RE: [TCML] Series Transformers

["Lau, Gary" <Gary.Lau@xxxxxx>]

2970V @800mA will be challenging.  Despite many who thought 4kV is too low for a coil, I built a very efficient coil using a 4kV/20mA NST, using a single static cylinder gap.  While it may be possible to get a static gap to work at 2970V, the 800mA part may be difficult to quench.  At that low a voltage, an RSG is probably not advisable.  You would also need a very large cap to utilize the full potential of that power supply.

Regards, Gary Lau
MA, USA

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Phillip Slawinski
> Sent: Tuesday, June 17, 2008 9:21 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Series Transformers
>
> I have a nice variac 20A rated and 0-270V output with 120V in.  I was able
> to wire them in series and ran them up to 2970V @ 800mA.  Is 2970V too low
> to run a TC efficiently?
>
> On Tue, Jun 17, 2008 at 5:13 PM, David Speck <Dave@xxxxxxxxxxxxxxxx> wrote:
>
> > OK, Phil,
> >
> > Looks to me like you want to step 110 VAC to perhaps 1100 VAC.
> >
> > If they are identical transformers, then their internal polarities are
> > probably the same.  (But you can't count on that!)
> >
> > Wire the two primaries in parallel.  Connect the high voltage output end of
> > one transformer secondary to the low voltage end of the other secondary.
> >  Power up the pair with a Variac set on a very low voltage, say one volt.
> >  Measure the voltage from the low end of the first transformer secondary to
> > the high end of the second transformer secondary.  If you have the
> > connection right, then you should see something like 11 volts.  You can then
> > crank up the Variac voltage slowly to confirm that you see a corresponding
> > increase in the output voltage at the high end.  Remember that Variacs do
> > not provide any line isolation, and even at a one volt output, the "hot" end
> > of the Variac output is still at line potential and can deliver a lethal
> > shock.
> > If you don't see any voltage, check the output across each of the
> > transformers, to make sure that one of them is not open.  If both are
> > putting out about 5.5 volts individually, then you have the secondaries
> > connected wrong.  Just swap the link between the two secondaries to the
> > opposite secondary pin on one transformer (with the power off!) and recheck.
> >
> > A small word of warning -- many digital voltmeters (both cheap and
> > expensive) can give spurious readings when used to measure unloaded
> > transformers.  You may get much more sensible readings if you parallel a 1K
> > to 10K ohm power resistor across the inputs to your voltmeter.  I once
> > wasted a half a day trying to figure out the transformer of a bandsaw blade
> > welder with a good Fluke DVM.  The readings were all over the map, and often
> > inconsistent and unrepeatable.  Putting a load resistor on the meter made
> > everything work out sensibly.
> > 1100 VAC isn't going to help you a lot for TC work.  You might be able to
> > get away with seriesing 4 transformers for 2200 volts, but at this point,
> > you will begin to stress the insulation between the windings and the cores.
> >  I've read of 4 and 6 MOT series strings, but the outer transformers on the
> > 6 MOT strings had to be run under transformer oil to prevent winding to core
> > voltage breakdown.  OTOH, the power output of a 6 MOT string (~12 KVAC at
> > 500 mA, if you use big MOTs), can compare favorably with a 5 KVA pole pig,
> > just not so nice and compact, but nearly free, if you can find the MOTs.
> > HTH,
> > Dave
> >
> >
> > Phillip Slawinski wrote:
> >
> >> Step Down transformers run in reverse.  The transformers have taps for
> >> neutral 100V 110V on the primary.  On the secondary neutral  420 480 550.
> >>
> >> On Tue, Jun 17, 2008 at 1:39 PM, David Speck <Dave@xxxxxxxxxxxxxxxx>
> >> wrote:
> >>
> >>
> >>
> >>> Phil:
> >>>
> >>> What kind of transformers?
> >>>
> >>> You can't series NSTs, 'cause they are center tap grounded.  You can
> >>> parallel them for more current, though.
> >>>
> >>> Dave
> >>>
> >>> Phillip Slawinski wrote:
> >>>
> >>>
> >>>
> >>>> Does anyone know of a way to wire two transformers so that they are
> >>>> oppositely phased?
> >>>>
> >>>
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