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Re: Grounded Transformers



At 09:25 PM 1/15/97 -0700, you wrote:
>Subscriber: tom_mcgahee-at-sigmais-dot-com Wed Jan 15 21:13:44 1997
>Date: Wed, 15 Jan 1997 18:04:01 -0500
>From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: 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)
>
>
>
Fr. McGahee,

I enjoyed your explanation there, it makes alot of sense. Could you please
explain what is going on when I put my caps across my neon outputs, everyone
here has explained this to me but another view don't hurt. I have my caps
across the neon outputs and two dinky series gaps on each HV line to the
primary. I am getting results with this configuration but as everyone has
told me I will blow my neon. WHat is going to blow it, the caps are not a
dead short, when I move to the traditional method of putting the caps in
series on one sire and the gaps on the other I get zero results and the gap
glows with a orange flame arc.

Thanks
PS by way of private mail (off topic here) I would really like to hear about
your lasers and other fun things:)))

Thanks again!

                                       Regards,
                                            Kevin Nardelle

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related with lotsa pictures and links.
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