Transformers in Parallel

To: tesla-at-poodle.pupman-dot-com
Subject: Transformers in Parallel
From: Tesla List <tesla-at-stic-dot-net>
Date: Wed, 14 May 1997 20:33:07 -0500
Approved: tesla-at-stic-dot-net
Subject:  Transformers in Parallel
  Date:   Wed, 14 May 1997 17:54:41 +0500
  From:   "Alfred A. Skrocki" <alfred.skrocki-at-cybernetworking-dot-com>
    To:   Tesla List <tesla-at-pupman-dot-com>


     About a week ago some one inquired as to how I got the oil 
transformers to hook in parallel, they stated that they tried to hook 
their oil transformers in parallel and it just seemed to kill the 
output of each transformer. It is very likely that the two 
transformers were wound differently one with primary and secondary 
wound clockwise the other transformer having both it's primary and
secondary wound anti-clockwise. If this were the case then the two 
transformers would buck each other and end up with no significant 
output. To properly phase transformers in parallel, be they neon
sign transformers, oil burner transformers, microwave transformers
whatever (realizing that if your transformer is not current limited 
that you MUST put a choke is series with the primary circuit). 
Connect the primaries in parallel and then connect one terminal of 
each secondary together, leaving the other secondary terminals open. 
Then using an insulated rod, wood or plastic, about three feet long, 
fitted with a test prod and lead, connect the lead to one of the 
unconnected secondary terminals. Holding the rod at the far end , and 
with an assistant turning on the power, approach the other 
unconnected secondary terminal with the test prod tip. If on near 
contact a large spark is observed, it is a clear indication that the 
phasing is wrong. If no spark is observed, the phasing is correct and 
the parallel connections may be completed. In the case where a large 
spark was observed, reverse the primary connections to one 
transformer and the phasing will now be correct. Recheck with the 
above procedure just to make sure. Note: neon sign transformers and 
oil burner transformers are usually obtained as used from salvage and 
frequently have one side of the secondary shorted to ground and as 
such won't work connected in parallel. You can test for a short on 
one side of a secondary by using the same insulated prod as above but 
this time connect the lead to the transformers case, if you don't get 
a spark on contact to one side of the secondary the that side is 
shorted. The majority of time this is due to what is called carbon 
tracking and can be corrected. To correct the short you can cook the 
transformer in an oven at around 400 degrees until you can hear or 
feel the tar sloshing inside when you lift one end (use a pot holder) 
then remove the transformer from the oven and allow it to cool and 
test both sides of the secondary to the case again, if both sides now 
arc you restored the transformer, if not then the transformer 
probably has a damaged secondary that would have to be rewound, which 
is usually not worth the effort.

                               Sincerely

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                           Alfred A. Skrocki
                   alfred.skrocki-at-cybernetworking-dot-com
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