"Ian Malcolm" wrote in message
...
A friend has just had his Ingersoll Rand compressor up and die on him.
Before it did so, it had the habit of dineing out on 140 uF motor start
capacitors. This is in 50 Hz 240V land. The old motor was 2Hp, Cap start/
Cap run single phase, and the final death throes were when it decided to
eat the 32 uF run capacitor as well. The potential relay appears to be OK
(at least its not welded shut and has continuity on its windings) and I am
suspecting a shorted turn in the old motor's start winding.

The compressor is rated at 150 Psi but the cutout is set to 100 psi.
It came with a three phase motor that we are hoping to put back on it.
Details as follows (the 'HH' is a symbol with both Hes sharing one
upright, 'O/' is the slashed greek O for phase angle):

HOLEC 'HH' heemaf
KMEP90L4/166/1
COS 'O/' 0.84
50 HZ 1400 rpm
1.5KW 240/415V 5.7/3.3A

I've allready rewired it for 240V delta and pullrope started it off load ,
then tried 18 uF of run caps to make it self starting and it seems pretty
happy but still off load. I would appreciate advice on choosing a
suitable single run capacitor to also act as a start capacitor. as we hope
to be able to loose the potential relay and electrlytic as bioth are too
high maintenance. I belive 25 - 30 uF per horsepower is normal and I
presume 60 uF would be better than 50 uF for starting torque.
I also know that all caps over here need to be 20% bigger to get the same
reactance at 50 Hz as most of you get at 60Hz. Would there be any
advantage in going to say 80 uF (2x 40 uF caps work out much cheaper than
one 60uF!) I've only got 18uF of run caps around the workshop and none of
them are over 2 uF so we really need to get pretty close on the first shot
as I'll have to order them.

Its going to be occasional use as a backup compresser, tyres etc, small
air tools, maybe some spraying. Are we wasting our time expecting 2/3 the
power, self starting off a static single capacitor phase converter?

I am going to measure the winding resistance and inductance today if I can
manage it so I have a little more data. I understand series resonance is
*A BAD THING* for a motor winding and I want to avoid it.
--
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

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.
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.

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.

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.

Another thing I'd consider is the pulley size. A smaller motor pulley
would lessen the load on the motor.

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.

Jerry