> Which raises another question...I have heard that over the
> long term, the
> failure mechanism of high power, high voltage transformers
> usually finds
> heat as the antagonist. What is the mechanism?
Most HV transformers fail from insulation failures.
There are two common mechanisms involved.
One is the insulation between layers cracks over time. This
can be aggravated by heat, but it is really just a long
drying process and poor materials. It will go sooner or
later anyway.
The other is corona. This is very common and was problematic
in some transformers Heath used. It has nothing to do with
heat, it is a voltage gradient problem around the wires. A
tiny bit of corona will slowly eat through the insulation.
Even if the insulation is very good, it doesn't stop the
corona. Moisture does aggravate this.
One example of this occurred in some sample transformers
from a place called "Better Coil". When they dressed the
leads soldered to the HV winding they left long thing spikes
where they didn't trim the wires. The thin spike produced a
very tiny corona that slowly carbonized the paper, and then
the carbon started conducting. The end result was a long
carbon track across a distance that never should have been a
problem if they would have twisted, cut flush, and then
fully tinned or flowed the wires. Instead they probably cut,
tinned, twisted, and then soldered leaving sharp points.
Heat certainly hurts electrolytics, but we have to be
careful with the cures. One west coast amateur suggests
reducing bleeder dissipation, but in most cases the bleeder
or equalizing resistor's heat never makes it to the core of
the capacitor. The most common premature failures come from
using carbon equalizing resistors, since they age downwards
in resistance until they eventually fail and might even
split open. No one in their right mind should ever use a
carbon equalizing resistor.
73 Tom
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