gradstdnt sez:

". . .currents for each leg are not as nicely
balanced. . . . "

You should not be measuring current in the 3 legs when attempting voltage
balance. It will totally confuse you! The current flowing in a 'rotary
phase converter / parallel load' is very complex. Some of the current looks
like it should be flowing in 2 directions, against itself. Of course, this
doesn't actually happen, but the reactive currents combine, add, and
subtract in the 3 legs in ways that make them an extremely unreliable
indicator of balance. Stick with voltage balancing techniques and leave the
clamp-on ammeter for the final power factor adjustments.

Bob Swinney





"gradstdnt" wrote in message
om...
Gary Coffman wrote in message
. ..
On 23 Mar 2004 06:53:03 -0800, (gradstdnt) wrote:
I tried doing this just this past weekend when I balanced out my 7.5
hp idler. Adding the PF caps didn't decrease my line current, it
increased it.

I tried a multitude of run cap and then PF cap combinations while
balancing my converter to run my 5hp lathe. I found my best
combination for loaded balance was 120 mf between L1 and L3 and 80 mf
between L2 and L3. L3 being the manufactured leg. This yielded 243v
between L1 and L3 and 245v between L2 and L3. Incoming line voltage
is 244v. Likewise my currents for L1 are 8.1 amps, L2 7.8amps, and L3
7.7 amps. This to me is pretty close to generating real three phase
power. Unloaded L1 to L3 and L2 to L3 voltages are up around 270
volts.

That's not good. You're using too much tuning capacitance. You want the
unloaded voltage balance to be within 10%, and the loaded balance within
10%. You have the loaded balance within 1%, but the unloaded voltage is
too high. You could overstress circuitry connected to the wild leg when
operating lightly loaded. Let the voltage swing above and below nominal
by about the same amount when going from unloaded to loaded. Then
you'll find that a power correction cap will reduce the primary current.

Gary

Well, as it is I'm just a tad over 10% on the high side at 275 volts
unloaded. Converter runs very smooth and quiet when the lathe is
running. Haven't tried running lighter loads yet. When idling
unloaded, it does make a bit more noise. I'm also have a feeling that
it will run a bit warmer when idling unloaded for a period of time. I
tried a configuration with 90 and 60 mf run caps. Runs a but quiter
unloaded but doesn't change much when loaded. My loaded voltages
decreased as expected, The current to power the converter while under
load went
up. By adding a 10 mf PF cap, I was able to decrease my unloaded
idling current to it's lowest value out of all configurations. So
your statement above is quite correct. My loaded current was still
higher then when nicely balanced under load. The idling current in the
third leg also went from 18 amps down to 13 amps. So according to
another poster, I'm not blanced as well as I could/should be???

I'm up in the air on where I should settle on balance. Having things
nicely balanced under load or slighty out of balance both unloaded and
loaded. Pros and cons of each? I have a 2 hp mill drill and a 1/2 hp
pedistal grinder to power as well. I realize that they will not be
balanced under load as my lathe, the biggest motor to power.

Thanks again for everyones input and expertise on the subject.