On 2017-06-10, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
On 2017-06-09, Jim Wilkins wrote:

[ ... ]

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.

Try this. It's too big for me to check with my limited dialup
bandwidth or monthly cellular data allocation.
http://lcweb2.loc.gov/master/mbrs/re...og%20P258G.pdf

That is offensively big. 57 MB total download.

From my download of the Powerstat catalog C7002-1:

"CONSTANT CURRENT LOAD: Output that can be carried regardless of
output voltage setting.

CONSTANT IMPEDANCE LOAD: loads such as incandescent lamps or
resistance heaters in which the current drawn is approximately
proportional to the applied voltage, increasing to maximum current at
line voltage. These ratings apply only to units having maximum output
voltage limited to line voltage."

Those passages do not appear in this catalog. After failing a
search for the phrase "CONSTANT CURRENT LOAD" I backed off to just look
for "CONSTANT" (case insensitive), and the typical references were like
this example:

================================================== ====================
TYPES 217-2, 217U-2, Q217U·2 AND Q217UM-2

When series connected on 480 volt, 60 cycle single phase lines,
the output is 0-480 volts, 4.0 amperes. When open-delta con
nected from a 240 volt, 60 cycle three phase input, the output is
0-240 volts, 4.0 amperes. For a constant impedance load the
allowable output current at the maximum output voltage position
is 6.3 amperes.
================================================== ====================

So -- you are allowed more current at full voltage than anywhere else on
the winding. (Assuming input is also at the full voltage tap.)

And I disagree with them calling incandescent lamps and/or
resistance heaters "Constant Impedance". Easy to demonstrate with an
automotive lamp. Measure the resistance when cold, then attach to full
voltage and measure the resistance and from that and the voltage,
calculate the resistance. Lamps are commonly used for an approximation
of a constant current source -- and heater elements. I've seen the
latter in old tube radios to limit the surge into the filaments when
starting cold, and adapting to varying line voltage.

Their tech support confirmed the catalog's 5A Const I, 7A Const Z
rating of a Type 21 whose nameplate gave 3.75A as the limit.

When I tested a type 10 at its 3.0A Const Z rating an IR thermometer
indicated the brush temperature as 100C. It was driving a 24VAC 250VA
control transformer with a rectifier / capacitor output. The
combination is well matched and makes a decent 10A variable power
supply and battery charger. It will provide 15A briefly, until the
Powerstat and transformer primary overheat.

O.K.

I added the output capacitor to stabilize the readings of digital volt
and amp meters.
http://www.ebay.com/itm/DC-0-30V-0-1...#ht_1505wt_868

-jsw

O.K.

Enjoy,
DoN.

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