wrote in message
...
On 10 Jun 2017 03:00:12 GMT, "DoN. Nichols"
wrote:
On 2017-06-09, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
On 2017-06-08,
wrote:
[ ... ]
I say "for house use" because there are also 240VAC industrial
versions, which have an additional (center) tap. You can feed in
120
VAC between one end and that center tap, and get up to 240 VAC
output
(at the cost of lots more current from the 120 VAC line than you
feed
out at 240 VAC, of course.
The Superior Electric "Powerstat" is pretty much the same,
including similar voltage taps.
The maximum output current decreases considerably as the brush
moves
above the line voltage. My 10A 120/240V Powerstat 236 is rated for
only 4A at 120V in, 240V out. Trying to pull more current loads it
down.
O.K. That makes sense. I've never had to deal with that
problem. And I don't have one on hand to read the current limits
from
the data on the terminal block.
O.K. Found some illustrated in eBay auctions. W5H series is
normally 5A at 120 VAC, and this is 2A at maximum output voltage.
(The input and load currents add in the part of the winding below
the
120VAC input tap.
The large rotating plate in the back is the heatsink for the brush
and
is at output voltage. On some types the shaft is also at output
voltage and needs a well insulated knob and clearance from a panel
mounting hole.
The GR (General Radio) Variacs have a steel shaft with a black
Bakelite shell, so it is insulated from the rotor and brush
assembly.
Now if somebody replaced the original shaft with a plain steel one,
all
bets are off. :-)
And, of course, if you have a three phase assembly, the shaft
*must* be insulated, since it drives all three brush carrying
rotors.
Here is a rather extreme 3-phase one on eBay:
Auction # 162543903031
It has two Powerstats wired in parallel per phase, for six in total.
Max 90 amps out @ 240 VAC.
The shaft appears to be bare metal, so there must be insulation
where each rotor connects to the shaft.
Enjoy,
DoN.
The rating of an autotransformer is generally NOT in amps, but in VA
(Volt Amps) All 4 of my variacs and powerstats are rated in VA or
KVA.
They DO also have acurrent rating, but that is the maximum INPUT
current limit. I have them from 210/220va (210 on 50Hz, 220 on 60)
to
3.5KVA, a mix of Powerstar and Variac brands..
If you have a 1200va powerstat it is food for 10 amps in at 120
volts
and only 5 maximum at 240 volts out.
The core and winding's VA rating has a more comfortable margin than
the brush's current rating, the one I observe since replacements are
hard to find and expensive, and difficult to make because the carbon
is brittle. I could only mill into an edge, milling out chipped it
off.
The Powerstat chart gives both current and KVA ratings, at constant
current and constant impedance, for metal panel (heatsink) and
non-metallic or bracket mountings.
-jsw