Addendum to previous post:

It might be necessary to connect the capacitance to the compressor motor
with a separate switch or set of contactors, connecting extra capacitance
only during the difficult starting interval of the loaded compressor. This
in order to prevent possible excessive voltages after the compressor comes
up to speed. Those voltages will be felt by the idler as well as the
compressor. IMO, it would probably be well to limit this "running" voltage
to 15 % over line in.

Bob Swinney

"Bob Swinney" wrote in message
news:TcCPb.98404$sv6.406723@attbi_s52...
Jeffrey sez: "Adding more capacitance accross the compressor motor would
be
no different
than having them in the phase converter as far as the compressor is
concerned, right? But other connected loads on the converter wouldn't
see
the high voltage, so I guess that may be an good option."

Adding capacitance directly across the compressor motor would be anagalous
to adding enough capacitance to the idler motor - except - that amount of
capacitance would not contribute to excessive voltage on the idler motor
as
it would if it was placed there. Your symptoms, I believe, indicate there
is insufficient total capacitance to facilitate starting a heavily loaded
compressor. However, we cannot add enough capacitance to the idler motor
to
handle the heavy load without excessive (over 15%) voltage across the
idler
motor during standby. Placing the caps directly on the compressor motor
would (should) be a way of overcoming the excessive starting load, while
not
causing excessive strain on the idler's insulation during standby.

Bob Swinney




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

"Bob Swinney" wrote in message
news:0kcPb.90394$sv6.297022@attbi_s52...

Are you certain the wiring connections are all tight? Loose fitting
temporary clip leads could be a problem.

I removed all the wire nuts from the compressor motor, straightened the
strands and redid them. Didn't make a bit of difference that I could
tell.

Are you using at least #14 wire
between the rotary and the compressor motor? Use #10 here if running
more
than a few feet.

I have the compressor connected to the phase converter with 10 ft. of
10-4
AWG SO cord. I wanted to use 8 AWG, but I had the 10AWG on hand and I
was
anxious to try out my new toy. Now I think I will buy some 6 AWG. The
converter is wired with left over 8AWG THHN except the caps, which are
paralleled with 10AWG to terminal strips. I am thinking of going to
copper
bus bars instead of terminal strips.

Are you really sure the compressor is properly "unloading"
for starting. 135 psi sounds like a high pressure to start against.

The compressor is unloading. At first I thought it wasn't, but the
unloader
exhaust is plumbed into the air intake and is silenced by the air
filter.
I
removed the line to the intake and I could hear the "pssshh" when the
motor
shut down at 175 psi. But...,this is a mechanical unloader which uses
weights that are activated by centrifugal force to close the unloader
valve
(which is like a schraeder valve). After about 3 or 4 revolutions, the
weights are advanced enough to close the unloader valve. I removed one
of
the two adjusting shims and it didn't seem to help. I will try removing
the
other. I doubt if it will help, though, as the motor needs to overcome
the
pump inertia and come up to full speed before pumping any pressure. A
friend
at work has a big 3 phase compressor running from a home made phase
converter and he says his unloader takes a lot longer to close. I
suspect
he
has a oil pressure activated unloader. I was thinking (and another
poster
has just suggested it), that I should use a time delay relay and a
solenoid
valve to delay the unloader's closing until the motor is up to speed.

The other thought would be to add another phase converter or use a
larger
idler motor. I read on the GWM phase converter site that ePact motors on
compressors may not work very well with rotary phase converters and may
require a 4X idler size. I even thought of using 3 separate identical
phase
converters with the generated leg going to a different terminal on the
compressor. But this sound like a lot of complicated contraptions.

The last solution that I toyed with is to reduce the motor pulley
diameter.
The pump on the compressor is used with 5, 7.5 and 10HP motors with
different sized pulleys. By dropping down to the 7.5 HP size pulley, my
CFM
would go drop from 35 cfm to 27 cfm at 175 psi. Hopefully the motor
would
have enough torque to spin against the load. This kind of negates
getting
such a big compressor, but at a distress sale, I got a new deluxe 10HP
with
intercooler and after cooler for the same price that I would have of had
to
pay for a 7.5HP single phase compressor with smaller tank.

What follows is opinion and is unsupported by any practical experience
on
my
part:
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.

My only other 3
phase load at this time is the 1HP Bridgeport mill, which spins up
nicely -
even in high gear!

Thanks,
Jeff Borning