"Ian Malcolm" wrote in message
...
Jerry Martes wrote:
Ian

You have several interesting things going on in this post. I
understand that you have a single phase motor that you suspect to be
"dead". And you want to replace it with a 3 phase motor that has no
idler so it must start and run from single phase. In addition, you want
information on "balancing capacitors" for the 3 phase motor, after is
gets up to speed.

It would be difficult for me to agree that the single phase motor is
"dead". I would inclined to try evaluating the single phase motor. It
isnt normally difficult to disassemble an induction motor with no
centrifugal switch.
I have'nt personally inspected it internally. My friend has and couldn't
see anything obvious. The bearings are good, the centrifugal fan and
shroud are present and reasonably clean and it hasn't let the 'holy smoke'
out. I have seen it eat a start capacitor, the run capacitor survived
that time. The compressor pump is in good condition and has a working
unloader.
Also, it is possible that the capacitor that is being eaten is
objecting to the voltage across it. That could be the result of alot of
things.
Well these are 400VAC rated start caps on a 240VAC system. The motor
starts and runs, but blows the vent on the Start capacitor on the first
start. I belive the potential relay fails to cut out. I can put that on
a variac and see what it takes to keep it in, and how much current on its
current winding but I dont want to blow another start cap. There are too
many of them lined up on the window sill allready. Would it be worth rope
starting it and checking the voltage on the start winding with that
winding isolated? I hope that might give me some indication of a shorted
turn.

You are *not* wasting your time by designing a start up system so
that 3 phase motor can get loaded to even 3/4 its name plate rated HP.
You are aware of the need for cooling the motor. It probably wont run
at full (3/4) HP for long anyway. You can build something to
automatically spin up the 3 phase motor from single phase.

We are confident we can get it to spin up OK, it shouldnt see much load
for the first few turns unless someone has turned it off at the wall and
forgotton to trip the unloader. I have allready suggested additional
cooling.


If you are right when you estimate that the compressor will not need
full motor HP when pumping to only 100 PSI, your main problem will be to
include an excellant unloader. Also, it would be prudent to also
include an overcurrent cut out for that condition where the motor gets
stalled.
Yes, I plan to rework the old control box which includes a circuit
breaker.

Another thing I'd consider is the pulley size. A smaller motor pulley
would lessen the load on the motor.
The other motor is 2900 RPM and each has its own pulley. I'd look at
going a third smaller but delivery is more important than pressure. We'd
settle for backing off the pressure switch to 90PSI.

It reads (to me) like you expect to improve the power delivering
ability of the 3 phase motor that drives the compressor pump, by
including balance capacitors. That could easily be a big project. I'd
consider the small amount of power increase associated with balancing
would make that time poorly spent.

Well I am just trying to get a feel for what I can get out of it without
breaking the bank. If we can get 75% rated power, we'll be rather happy.
What approximate power increase can we expect from decent balancing?

Thanks for your assistance. We are going to have a shot at it. Plan B is
buy in new control gear and plan C is replace the motor. I dont like plan
C :-(
--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk [at]=@, [dash]=- &
[dot]=.
*Warning* SPAM TRAP set in header, Use email address in sig. if you must.


Ian

I'm thinking that your main problem will be to get the 3 phase motor to
start spinning when fed single phase.
I'm thinking that the start winding is commonly cut off when the motor
reaches about 80 % of its normal, unloaded, RPM.

Although I wouldnt have courage enough to advise you to dig into that 3
phase motor to make it a better single phase motor, that can be done. I
have gone inside the 3 phase motor and disconnected the unused, third stator
winding and use it as a start winding across the single phase line with a
start capacitor in series. The start torque is quite high when that is
done. But, if you can get the compressor to start with the "phase-O-matic"
approach, thats likely to be much more satisfactory.

I wonder if you have considered increasing the volume of the air supply
tubing between the pump and the check valve in the tank. That allows the
pump to be rotated more revs before the load kicks in. But, maybe you have
an unloader that isnt dependent on air pressure.

Jerry