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Re: VA vs. Watts (was Calculating VA)
Andy, Brya, Reinhard,
VA (Volt Amps) can be interchanged unless you are dealing with a
reactive circuit. Such as a coil or cap. Then you should use the real power,
and apparent power formulas to figure what you want. The more power you have
the better, in most cases. If you want to use VA and watts interchangeably,
you need to use a PFC(power factor correction cap) to cancel out the
inductance of the transformer. As for the VA rating of a transformer you can
draw 60mA from a 15Kv transformer if you use a 250000OHM resistor. It better
be rated at 900w though. It was really fun to watch a .5W 1M OHM resistor
across one of my 12KV NSTs. Since your NST is shunted the voltage will lower
if you hook a load that would try to draw more then 60mA to it. the shunts
cause the transformer to go into saturation early. Else the current would go
high enough to burn the windings. This is good to demonstrate with a small
12V 1A(12VA) non shunted transformer. Hook a 12 OHM load to it then a 6 OHM
watch the voltage drop as the transformer saturates.
> Original Poster: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net>
>
> Hi Andy, Bryan,
>
> > Original Poster: Bryan St <warp27-at-juno-dot-com>
> > > Original Poster: "Andy Cleary" <gemware2-at-dreamscape-dot-com>
> > >
> > > How does one go about calculating the VA rating for a
> > > transformer? Is it
> > > logical to assume that it is Voltage * Current? (15000 V * .060 A =
> > > 900VA).
> > > Would that be correct? If so, would that mean that a smaller VA
> > > rating is
> > > better? Also, how is power calculated (for Watts)? I keep
> > > forgetting.
> >
> > Andy
> > VA is power
> > VA=watts
>
> For most the term V*A is interchangeable with Watts. However,
> as far as I know VA is used for devices which do not *consume*
> power (like a transformer), whilst Watts are used for devices
> that *consume* power (like a power drill).
>
> Now back to Andy´s question. Yes your calculation is correct,
> but there are two "buts", which you should consider (and no,
> bigger is almost always better, if your coil can handle it).
>
> a.) The 900VA is the output capability of your xformer. As each
> xformer has some internal losses, you can easily exceed this
> by a large margin (depending on how good your xformer was
> designed) on the primary side. You should also remember that
> your transformer will either give you 15kV (no load) OR 60ma
> (dead short), but not both at the same time. Interestingly
> enough, however, VA is really V*A in these cases.
>
> b.) Running a mains resonant cap will increase the "effective"
> current draw possible from your xformer as XC cancels out
> XL. You are cheating the magnetic shunts in your xformer,
> so it CAN put out more power than it´s faceplate rating.
>
> Coiler greets from Germany,
> Reinhard