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Grounded Transformers



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Just joined the Tesla discussion group here and noticed an on-going
discussion concerning Transformers and grounding. So please excuse me if I
repeat anything anyone else has said.

Neon sign transformers usually have two separate secondary windings. To
reduce the electrical stress on these windings, and the difference in
potential that exists between various parts of the transformer (especially
the primary and the case), one end of each secondary winding is connected
to the case. This does several things. Firstly, it effectively connects the
two windings in series, so that the potential difference is now the sum of
the two sets of windings. In other words, two 6KV windings now have a total
difference between the two HV windings of 12KV. Secondly, since one side of
the AC line resides at NEUTRAL and the transformer case is usually
connected to safety ground (which for all practical purposes is usually
considered to have the same zero potential as the neutral line, barring any
ground loops and such.), the maximum difference in potential that exists
between the primary and what I will call the Bottom of the secondaries and
the case is the 120VAC that is applied to the Hot line. 

The Bottom of each Secondary is the side physically closest to the iron
core. It is usual practice to provide SOME insulation between the Bottom of
the winding and the iron core, but the designers really aren't concerned
about insulating the Bottom, since they assume that THAT will be directly
connected to the Case. Now, because each secondary only produces 6KV with
respect to the supposedly Grounded Case, the high voltage insulators that
hold the terminal bolts only have to withstand 6KV. Being nice engineers,
they make it so they can easily handle about 9KV.

The Primary is assumed to be 120V above the case voltage. So they are nice
and insulate it for at least 1KV. (The insulation I refer to is the
insulation between the Primary winding and the Core, which is also
connected to the Case).

Then along comes a hobbiest or experimenter (wonderful fellows like you and
I), and they get the bright idea to FLOAT the case. Let's see what happens
if we are nice and JUST decide to ground reference the Left Secondary so
that the Right Secondary is 12KV above Ground. The voltage at the Left
Terminal is 0 since it is Ground Referenced. So far so good. The Right
Secondary HV Terminal is now at 12KV with respect to ground. YEH! Good
Stuff! The only problem is that the voltage at the Case is now 6KV. The
engineers who designed this sucker didn't know we were going to do this!
Deep inside the wonderful tar-filled transformer case is a primary wire
that the engineers thought would NEVER go more than a couple of hundred
volts above the case and core voltage. Which was assumed to be zero,
remember? It is not just that the dinky little porcelain insulator for the
primary wires does not look like it was designed for 6KV. (It wasn't). The
far greater danger lurks deep INSIDE the transformer. The ENTIRE primary
circuit is at risk. Anywhere one of the wires comes near the Case OR the
Core, it is at risk. The Primary is Wrapped Around the core, which is
connected to the case. Oh Happy Happy Joy Joy! Now, IF the wires Just
Happen to have some of that good insulation spaghetti stuff on them, and IF
they never get closer to ANY other piece of metal in this whole cramped box
of wire and metal core, and IF the tar has no air bubbles, and IF you are
an extremely lucky fellow, then you can probably run this thing for 
several years without a problem. But more likely it will poke an itty bitty
teeny weeny hole in the insulation somewhere and decide to self-destruct.

So far I have assumed that the transformer is connected to ground at one
electrode, and that the 12KV electrode is just sticking around in the air
generating ozone. Ah, but now we attach the sucker to a Disruptive
Discharge circuit containing the most humongous high voltage capacitor we
could buy, build, borrow, or steal. We charge the capacitor(s) up and dump
the charge through a spark gap into a wonderful high Q primary which is
connected to a tuned secondary. The beast bursts into magnificent
resonance. Tremendous back emf's are produced. (We gloat over the fact that
if we drop a short across our primary we get these really fat, white,
terrible SPARKS about an inch long). We Tune Up the Tesla Coil. Then we
wonder what happened when the sparks suddenly cease entirely, or whimper
down to mere whiskers of their former selves. Let's see, add the in-phase
voltage from the Tesla primary back emf to the 12KV already at the
transformer's Right Secondary, and we get something like an additional 12KV
jumping around that top insulator. So it Sometimes hits 24KV at that
transformer secondary. Assuming ONLY simple voltage divider stuff needs to
be taken into account (a RASH assumption in ANY disruptive circuit!), we
come up with a very conservative voltage between the case and the primary
AC line of about ummmm, 12KV. Heck, that's more than a single SECONDARY was
RATED at for breakdown. And this is what we have between the Case and the
120 VAC mains voltage Primary! Ooooops!

Yes, but I have witnessed people float TWO 15KV transformers and run the
primaries in anti-parallel to get 30KV. It worked. Until it stopped, that
is. It ran for about a month, being used only a few minutes each day. From
day one it had this irksome buzzing sound like a mosquito trying to die.
The guy had it connected to a bunch of 1B3GT high voltage rectifier TUBES,
each with its own private filament battery, and he was trying to produce
X-rays from an old Cenco X-ray tube. (The 1B3GTs produced more x-rays than
the tube!). It was quiet enough that you could hear that something Nasty
was going on deep within the Neon Sign Transformers. This same fellow
actually tried a weird arrangement in which he tried to boost four 12KV
transformers to 48KV. All he managed to do was destroy four perfectly good
transformers.

I'm not saying that floating a transformer doesn't work. It does! I'm just
saying that many times it fails miserably after it Does OK For Awhile. And
when it DOES work, you are always sweating, waiting for the moment when it
finally succombs to the basic laws of physics and goes belly-up. The only
saving grace is that Neon Sign Transformers don't explode. Thank God for
Tar.

Hope what I wrote helped somebody out there!

When all else fails, try something else!
Fr. Thomas McGahee
(Yep, I am a 50 year old priest that has taught electronics for 30 years. I
am a dyed-in-the-wool Amateur Scientist, having built numerous Tesla Coils,
X-ray machines, Lasers, and literally hundreds of other fun things. I hope
I can learn something from each of you, and contribute something in return)