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Re: variac survival
Subject:
Re: variac survival
Date:
Thu, 27 Mar 1997 01:06:43 -0500 (EST)
From:
richard hull <rhull-at-richmond.infi-dot-net>
To:
Tesla List <tesla-at-pupman-dot-com>
snip
>insulation between windings and core. Another way that variacs get
>destroyed is by supplying too much voltage to a transformer causing the
>core
>to saturate which causes high current spikes. (Most 120 volt
>transformers
>cannot tolerate more than about 145 volts input before saturation
>becomes too
>much of a problem). This can cause the brushes to "stick", making the
>handwheel hard to turn, and particles of copper can get "blasted" out of
>the
>variac winding where it contacts the brushes.
>
>Towards optimal coiling,
>
>John Freau
John,
I have repaired a fair number of variacs and brought a good number back
from
the grave. This "sticking" you note is as common as the sea sickness,
and
as you say, is caused by large peak over currents.
Heating of the turns on the core cause the wire which is captured in
shellac and or epoxy to rapidly expand and this bends the perfectly flat
surface turns for wiping up a bit and breaks loose the varnish. The
wire
later contracts, but stays bent up slightly and the damage is done.
In severe cases, where the wire is not fused or opened by the over
current,
the "humping" of the flat wiper area can be so severe that a brush can
be
broken if forced to track over this area or the wheel will no longer
travel
past the damaged turns until repair is effected.
Richard Hull, TCBOR