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Re: [TCML] Primary short
Hi Matt
Just thought I'd let you know the outcome of the "shorted primary", since
you were kind enough to help me diagnose the problem. The primary was OK at
.7 ohms, and the sec was OK at 8k ohms both sides. No voltage at the
secondary withe primary voltage applied at 120 volts! Opened up unit, and
began chipping out tar with a hammer and screwdriver, and finally got enough
out to remove the transformer. 6+ pounds of tar chips. During the course of
chipping, even tho I tried to be as careful as I could, I did strike the
secondary windings, thereby opening the secondary. (DRAT)! I lifted the
unit out, and upon careful examination of the secondary windings I found a
charred section on the side of one of them which showed flakes of
insulation. I carefully removed them, and wound up with a crater close to
the core. Definitely a carbonized area. This was not a crack, Definitely
a crater. This was definitely a total loss! Awful feeling after months of
work. All I have left is the two insulators. The primary coils looked like
they had been subjected to excessive heating (not caused by me) and showed
some flaking of the insulation. #16 copper wire, only good for antennas
now.
Before you ask, I used a Terry filter as prescribed, using the appropriate
components. Also the safety gaps spaced 1/4 inch each side to ground. Again
spacing determined by setting to fire with transformer alone at 120 volts.
In conclusion, the only other piece of info you must know is this unit was
old and used. It showed signs of paint flaking on the surface of case, and
severe corrosion of the steel at bottom. However it did produce sparks
initially.
Guess I start hunting for another. Thanks again.. Dan
----- Original Message -----
From: <mddeming@xxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, January 19, 2010 11:44 AM
Subject: Re: [TCML] Primary short
Hi Dan,
It is highly unlikely that the problem is on the primary side of your
transformer. It is very possible to have a high voltage short in the
secondary of a transformer and still see normal resistance when it is looked
at with the 1.5 to 3V supplied by a voltmeter. In a case like this, there
may be a bare spot in the insulation on the windings in one or two different
layers of the secondary, but the wires are not touching. However, when the
secondary voltage gets high enough, an arc occurs between the two layers, or
windings to core, or windings to case, which shorts the output (carbon
tracking). A voltmeter will only tell you if a) one or more of the secondary
windings have melted and left an open gap (infinite resistance), or b) if
two layers have "spot-welded" together(lower resistance on one side).
The only way to verify this, (short of running the NST power up until you
see the smoke) is to measure the output voltage from each output terminal to
ground and you will see them start to diverge quickly when the arcing
starts. For this you will need either a voltage divider or HV probe with
built-in divider, or an old analog meter such as my Triplett 666-HH which
has a built-in 0-5000 VAC capability
thus allowing me to check each side of a transformer with up to 60% power
applied. (15 kV X 60% = 4500 volts each side) This is almost always enough
to bring out any problems.
Hope this helps,
Matt D.
-----Original Message-----
From: ag2z@xxxxxxxxxxx
To: tesla@xxxxxxxxxx
Sent: Mon, Jan 18, 2010 10:42 pm
Subject: [TCML] Primary short
Has anyone out there ever had a 15kv/30ma NST suddenly develop a shorted
rimary during a short test? Less than one minute. The resistance of the
rimary became .7 ohms. Secondary is intact on both sides. This is an old
ardiner NST with spot welded bottom plate. Primary connections are along
the
ower part of case making access very difficult. Thanks. Dan
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