"DoN. Nichols" wrote in message
...
On 2017-06-10, Jim Wilkins wrote:
"DoN. Nichols" wrote in message
...
On 2017-06-09, Jim Wilkins wrote:
[ ... ]
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.)
I think what they mean is you will get the most current at full
voltage, but could draw the same 6.3A maximum current at a lower
setting if necessary. You can NOT get that current above line voltage
in the boost configuration, in that case the limit is 4A. I don't know
if you can pull it below the input tap in the boost configuration
where the volts per turn is higher.
At least that was the story when I was learning to design industrial
controls. The derating curves don't reduce to a simple, easily
remembered explanation or a single number on the label.
I've measured the iron loss on a few cores. It rises rapidly above the
spec sheet voltage and frequency, they don't make good speaker
crossover inductors.
Electrical engineers aren't taught the subtle imperfections of
components and electricians fall short on the theory. I sought to fill
that gap and become able to turn a scribbled schematic into a fully
documented working machine.
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.
This is Staco's explanation:
http://isefaq.com/package/index.php?...nsformers.html
This addresses the limits to brush current, my concern when using them
for bench testing:
http://variac.com/ShortTermOverload.pdf
The brushes drop a volt or so which limits circulating current in
shorted adjacent windings. When I was looking for carbons to cut down
I found that brush composition should be more or less matched in
resistivity to the working voltage, to reduce motor commutator
sparking. Presumably the original designer chose the proper cross
section to control the current density.
In the 1980's I used a 4W night light bulb as the Zener limiting
resistor in a test fixture that had to operate from an input that
varied from ~20V to 80V. The 80V entered the rotating assembly
through slip rings so I couldn't just add an external power supply.
Today I could use a cheap switching supply to drop the voltage.
-jsw