Re: [TCML] Primary short

[mddeming@xxxxxxx]

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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