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Re: Overdriving potential transformers

>Subject:       Re: Overdriving potential transformers

>>From scotty-at-wesnet-dot-com Thu Mar 28 12:44 MST 1996
>>Received: from billboard.wesnet-dot-com (billboard.wesnet-dot-com [206.21.6.2]) by uucp-1.csn-dot-net (8.6.12/8.6.12) with ESMTP id MAA11346 for <tesla-at-grendel.objinc-dot-com>; Thu, 28 Mar 1996 12:27:10 -0700
>>Date: Thu, 28 Mar 1996 14:00:54 -0500
>>From: Scott Myers <scotty-at-wesnet-dot-com>
>>Mime-Version: 1.0
>>To: tesla-at-grendel.objinc-dot-com
>>Subject: Re: Overdriving potential transformers
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>>Hi Steve,

>> How much current can I get from my 1.5kVA, 14400 potential transformer
>> before some kind of catastrophic failure occurs - 2kVA? 3KVA? more?  I
>> have assumed that as long as I don't exceed the rated voltage, a PT would
>> be almost as bullet-proof as a pole transformer and would be pretty much
>> limited by the temperature rise (i.e. - turn it off when it starts
>> getting warm).  Is this a reasonable assumption or am I going to quickly
>> end up with a dead PT by running it at a higher current than what it's
>> rated for?

>I had the same question a couple of months ago.  I had to call the
>manufacturer and inquire on the internal wire size.  I then had
>to ask what the max operatin temperature is.  They went into a formula
>(which I don't have handy) for their PT.  I'm sure these figures would
>be different for each brand and model of transformer.  I suggest you
>contact the manufacturer and give them your model number.  Their
>engineers should be able to give some insight.

>Scott Myers

Hi Guys!,

My experience with potential transformers came before I learned how 
not to blow up a neon xfmer.  The potential xfmer (solid potted core) 
will self destruct just as easily as a neon if proper holdoff chokes 
are not used.  I would suggest to the inquiring party that for 
current capability assessment, put a shorting AC current meter across 
the H.V. output.  Place the primary on the output of an appropriate 
variac and 'dial up the power slowly".  It would also be a good idea 
to monitor primary potential transformer current here as well.  Buzz 
this sucker up to what you expect for output current while watching 
the primary current.  If your variac is big enough that you can melt 
the potential xfmer before strressing the variac, that is better.  For 
a very brief testing cycle, push the primary current into the 
potential xfmer really high and monitor the secondary current until 
it stops climbing.  Back off on the primary current by 50% of what 
was necessary to reach this secondary saturation value.  Let the test setup
run for a while at this variac setting with the potential xfmer 
delivering current into the shorted output ammeter load.  Monitor the
temperature of the windings and the core. If your potential xfmer starts to
smell, back off more on the input power. 

This test setup will tell you what the long term continuous load rating of your
xfmer is by watching the ultimate temperature rise. The absolute 
maximum short term overload current available has also been 
determined.

Happy coiling, R.W.S.