"Ignoramus23077" wrote in message
...
On Wed, 27 Jul 2005 15:52:12 -0500, Robert Swinney
wrote:
Iggy sez:

"I will use 3 of my 92 mF capacitors, connected in parallel, between
leg 1 and leg 3 of the 10 hp idler, if I use a 10 HP idler."

That would be about right for a 10HP idler motor unbalanced RPC. It is
close to the rule of thumb for 30 uFd per HP. It will probably make you
a
fine RPC. Now, here is where a problem may occur:

You specified a "Hanrahan" Self-starting RPC. It is unlikely a 10 HP
motor
will start reliably - i.e., without excessively long spin-up time drawing
heavy current and popping breakers, etc.

I thought that meeting the spec of capacitance is enough to get the
motor to spin up quickly. It will be on a 60A circuit. I just bought a
60A breaker at Home Depot.

IMO, you will need to include a
capacitor start circuit. It doesn't have to be elaborate with voltage
sensing relays, or such. Between the same line side and 3rd leg where
the
run capacitance is connected, connect a momentary push button and some
electrolytic (start) caps. You will need in the neighborhood of 700 -
800
uFd. I would suggest trying 1 of the 270 - 324 uFd nom., start caps as
sold
by Grainger. That would give you around 600 uFd counting the 3 92's of
run
cap you have connected. If that isn't enough to start in less than 1
second
spin-up time, increase the amount of electrolytics. Never leave this
type
of RPC unattended. In the event of power failure, when power is
restored,
the motor would be attempting to start with insufficient start
capacitance
and might burn up and/or cause a fire.

Bob, I am confused, the author of the web article that I referred to,
considers his run capacitor of 30 mF/HP between legs 1 and 3, to be
sufficient. Are you saying that he is wrong?

The source you quoted in your post was re. the Hanrahan self starting RPC.
No, Jim Hanrahan is not wrong - far from it; but his paper cautions that
the usu. 25 - 30 uFd / HP may be insufficient for starting the idler,
leading to long start times and tripped breakers. Also, he mentions the
amount of (excess) run capacitance necessary for proper starting may lead to
high 3rd leg voltage. I'd advise you to go ahead and try it and gather some
useful information in the process. You do have a voltmeter don't you? It
may be possible for you to make a self-starting RPC that will be just fine -
there are slight differences in motors that may work in (or against) your
favor toward being truly self starting. Please let us know how it comes
out.

Bob Swinney