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Fixing Variacs





---------- Forwarded message ----------
Date: Fri, 10 Oct 1997 15:40:57 -0400
From: Thomas McGahee <tom_mcgahee-at-sigmais-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Fixing Variacs



----------
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: variac rewind 
> Date: Friday, October 10, 1997 2:42 PM
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Fri, 10 Oct 1997 08:08:21 PDT
> From: Mad Coiler <tesla_coiler-at-hotmail-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: variac rewind
> 
> List, 
> 
> One of my friends just got a hold of a reletively new looking but
burnt 
> out 'Light dimmer' from a school. Several windings on one end were
all 
> melted, and there were arc marks where the contact touched the
winding. 
> This contact wasnt making good conection. Anyways, it is a 50A
unit, 
> wound with #12 wire. I have two lbs of #13 enamel mag wire. Would
the 
> #13 wire be OK so long as it isnt run for long periods at high
current 
> levels? That is provided that the thing works at all. If I remember

> correctly the name plate says 50A / 8kVA. The next coil that I
might use 
> this variac on will be using probably three 12kV 60mA NST's. This 
> shouldnt be near the current limit of the variac anyways. But then
again 
> with resonant charging I have blown a 15A breaker with a single
15kV 
> 60mA neon. (not neccesarily a good thing!)
> 
> Any insight appreciated,
> Mad Coiler
> 
> 
Dear Mad Coiler,
On occassion I have partially reconstructed such variacs. I am wary
of doing any more work than I absolutely have to to achieve my
objective. 

Approach 1. Damage on low side. When I have a unit like yours where
there are just a few burnt wires at the low-side end, I first unwind
the damaged wires and splice in new ones. If I can get by without
actually having to have the wiper go over this repaired section, then
I don't even attempt to make that section bare where the wiper would
normally go. I add a mechanical stop so that the wiper cannot rotate
onto the repaired section low section. Simple, easy, does the job.
The down-side is that you do lose some variability. When this loss is
on the low side, then I generally tolerate it. If it is on the high
side, then I use a different approach.

Approach 2. Damage on high side. Remove the damaged high side
windings. Attach a wire from the last good winding to the high side
terminal. Add the new replacement windings, but do NOT attach them in
any way to the original high side windings. Instead, remove the wire
that goes from the low side winding to the low side terminal from the
low side terminal. Connect the wire just released to the high side of
the new wires just wound on. Connect the low side of the wires just
wound on to the low side terminal. Make a mechanical stop so that the
variac cannot go past the original wires onto the new wires.  What
you have just done is "shifted" the winding so that the new section
is actually part of the low side. Now instead of being able to go
from 0 to 120% you can only go from, say, 10 to 140%. Which is
generally adequate.

After winding, all added wiring should be fixed down with something
that can take the heat. A variac can get VERY hot!


As a little aside: most damaged variacs that have come into my hands
were given to me because somebody wired them up wrong and was trying
to feed voltage IN through the wiper. As it approached the end, the
current would just go through the roof. But I don't complain. Their
loss was my gain. I do, however, show them the proper method for
connecting up a variac. I had one physics teacher that was using
variacs as if they were rheostats. Made for some interesting lab
results!!

Hope this helps.
Fr. Tom McGahee