Re: inductive ballasting

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

In a message dated 10/27/01 11:18:21 AM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:

> I am looking for a cheap heavy duty welder to ballast my pig. I 
>  currently only have a 140A welder in my garage. I was wondering if this 
>  would be sufficient for low power testing? I am worried that it is too 
>  small and the core will saturate - causing effectively no ballasting at 
>  all. 

Greg,

It's my understanding that saturation is caused only by over-
volting, not by excess current.  Excess current simply increases
the losses, but doesn't cause saturation.  So my guess it the
welder will work.  More below.

>  Any advice would be great. I have read stories in the archives 
>  about difficulties encountered when using purely inductive ballast at 
>  high power levels, such as difficult to control variacs, etc. Is this a 
>  really big problem? How exactly does one combat this with resistive 
>  current limiting? 

Pure resistive current limiting is inherently very lossy.  But some
resistive ballasting can be added to inductive ballasting to damped
the unwanted resonances.  This can be done by placing resistors
in series or parallel with the inductive ballast.
>  
>  If I was to use the 75kVA pig, I don't want the worst case scenario, 
>  whereby the variac swings to full power, the welder can't ballast the 
>  pig, the high current surge fuses the circuit breaker and the whole 
>  house explodes, along with all the pole pigs within a few blocks :) :). 

I think a lot depends on the value of the primary tank capacitors.
Large capacitors will want to draw more current, and any problems
will be magnified.  I don't think the system will tend to go out of
control.  I do however prefer to use large cap / low bps systems
for highest "efficiency". 

      http://communities.msn-dot-com/TheWorldofTesla/

>  I would like some sound advice on this subject. Exactly how strong is 
>  the force created on the variac?

I've never heard of a case where magnetics forces destroyed a variac.

The whole subject of ballasting is filled with a great amount of
controversy and disagreement.  Some have good results with
welders, and some don't.  Some have good results with variacs,
and some don't.  Some have had to cut a slot in their variacs,
and some have not.  Some have had to add resistive ballasting
to their inductive ballasting and some have not.  My guess is
that some coils may not be running as efficiently as they could,
ballast-wise, but the operators might not be aware of that.

In the one case when I tried using a variac as a
ballast, the coil drew twice as much power to get the same
spark length, as compared to when I used my homemade
inductive ballast.  I only needed to use a few turns of the
ballast.... and that's what probably caused to the problem...
saturation in other words.... too much voltage for so few
turns.  It was a 22 amp variac, and the coil drew only 5
amps or less.  I know too, that some coils permit a greater
portion of the variac winding to be used, and these may
fare better.

Cheers,
John Freau

>  Cheers,
>  
>  Greg Peters