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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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