"Ignoramus19023" wrote in message
.. .
I am considering adding some run capacitors to my self starting RPC. I
am reading Jim Hanrahan's article at

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

and I am confused by something. I understand how self starting
RPCwould start with one cap between one leg 1 and leg 3 (the generated
one). That's how mine is wired. Jim makes a point that it works, but
makes unbalanced voltage.

But why would it start is capacitors are connected between 1-3 AND
2-3, like in this pictu

http://www.metalwebnews.com/howto/ph-conv/fig1.html

I cannot see how it would create assymmetric fields needed to spin up
the motor. Is that because capacitances across legs 1-3 are greater
than capacitance between leg 2-3?

A 3-phase motor will start on single-phase current because the (3rd leg,
middle leg, whatever) has phase-shifted current applied to it. That leg in
a 3-phase motor operated on single-phase, has phase-shifted current applied
via the start cap. That current is "sufficiently out of phase" with the
current in the main winding to provide a starting field, if you will, for
the main field to operate against, thus causing start-up. Remember, we are
talking about a single-phase motor. After the starting interval, the motor
will continue to operate as a single phase machine.

The start cap, if left permanently in place, forms a "self starting" RPC. A
rather large amount of capacitance is required for starting any motor, be it
single-phase or 3-phase, when starting on single-phase current. Therefore,
a 3-phase motor running with an over-large starting capacitance, permanently
in place, is likely to exhibit severly unbalanced running voltages. This is
why serious RPC builders always seperate the starting and running functions.

Assymetric fields are necessary for starting, as above. The "running
current" flow paths in a RPC are quite complex and are also assynmetric even
in a tuned, voltage balanced RPC because of direction of rotation, among
other things. Suffice it to say, the current flow in a RPC and its load
requires "steerage" (think series resonance) in order for the fundamental
single-phase running current to be guided into paths that emulate 3-phase
conditions. Remember, you are still dealing with single-phase power. A RPC
does not "generate" 3-phase power - it merely performs adjustments in a
fundamentally single-phase scenario which emulates 3-phase.


I could try to use run caps at run time and start caps at start
time. In fact, I won a time delay relay for $9 on ebay yesterday, so I
could set the RPC to start on start cap (both caps between leg 1-3)
and then reconnect the same caps to become run caps, one between 1-3
and another between 2-3. Same TDR could, then, turn out output current
aftet time delay, allowing the RPC to spin up and switch to the run
mode.

By all means do "use run caps at run time and start caps at start time".
Forget about reconnecting start caps and using them as run caps. Either use
a simple push-button switch to temporarily connect the start caps or a NC
potential relay that senses 3rd leg voltage to open up the start circuit.
Then leave the start circuit alone. Period.


As you can see, I am quite confused, but am willing to experiment. I
have 4 unused Furnas 75 A contactors that I can wire, with the time
delay relay, to do just about anything.

Idler: 10 HP

Capacitors: 92 mF each, 535 VAC rated, oil filled. I have 5 total, and
use 2 for the starting leg, so three are unused.

It occurs, you are attempting to use components that may not be appropriate,
or the best way, just becasue you got them on the cheap. No amount of
aimless, and possibly dangerous experimentation, with the wrong things can
necessarily force success.

Bob Swinney


i