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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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