Re: PTs

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Chris,

On 14 Apr 01, at 19:57, 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?

An ideal transformer shouldn't have any resistance in its windings at 
all should it? You don't want it to dissipate power. It is actually 
the primary inductance that limits the (unloaded) primary current. 
You may have a winding short somewhere (in any of the windings) or 
you may be switching on (coincidentally) at a peak half cycle which 
attempts to magnetize the core in the direction it was already 
magnetized at last switch-off (not terribly likely if the breaker 
remains in for a whole 2 seconds). Remanent magnetism is sometimes 
the culprit. Question: does the core hum loudly and continuously? 
Does it initially hum and then go quiet? If the first, a winding 
short or externally shorted winding is most likely the problem.

Malcolm