DrollTroll wrote:
"Jon Elson" wrote in message
...

DrollTroll wrote:

Awl--

What's more important when adjusting capacitor values:
Getting equal voltage between each pair of legs, or balanced current in
each leg? Seems like you can't have both.


Balanced current in the MACHINE TOOL motor, or in the RPC? You don't care
about balanced current in the RPC, and it won't be balanced, because each
winding is doing a different job. (The 240 V line windings are running
the RPC, the 3rd leg is providing the shifted phase.)

What you are concerned about is getting roughly equal current in the
machine tool motor. And, equal voltages on the line DO NOT guarantee that
you have the proper phase shift of the generated leg. The voltage from
the 240 V mains ceter tap (neutral) to the generated leg should be at 90
degrees to the mains, and about 207 V. Without an oscilloscope, however,
it is pretty hard to measure phase shifts. If the machine tool motor
draws equal current on all 3 legs, with roughly balanced L-L voltages,
then your phase shift is right.

As long as the current in the generated leg of the RPC motor is not
excessive, I'd ignore it. If it IS much above the nameplate rated amps,
then you need to reduce the caps to bring the current down.

At equalized voltage, the current through the generated leg is through
the roof.

I'm not too surprised, but what is "through the roof"? twice rated
current? More?

At equalized current, if line L1-L2 is 240, then L1-L3 and L2-L3 are
about 230-233, which is not so bad.


What are you complaining about? That is quite good for an RPC.


Well, I don't know enough to know if I *should* be complaining or not.... so
I just complain reflexively....

I bought a bunch of cheapie analog VOMs (identical), and now have them
permanently wired among the three legs--convenient. I have clamp-on
ammeters now, but I also plan to get 3 cheapie digital clampons (about $20
each, if I'm lucky), and have them installed permantly as well.

A 10 hp baldor Super E will read over 20 A in the 3rd leg, with closely
matched voltages (more capacitance)--idler only.
With matched currents (much less capacitance), it's as above, 240, 230-233.

A 10 Hp 3-phase, 240 V motor would normally have a full-load
line current around 33 A. So, 20 doesn't sound like much of a
problem. When the RPC is idling (no load motor connected) then
ALL that current is going through the balancing caps, make sure
they can handle that much current.
I will wire up 3 ph resistive loads today (5 A heating elements), in both
delta and wye, and adjust the caps see how the currents/voltages respond *in
the resistive load*. Is this a good test method? Will the delta/wye
config of the load affect things?

I "think" you are overthinking this! Make sure the idler motor
and cap bank are not overstressed when idling, and make sure the
load motor currents are roughly balanced (+/- 10%) and that the
machine tool starts smoothly in all speed ranges. If it meets
all these needs, you are set!
Also, what do you think of the comment to add capacitance between L1 and L2,
and that the current through this capacitor "is the only effective
measurement you can get with a clamp-on ammeter in a rpc"?

I did notice that 25 uF between L1 and L2 drops the current in L1 and L2 by
about 2 amps, without affecting the current in L3 (no load).


Not really sure, but I don't think it matters much.

Jon