Grant sez:
I think you need to check current and voltages on your 3 legs. And your
10 hp motor *is* wired for 220, right? You may need to borrow a
peak-detecting
ammeter.

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.

IMO you should:

Install aprox. 750 ufd of run cap between 1 side of the input 240 V line and
the 3rd leg of the idler motor. Make sure the run cap is connected across
the same leg (phase) as the temporary start capacitor. Check voltage across
the 750 ufd cap with the idler running unloaded. If that voltage is more
than 15 % higher than the input line voltage, reduce the cap amount until it
comes down to aprox. line + 10 to 15%. Check the voltage with the load
motor running. It should then be aprox. the same as line in V or slightly
lower. This arrangement gives an "unbalanced" rotary phase converter.

Get it up an running satisfactorily in the unbalanced mode. Then, later you
can manipulate the amounts of capacitance in both legs to achieve the Fitch
style balanced rotary phase converter. This will probably require more
capacitance in a 60/40 arrangement between the two phases - that is from
both sides of the input line to the 3rd leg. As before, check voltges and
strive to achieve aprox. line voltage across all 3 "phases"; making sure
neither phase goes higher than 10 - 15% greater than line in with the load
disconnected.

Bob Swinney






"Grant Erwin" wrote in message
...
If I ever decide to do sandblasting, a gas-powered compressor on a trailer
is what I'm going to get. I had one once and even *it* could barely keep
up.


Grant Erwin

Jeffrey M. Borning wrote:

I built a rotary phase converter using a 25HP motor. I am using 400uf Cp
and
20uF Cs with no Cpf. It just barley starts my brand new Ingersoll Rand
10HP
compressor with an empty tank but will pump it up to 175psi. It won't
start
the compressor when it cycles on at 135psi. The compressor turns slowly
and after about three revolutions it slows down and the overload trips.
I
checked the unloader and it is working. I tried starting the compressor
motor without the belt on and it snaps right on, but apparently with the
inertia of the flywheel and pump, the motor draws too much current. It
pulls down V1-3 and V2-3 to about 140 volts before it kicks out the
overload.

Do I need a different or bigger idler motor or is my compressor motor
just
that hard to start?

I thought that a 25HP idler would be big enough but evidently not. The
specs
on the idler a Linclon A.C. Motor, Lincguard Dripproof, Frame 284T,
1765
RPM, 25 HP, Insulation B, 230/460V, 62/31A, 40º Max Amb, 60 Hz, S.F.
1.15,
Time Rating Cont., NEMA Code G, NEMA Design B. I have heard that some
motors
do not make good phase converters, maybe this is one of them.

Any insight on this will be greatly appreciated. I bought the large
compressor specifically to do sandblasting and now I can't get it to
work.

On a positive note, the phase converter seems to power up my 1HP
Bridgeport
OK. 8)

Jeff Borning