Re: H.V. Transformers (parallel)

To: tesla-at-poodle.pupman-dot-com
Subject: Re: H.V. Transformers (parallel)
From: Tesla List <tesla-at-stic-dot-net>
Date: Sat, 10 May 1997 00:10:05 -0500
Approved: tesla-at-stic-dot-net
Subject:  Re: H.V. Transformers
  Date:   Fri, 09 May 1997 20:57:18 -0400
  From:   "Daryl P. Dacko" <mycrump-at-cris-dot-com>
    To:   Tesla List <tesla-at-pupman-dot-com>


At 11:39 PM 5/8/97 -0500, you wrote:

>how did you get the oil transformers to hook in parallel with the center
>tap grounded.  I tried to hook mine in parallel and it just seemed to
>kill
>each other, no matter how I tried to hook them up. I even tried to
>connect
>the cases so the center taps were connected together, too.  
>
>Confused
>
>LT

I've seen two types of OBT's, the first uses a grounded case and a
single
insulated terminal, and the second uses a floating case and two
insulated
terminals.

I've never seen a OBT with a grounded center tap - this seems to be used
only on neon transformers. ('tho now that I've said this, 837 people
will
mention that center taps are common where they live... ;'}

I usually tie the cases together first (no matter which type they are)
and then start the sorting process. (This is to take care of leakage
current...)

Let's say you have three of these puppys to tie together, giveing you 
10 Kv at 69 Ma.

First check that all of them work ok to begin with, a single dead 
transformer will give you fits when you try to phase them. Just draw 
an arc off each one and make sure they're about the same 'heat'.
Toss out or re-work the bad ones.

Mark one of the OBT's as the 'master', this will be the phaseing
referance for the rest of the transformer bank.

You should also make sure you positivly mark each lead, primary and 
secondary, on all transformers, so you can keep notes as to which goes
where.

I used an two-drawer fileing cabinet for my 'transformer bank', and
physicaly placed all six transformers in their final positons.

Figure out the easyest way of runing the 10 KV lines (since this wire
is most expensive).

Wire 120 V to the 'master' transformer, and never unhook it again
(lest you forget what the phase is !)

Now wire up 120 V to the 'slave' transformer and write down which 
primary lead goes to the Neutral and which to the Hot.

If you are useing the 'grounded case, single hot terminal' OBT type
things are easy now, since one side of the secondarys are all tied
together...

Hook an insulated wire up to the master OBT and tape it to the end
of a long plastic rod.

Apply power and see if you can draw an arc to the HT terminal of the
slave OBT.

If you can't, then the phase is correct - just mark each primary wire
as to which is hot and which is neutral, remove it from the system and
move on to the next OBT...

If you can draw an arc, the phase is incorrect - you need to reverse
the primary leads on the slave, and try it again, and if you can STILL
draw an arc, one of the OBT's is dead/shorted/breaking down inside/etc.
In this case, just remove the slave OBT and try another - if it happens
again, either the master is bad, or (unlikely) you have two dead slaves.

When the OBT's are 'out of phase', you really have made a 20 KV OBT
with a center tap !

If all of this gives you a headache, I urge you to work it all out
on paper beforehand...

The key to all of this is careful notes and takeing no chances !

If you have two hot terminals, you'll need to mark them as 'one' and
'two' or whatever, to keep the phaseing straght.

One possible wrinkle, if you have an even number of OBT's, is to wire 
them up so that you can reverse the phase of one half of the OBT's 
with a switch. That way you can remove a shorting jumper tieing the
two banks together and have an instant 20 KV -at- 1/2 the amps you had
before. 

I tryed this with my last transformer bank, and it worked pretty well,
except that I got a lot better spark with only 10 KV, so I hardly ever
used the switch...

Corrections and oversight eliminination gladly welcomed !

Daryl