"Jeffrey M. Borning" wrote in message
...

"Bob Swinney" wrote in message
news:bISOb.99680$8H.144430@attbi_s03...

Agreed! What is the function of the 400 ufd Cp? Is that the start
capacitor for the 25 HP (idler) motor? What about the "20 ufd Cs"? Are
they run capacitors? If so, they aren't nearly large enough for a 25 HP
idler and 10 HP load motor. Theory has it that run caps should be at
least
25 X 30 ufd or 750 ufd. Insufficient amount of run cap will give the
symptoms you mentioned, esp. when trying to run a loaded air compressor.


Woops, I made a typo on previous post. I should have said a 200uF Cs
capacitor.
(Cp is between L1 & L3, Cs is between L2 & L3, Cpf is between L1 & L2.)
I have 1800uF worth of start caps between L1 & L3 which I momentarily
switch
on to start.

The voltages I measured with different caps are as follows:
Cp Cs V1-2 V1-3 V2-3
0 0 238 211 204
100 0 238 222 210
200 0 239 237 217
300 0 239 246 220
400 0 239 256 230
400 100 240 256 244
400 200 240 257 255
400 300 240 258 265
400 400 241 258 276
500 200 239 267 261
500 300 240 268 271
600 100 241 276 259
600 200 240 276 268
700 100 241 287 267

I didn't go with all 800uF on Cp as the V1-3 was getting pretty high.

Jeffrey,

I'm sorry to say I am pretty much drawing a blank, here. From your
description it appears you are working a balanced (Fitch type) rotary phase
converter with Cp between L1 and L3 and Cs between L2 and L3. L3 is the
3rd leg or "manufactured" leg. The 240-V "line in" is on L1 and L2. L1, L2
and L3 are parallel connected to the air compressor's motor.

Are you certain the wiring connections are all tight? Loose fitting
temporary clip leads could be a problem. Are you using at least #14 wire
between the rotary and the compressor motor? Use #10 here if running more
than a few feet. Are you really sure the compressor is properly "unloading"
for starting. 135 psi sounds like a high pressure to start against.

The next to last line in your chart shows Cp of 600 ufd and Cs of 200 ufd
with V1-3 at 276-V and V2-3 at 268 -V. The 276-V is about 15% higher than
line in V which is near the practical limit with V2-3 (268-V) a little
lower. You might try increasing Cs to 300 ufd to see if it raises V2-3 up a
little. Do not do this if V1-3 or V2-3 goes over 276-V (the upper 15%
limit). The obvious reason for this is to keep the unloaded idler motor
voltages from going high enough to damage insulation. Stay tuned - there
may be an alternate solution.

What follows is opinion and is unsupported by any practical experience on my
part:
Loaded air compressors draw extremely high starting currents and your idler
motor to load motor ratio is about 2-1/2 : 1, which may not be enough for a
loaded compressor. Consider that more run capacitance may be necessary but
that you are already at the practical limit with the amounts of capacitance
you have tried. Consider addding permanent capacitance to the air
compressor motor, connected across L 1-3 and L 2-3. Make sure this
capacitance is connected directly to the compressor's motor, downstream from
the compressor's on/off switch. Begin with, say, 200 ufd in each leg.
Observe the compressor's starting "labor" and increase capacitance as
indicated. If enough capacitance is added to facilitate proper starting,
make sure that running voltages are no more than 15% higher than line in.
Good luck!

Bob Swinney





With Cp of 700 and Cs of 100, the compressor currents at 150psi are
I1=21A,
I2=29A, I3=30A. This is with the compressor starting with no pressure in
the
tank. It still won't start at 135psi.

I can email anyone an excel file of this if the columns don't come out
right.

Thanks,
Jeff Borning