"Denis G." wrote in message
...
From reading previous RCM discussions of this topic, it seems to
sometimes generate as much heated discussion as politics or religion.
For that reason, I’ll disclaim being an expert and “tread lightly.”
There was mention of running two 5 hp idlers and why they behaved so
differently from one 10 hp idler. I assume that the two 5 hp idlers
were running freely and not connected to each other. I believe that
would be like trying to hook up two independent gas-powered 120 VAC
generators and trying to get 240 VAC (single phase). I don’t think
that it will happen because the generators would not likely be in
phase with each other. Same goes with trying to run two free-wheeling
5 hp idlers. The currents and voltages would not add in phase and
would probably bounce back thru the mains or otherwise dissipate by
heat in the windings. More properly done, the motors shafts should be
connected to each other with the proper angle between them so that
they are acting in concert (like a single idler).
As far as using a resistive load to test the RPC, I don’t think that
it will help. What you want to drive with an RPC is a motor which is
a reactive load. A resistive load always has a power factor of 1,
whereas un-tuned reactive circuits generally have a PF 1. The motor
reacts with the idler causing phase shifts between the voltage and
current curves and this affect how much real power can be used by the
motor. If the PF=0.7, then you need to have 1.4 x the current and
double the input power to get the rated output power.
================================================== =
See my data post.
I used purely resistive loads, bec I thought this would *simplify* the
problem!
No power factors, phase shifts, no bouncing back and forth between two
"co-generators"--which is what 3 ph motors in parallel do.
========================================
If you had the right tools (multi-channel oscilloscope, current &
voltage taps, etc or LabView data acquisition setup) you might be
better equipped to see what is happening in the circuit. If you had
lots of cash, I’d get one of these:
http://www.duncaninstr.com/power-quality-analyzer.htm
I bought an old watt-hr meter at a flea market and have used it to
verify that I needed to buy a new refrigerator. Maybe you can find
one and monitor your RPC performance. Since you probably can’t afford
a dynamometer, you would need to make sure that you set up your 3
phase motor with a constant workload (like a circulating water pump
and reservoir). If you measure your electrical use over time vs. the
amount of work done, you could use this as a tool to adjust the
capacitance and get some idea of improving the efficiency and PF of
your RPC. Even without a watt-hr meter, you can still make
measurements on the legs of the RPC, but I think that you need to set
up the motor with a constant mechanical load to see the effects of
changing capacitance values. (Measuring voltage and current (RMS
scalar values) will only tell you the “apparent power” used in each
leg and not the real power delivered in producing work. (see
references). I think that even using the watt-hr meter will just give
you an idea of the “apparent power” used in the system, but it is
roughly analogous to the measurement of efficiency, so it could be
thought of as measuring “input power”.)
========================================
There was a guy here about a year ago I think who gave a spectacular and
detailed description of how watt-hour meters work. A real keeper, which of
course I didn't keep. It was in, iirc, partial response to one of my
queries, made under the nym Proctologically Violated. No foolin.... (ahm
tryna clean up my act a little under Droll Troll. g).
I think he was involved with building them or designing them, and he said
that regardless of power factor, they do in fact measure *true* power by
inducing eddy currents or hysteresis or sumpn in the metal rotor, which
somehow made phase angles irrelevant. A bit over my head, but worth
googling, if you can wade through all the spam.
I haven't yet checked out your last link--hopefully it will be consistent
with what I've cited. g
--
DT
Here are more links (if you are academically-inclined or just want
torture):
Wattmeter and power factor:
http://en.wikipedia.org/wiki/Power_factor
http://en.wikipedia.org/wiki/Wattmeter
3-Phase power measurements:
http://www.6mgb.com/E-News/08-04-01/...-493_short.pdf
http://www.manumesure.com/fiches/pdf...ical_power.pdf
https://www.ohiosemitronics.com/pdf/...-method(E).pdf
Wattmeter – construction and principles of operation:
http://www.usbr.gov/power/data/fist/...10/vol3-10.pdf