I do *not* like leaving the larger start cap in the circuit. It usually will
buzz loudly. I had a buddy with a phase converter set up like that and it
was awful to work around. Finally I introduced him to Bob Powell who reworked
his converter to take the start cap out of the circuit (don't know how, but
knowing Bob, it was most likely a potential relay) and now it's nice and quiet.
In my experience potential relays are extremely robust and work just fine.

Your start capacitor needs to be big enough to reliably start the motor while
small enough so that too much current doesn't flow for your wiring. Bigger
means more current means faster starting for more initial cost. Smaller means
slower starting with less current for less money. Start caps can be electrolytic
and are thus cheap. Run caps should be oil-filled and cost much more, but for
a 1.5kw motor shouldn't cost that much. I can go look up recommended start
capacitance per horsepower but so can you. If you don't like the new Google
groups try groups.google.com.au - the Aussies still have a good google going
down there.

GWE

Ian Malcolm wrote:

The compressor snip 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.