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Re: Variac current and VA rating
Original poster: "Jason Petrou by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>
Pete,
I think theoretically that should be fine - its the KVA not the A that
matters (i think! correct me if Im wrong). The point is that at 1/2 the
voltage double the current would be hard because at half the voltage only
half the current will be drawn by the same transformer (assuming the same
resistance) - but i see your point - try it to find out!!! Just to let you
know, Mike Tucknott was running a 16A variac at 40A for his big coil, and
this is fine so long as you keep the runs under 2 mins and leave plenty of
time for it to cool (or stick it in oil) so try it and if it gets too hot,
stop!
Jason
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, May 31, 2001 7:54 PM
Subject: 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
> >
>
>
>
>
>
>