RE: Unused Winding Autotransformer Warning
You've missed the mark on this one. I've been running 28a variacs as a
series inductor up to 50a without any problems. I also know that both Jeff
Parise http://www.teslacoil-dot-com/ and Bill Wysock http://www.ttr-dot-com/ run
them at KVA's into the two and three digit range respectively (10kva+ and
From: Tesla List [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, April 30, 2000 9:36 AM
Subject: Re: Unused Winding Autotransformer Warning
Original Poster: "Robert Jones" <alwynj48-at-earthlink-dot-net>
As a general rule you can not use a variac as a seies control element
close to its rated kVA.
Your using it as an inductor but you must still not saturate the core or it
will over heat Which means the magnetisation current. or voltage can not be
exceed. The magnetisation current for the whole winding may be less than 10%
of the rated current. So if you have 10A unit say 1.2kVA (assuming 120V
in). the magnetisation current my be only 1A corresponding to 0.12kVA. If
you do the sums it turns out that you must not exceed this figure what ever
the position of the wiper is or the variac will get too hot and die. You
can usually tell when the core saturate because it makes a loader and
different noise than usual. More of a buzzing sound than a hum.
You can measure the magnetisation current by measuring its off load input
current in normal mode.
So to control 5kVA in series mode you may need a 50kVA variac or even
It is difficulty to overvoltage most iron core transformers (or auto)
because the saturation means you would have to put in a very large current
even to just double the normal voltage.
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: 28 April 2000 06:33
Subject: Unused Winding Autotransformer Warning
>Original Poster: "Jeff W. Parisse" <jparisse-at-teslacoil-dot-com>
>A while back, a post warned that a variac, used as a
>series current control, could develop an overvoltage
>condition (due to the autotransformer effect) on the
>"unused windings" and cause a burnout.
>I thought about this and performed an experiment.
>In this case, we're talking about a large variac used as
>a series current control (alone) with one end used as
>the input, the wiper used as the output and the unused
>end, well, unused.
>If the variac was to fry (I've fried several, including 1156's
>and 1256's I'm proud to say), it would do so from windings
>to case (read: console ground). The autotransformer effect
>should only be between the wiper and the unused end (I
>guess) not between unused end and ground. So...
>I set the multimeter on High Value Hold and read the voltage
>between unused end and ground and pulled between 50 and
>100 amps in a variety of voltage and "current control" settings.
>I did not read any voltage higher than the input voltage. This
>suggests to me that the "unused winding autotransformer
>effect" of a variac used as a series current control does not
>contribute to it's demise in Tesla coil service. Or my
>methods may be flawed.
>Jeff W. Parisse
>Director, kVA Effects