Re: PTs (also Ballasting)

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Chris,

I'm betting that you don't have a shorted PT - these are actually pretty
tough little beasts. During your test, if you simply connected the LV
side of the PT directly to the incoming 120 VAC mains, it's quite
possible that heavy "inrush currents" popped your breaker. This is
typical when you first connect large variacs or larger transformers such
as PT's to the AC mains. Inrush currents can be very large even modestly
sized transformers and variacs. Sensitive circuit breakers often respond
to this current surge by immediately tripping. Simply try running the PT
off a variac while measuring the input current (HV side open), then ramp
up the voltage. If your PT is operating correctly, it should draw
relatively little "magnetizing" current.

If you don't have a variac, try resetting the circuit breaker and then
power up the PT again. Inrush current is a function of where you power
up the PT relative to the phase of the incoming AC waveform. So the
probability that the inrush current will trip the breaker is statistical
- sometimes it'll trip, sometimes not. Certain types of circuit breakers
may be more sensitive to inrush currents than others. Usually resetting
and trying again, sometimes several times, does the trick. Putting a low
resistance power resistor in series with the PT will also help limit the
inrush current - the resistor can then be short circuited after you've
powered up the PT. 

Also, (in response to another posting) PT's will NOT work as inductive
ballasts unless you do some major surgery to "open" up the magnetic path
with an air gap. PT's are specifically designed to have low leakage
inductance in order to reduce measurement errors - they are in fact
measuring instruments. This property eliminates their use as inductive
ballasts without modification. I'm not aware of anyone who has actually
tried to "depot" one of these butyl-rubber encapsulated beauties, but I
suspect it would really be a nasty chore... :^)

Safe coilin' to you!

-- Bert --

Tesla list wrote:
> 
> Original poster: "Christopher Boden by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <chrisboden-at-hotmail-dot-com>
> 
> >Hi Chris,
> >
> >Unfortunately, it sounds to me like your big PT has an internal
> >short in the HV secondary winding. It certainly shouldn't be
> >tripping your mains breaker with no load on the output, unless
> >there's an internal short.
> 
> That's what I though, but what if it's just a matter of a REALLY low primary
> resistance? Couldn't that appear to the breaker as a short?
> 
> >And unfortunately, a butyl covered PT
> >is virtually impossible to tear into to repair ;-(
> >
> >David Rieben
> 
> Yup, but, even if it's got a HV side short, it's a beautiful bit of
> inductive ballast :)
> 
> Have fun!
> 
> Duck
> 
> Christopher A. Boden Geek#1
> President / C.E.O. / Alpha Geek
> The Geek Group
> www.thegeekgroup-dot-org
> Because the Geek shall inherit the Earth!
> 
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