Re: Variac current and VA rating

["Tesla list" <tesla-at-pupman-dot-com>]

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

I have reasoned this out and here are my conclusions.

in             )
-------------)           120vac input
                )    Y
                )
                )<------------  output tap
                )    X
_______)________        common

We know:
1)  The voltage varies from 0 at the common end to 120vac at the input to
say 140 at the max.
2)  All current cannot flow the same direction except when there is no load.
If it all flowed the same direction, you couldn't get more than no load
current.

Call the winds between common and the output tap X.
Call the winds between the output tap and the 120vac input Y.

Suppose that the output tap is at 60 vac and loaded at 6 amps.  The input is
therefore drawing 3 amps.  Then Y is carrying 3 amps in the down direction.
X is carrying 3 amps in the up direction.  In a sense YX is carrying 3 amps
and X is carrying 6 amps but the opposite currents cancel. leaving just the
difference.  I suggest that the current times the voltage over Y will equal
the negative of the current times the voltage over X.  VY*IY = -VX*IX

in             )<------------   output tap
                )   X
-------------)           120vac input
                )
                )   Z
                )
                )
_______)________        common

What about the case where the output voltage is greater than the input?

Call the winds between common and the 120vac input Z.

Suppose that the output is 150 vac at 4 amps (most variacs can't do this but
for ease of computation I'll use it).

The input current is 5 amps.  150 V * 4 A = 120 V * 5 A.

X current is 4 amps up.
Z current is 1 amp down.

VX * IX = -VZ * IZ		30 * (-4) = -120 * 1

There are always two sections of windings: common to the nearest of the
input or output taps and between the input and output taps.  The VA of one
section will equal the negative of the VA of the other section.  (one has
current flowing one direction and the other has current flowing the opposite
direction).

VA is conserved.

If I am right on this, then the output current measured on the output tap
wire will be in phase with the input current.

I have mostly ignored losses.

Ran my TC with 25 small gaps between brass balls.  Black and tan deposits
appeared very quickly and performance degraded.  I have had my fill of brass
spark gaps.

Also ran with a sucker gap which is GREAT!  Getting about 31 inch arcs to a
target, 29 inch arcs to the strike rail, and no change in performance no
matter how long I ran it.  I shielded the gap with a piece of 4" sch 40 PVC
and it glows in the dark when the gap is firing.  At least 90 degrees of the
PVC is cut away so it is open on the bottom.  (I need a digital camera)

Why do 2Kohm 160W resisters get so hot if the current through them is only
60ma?  If they get so hot with 7.2W, you could cook with radiant heat at
160W.  (This was during testing of the sucker gap)