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RE: Variacs in parallel ?



Daniele:

	I've been getting back into the world of Tesla Coil construction over the
last two months after about 8 years of complacency.  I don't have much
electrical experience so I can't say if all variable transformers can be
hooked up in parallel for increased power requirements, but the variable
transformers I'm dealing with can be.  In fact, I'll be stacking three
Powerstat 1256 and two 1156 units (separately) later today.
	You should probably ensure that the units you plan to hook up in parallel
are the same manufacturer/model.  You will need to stack your variable
transformers, removing the shafts from each, and pass one common shaft
through all of them.  Make sure all the brushes are aligned by moving them
to one side (against one stop) and clamping the common shaft in place
(usually done with set screw(s) for each variac).  Under ideal circumstances
this shoud do the trick, but in reality slight differences in voltage
between the outputs will cause circulating currents through the transformers
that can damage them.  Ten years ago I did this without using a common shaft
and observed some smoke before I got a chance to cut the power (no permanent
damage though).  Some people on the list have simply hooked up the common
shaft, the wiring, and given it a shot.  Due to the low duty cycle of a
Tesla Coil you may be ok.  To eliminate these voltage differences and
promote the longevity of your transformers, however, you will need to use
paralleling chokes to equalize the volatages.  Your best bet would be to
contact the manufacturer of your variable transfomers and purchase them
outright, along with hookup info, but they are amazingly simple devices and
could possibly be homemade (although I'd prefer to purchase a used, tested
one myself).
	Let me know if you would like me to provide you will diagrams/pictures off
the list to help you out.  If so, also let me know if you can receive E-mail
attachments.  The number of required chokes varies according to who you talk
to, but the diagrams I have show one choke for two transformers and three
chokes for three.  The hookup info I can provide is said to be specific to
Powerstat units, but I have seen that at least one other manufacturer (Power
Designs which manufactures the Variac brand name of variable transformers)
also uses the same hookup.

- Jeff Blodgett
jeff-at-whitemountainpics-dot-com


-----Original Message-----
From: Tesla List [mailto:tesla-at-pupman-dot-com]
Sent: Tuesday, April 18, 2000 8:56 AM
To: tesla-at-pupman-dot-com
Subject: Variacs in parallel ?


Original Poster: "Daniele Bortoluzzi" <dbortoluzzi-at-sogeda.it>

can I put smaller variacs in parallel to withstand more power ?

ciao all

daniele