Re: DANGER! Strange transformer problem

On Thu, 24 Jun 1999, Tesla List wrote:

> Hi John, Terry, all,
> Snip
> >I know something strange is going on here but I can't figure out
> >what.  Saturation would be the first thought but the variac should
> >handle it since it's rated for 25 amps.  I thought the variac might
> >be breaking down but today I borrowed a 5KV hipot set and tested
> >it.  Nadda.  Some sort of resonance comes to mind but I can't
> >magine where, given only stray capacitance in the circuit.  I've

The system may not be resonating at 60Hz.  This would require
alot of stray capacitance,  and would probably be fatal.

However,  it could be resonating at an odd harmonic of the
supply frequency,  such as 180Hz or even 300Hz.  This can happen
because these frequencies are quite strong in the line current,
and little stray capacitance is required.

This effect is well known to power companies,  and is a hazard
of running large transformers with no load.  The losses in such
equipment are so low that there is nothing to damp these
spurious resonances. (A little resistance in the line may help ?)

Also,  as the voltage builds up,  the transformer will become
more non-linear as its core saturates, and will add too these
3rd and 5th harmonics !   Does the growl build up slowly ?

You said you had observed voltage waveforms.  Did they contain
LARGE quantities of 180Hz or 300Hz,  at the "bad-spot" on the
variac ?
					- Richie,

					- In sunny Newcastle.