Re: current limiting on pole transformers

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: current limiting on pole transformers
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 25 Jul 1997 04:43:16 -0500
Approved: tesla-at-stic-dot-net

From: 	Rodney Davies[SMTP:rgd872-at-anu.edu.au]
Sent: 	Thursday, July 24, 1997 8:04 PM
To: 	Tesla List
Subject: 	Re: current limiting on pole transformers

hi Bert, Rodney, All,

On Thu, 24 Jul 1997, Tesla List wrote:

> 
> From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
> Reply To: 	bert.hickman-at-aquila-dot-com
> Sent: 	Thursday, July 24, 1997 8:58 AM
> To: 	Tesla List
> Subject: 	Re: current limiting on pole transformers
> 
> Tesla List wrote:
> > 
> > From:   Rodney Stapivic[SMTP:rstapivic-at-sprynet-dot-com]
> > Reply To:       rstapivic-at-sprynet-dot-com
> > Sent:   Wednesday, July 23, 1997 6:57 PM
> > To:     tesla-at-pupman-dot-com
> > Subject:        current limiting on pole transformers
> > 
> > I have recently purchased a 14.4kv, 10kva single phase pole transformer for
> > a good price. I understand current-limiting due to working with high power
> > radar systems for a defense contractor, but my question is why under-
> > saturate the 240v side? the transformer I bought is rated  at 42 amps max
> > on the 240v side if used as an input. According to the specs from the
> > manufacturer this is the max current it will draw when fully saterated. If
> > I hook this one up to a 60 amp service and somewhere get a 50 amp or more
> > 240v variac (does anyone have one for sale or a source?) I should not have
> > a problem I would think. I'm trying to get away from welders, convection
> > ovens, and I really don't want to use neon sign transformers. Any help is
> > appreciated
> > 
> > thanks
> > Rodney
> 
> Rodney,
> 
> Transformer core saturation does NOT limit primary current - quite the
> opposite! Once you saturate the transformer, your primary current rises
> dramatically, irrespective of the secondary load. If you don't have some
> type of external current limiting in series with the primary, it can
> easily draw more current than the total rated electrical service can
> supply to your house. BTW, a similar thing will occur when you short
> circuit the secondary with a spark gap - running with no ballast will
> result in you drawing HUGE amounts of primary current - you could
> literally "take down the neighborhood" with one of these if your supply
> mains circuit breakers failed to open quickly enough! If you're gonna'
> run a pig, you NEED a ballast of some sort. 
> 
 This is true. I'm currently in the process on desiding the more
economical approach to current limiting my pole pig. So far, since we're
basically on a $0 budget, we're taking the resistive ballast approach.

Getting a whole heaps of electric stove elements, light bulbs, whatever,
with reasonably high resistance, and will be placing it in series with the
low-voltage side. Yeh, they'll get hot, but we'll deal with that
accordingly...

The other (more efficient and ideal method) is inductive ballasting.
However, I can't afford the rather thick copper-wire, former, or let alone
iron for the core I would need for a choke to make around (approx) .19
Henries to inductively ballast my pole pig down to 1-2KVA....but we're
working on it! ;-)

I'd sugest the resistive ballast method, however, it's perhaps not-so-safe
and may generate problems...serious or not, I don't know...I'm travelling
down the same yellow brick road at the moment...will know in a few weeks
or so...

Anyway, hope this helps..... anyone with other ideas/comments? Please?

Thanks!
Rod