RE: variac amperage control?

To: tesla-at-pupman-dot-com
Subject: RE: variac amperage control?
From: Toxandale-at-SPP-dot-org (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Sat, 27 May 2000 21:21:45 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

My experience with current control appears different from some others on
the list. I currently use the 1256 for current control. At full tap
(full winding usage) I get only a few microamperes of current flow, as I
steadily decrease the number of windings used the current increases
(less inductive reactance more current). Beware, the current increase is
not linear and the current will increase very VERY quickly about half
way through the winding. My experience has been 5 amps at about 75%
winding usage, 10 amps at about 65%, 15 amps at about 60%, and over 25
amps at between 45%-50%. After this, the current will go out of sight!!.
GREAT care must be taken that you are so familiar with your variac that
you will have it at full winding setting prior to powering up the unit.
If by chance you are a the no winding setting, then you will have a dead
short with no reactance, and in a microsecond no variac. With these
cautions taken, the variac makes a very nice current limiter. I have not
tested smaller variacs for this usage, so I cannot comment on their
performance.

There have been legitimate concerns about the open end voltage of the
unused portion of the windings. With the 1256 used at full amp setting
of 28amps, I have read roughly 540vac on the open end (240vac input)
which is high, but not destructive. I originally was using a high
current solenoid coil (10ga) that was tapped at various locations, but I
like the variac use better. It is a little bulky for this use, and
again, great care must be used (be sure to fuse it correctly!!!!) to
ensure full windings are used, and you don't get the wrong end of the
variac wired to the primary circuit of your coil.

Terry

> ----------
> From: 	Tesla List[SMTP:tesla-at-pupman-dot-com]
> Sent: 	Saturday, May 27, 2000 11:08 AM
> To: 	tesla-at-pupman-dot-com
> Subject: 	variac amperage control?
> 
> Original Poster: "Trent Mullins" <neontrent-at-earthlink-dot-net> 
> 
> Hey to everybody!!
>  
> I've heard that a variac can be used to regulate amperage as well as
> voltage.
> Can anybody explain to me how this is done?
> I just purchased a model 1256 from Dave Wightman for $50.00 and am
> considering
> another for 
> amperage control on a large coil that I'll build someday.  The variac
> steps up
> 120v to 280v w/o load, and 240v to 500v w/o load.
> I just got 78mA out of a 30mA NST for a very short time (didn't want
> to burn it
> up)
> With load the high end drops slightly.  The variacs are rated for 28
> amps and
> weigh about 80+pounds.
>  
> Thanks in advance,
>  
> Trent Mullins
> "Bendin' Glass 'n Passin' Gas"
> 
>