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Re: [TCML] Series Transformers

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.

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:


What kind of transformers?

You can't series NSTs, 'cause they are center tap grounded.  You can
parallel them for more current, though.


Phillip Slawinski wrote:

Does anyone know of a way to wire two transformers so that they are
oppositely phased?

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