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RE: Variac current and VA rating



Original poster: "Pete Komen by way of Terry Fritz <twftesla-at-qwest-dot-net>" <pkomen-at-zianet-dot-com>

Hello Shaun,

I am quite aware of the construction of variacs.  I dismantled this OLD one
enough to clean it up and dust the insides and loosen up the brushes which
were sticking.  It is rated at 2.5kva and 9 amps maximum output.  The
windings are shielded by a nice brass screen that is screwed to the
hexagonal top and base.  I just answered part of my question; 9 amps max
output.  I have acquired some 7.5 amp AC breakers and so I may use them on
the input and output of this variac just to be safe.  Still, I would like to
know the current in various parts of the winding during operation of the
variac at different output voltages.

I'll try to restate the question.  Suppose that the variac is hooked up to
120vac drawing 2 amps (240va).  For various output voltages: 90vac at 2 2/3
amps, 60vac at 4 amps and 30vac at 8 amps, what is the current in the
windings of the variac between the common tap and the variable output tap?
The current in the windings between the variable output tap and the 120vac
tap must be 2 amps.

No wires will attach to the current transformer, they will only pass through
the middle of the toroid core.  The wires from the transformer hook to the
meter.  The current transformer (CT) gives a 30:5 ratio.  30 amps through
the middle gives 5 amps to the meter wires.  So the hot ac wire to the
120vac tap goes through the CT  (maybe more than once) and the wire from the
variable tap is routed through the CT in the same direction as the hot wire.

Most of my 120vac wiring does not have slack enough to route single wires
through the CT.  I'll have to crimp some terminals to some extra wire that I
have and breadboard up the circuit.  I haven't had time yet.  I have been
thinking about the variac current but working on a suction spark gap.

For Mark,

I was aware that some people run variacs over the rated VA.  I don't need to
do that yet.  I have a 20 amp 140vac output variac also.  The basic question
is: for a variac input of 120vac at 3 amps and an output of 40vac at 9 amps
(ignoring losses), what is the current in the windings between the 0vac and
40vac points?

Regards,

Pete

BTW, my 12kv/60ma NST high power factor rated 400va draws 3 1/3 amps when
testing the spark gap (while arcing).

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, May 30, 2001 3:09 PM
To: tesla-at-pupman-dot-com
Subject: Re: Variac current and VA rating

Original poster: "Shaun Epp by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<scepp-at-mts-dot-net>

Hello Pete,

I the poor guy trying to unpot a MagneTeK Jefferson NST...    :^(((
while still waiting to see if I can reserect this soggy transformer I have
some answers for your variac question.

Well,   Power in must = power out   which is almost the same as the VA (volt
amps) rating.  Power (resistive load) & VARs (reactive load ... inductive/
capacitive) load.  so it would seem this is a posibility but, there is a
current limit for the wire (which I have exceeded in the past ... gives
smoke signals   :-0  ).  The 180 degree phase shift is the same as swapping
the output wires.  If your not already aware,  the Variac is an
autotransformer, meaning it's one continuous length of wire rapped around a
toroidial core of iron.  The connections to it are taps on this coil (single
layer as you see).  The wipper connected to center(variable) terminal wipes
across the top of the toroidial transformer / coil and you can get different
voltages.    Check to see if a current maximum is listed .. that would be
the indicator.  otherwise it shound work   if within VA and current limits
.... I wouldn't interconnect windings though !
              _______________________________
VA=    /    power(watts)^2 + reactance(VAR's)^2
         v

~^~^~^~^~^~^~^~^ MAY THE ARCS BE WITH YOU^~^~^~^~^~^~^~^~^~^~

Shaun Epp
Winnipeg, Manitoba
Canada
>Original poster: "Pete Komen by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<pkomen-at-zianet-dot-com>
>
>I have an old variac rated at 9 amps with taps for 120vac and 240vac input.
>I have read that the output current of a variac autotransformer is 180
>degrees out of phase with the input current and therefore tend to cancel
>each other out.  Is it possible to set the output for 120vac and draw 18
>amps when the input is 240vac at 9 amps without overloading any of the
>windings?  If the output current is out of phase with the input is it true
>for 120vac in and 120vac out?
>
>Pete
>