>bulb in series with the 12.6 volt side of a 12.6 volt transformer (or
T>other low voltage transformer). Connect bulb/low volt side to variac
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>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?
T>lamp was not there you would have blown a fuse. Once the core is
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.
| Bulbs of differing wattages will |
| beget different results in this |
| demonstration! Try it. |
[ ... ]
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>8.33 times the current in the second transformer's primary. I don't
T>think that the military stuff is built THAT much over the rated specs!
T>especially don't think that they have over-spec'd the size of the CORE
T>factor of 8.
With all due respect, sir (sincerely said and meant),
No, no, and no!
The secondary of the first transformer can deliver
only 1-amp. (assuming for the sake of discussion
it is a 1-kW transformer).
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
It's not a "factor of 8" as you write, but a factor
of =less= than one-eighth (1/8.333 to be precise).
T>833% over spec? I doubt it. But hey, the proof is in the doing of it.
T>you personally lashed together two 1KV transformers and gotten 8.33 KV
T>more than 16.66 milliseconds? Maybe Gary has done something along
[ ... ]
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.
I did =not= operate open-circuit or no load.
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.
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.
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
[ ... ]
T>This is true ONLY if you do not saturate the core of either
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.
[ ... ]
T>To anyone attempting to try this, I suggest you try to determine where
T>second transformer is going to saturate by using a variac and
T>input in series with a low wattage 110 v light bulb. When the light
T>begins to light, you have hit the wall. Any increase from this point
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>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
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 -