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Re: [TCML] Primary short
Hi Matt,
Thanks for the info. What I did was measure the sec resistance which
measured about 8 ohms either side to case. Then I fed the primary from the
variac with a clamp-on ammeter and slowly raised the input voltage to 120
volts, and measured about 4 amps which is about right considering that the
transformer is current limited to 30 ma. Secondary voltage was not
measured, but I had a jumper with a gap of about 1/16 inch which showed
nothing. I'm sure now that you are absolutely correct in your diagnosis re
an internal path in the hv winding. I guess my only choice now is to check
the archives re extracting and hopefully finding the carbon track and
eliminating it. Don't know whether I'll tackle it or not. Thank you 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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