On Sat, 06 Aug 2005 00:46:19 GMT, Ignoramus19023
wrote:

SNIP

For the run capacitor, try one capacitor or two capacitors series
connected to halve the effective value - whichever gives the best
voltage balance with full load on the load motor.


Looks like one run cap per side should be good, based on Jim
Hanrahan's writings. I'll see.


Do not place a capacitor across L2 L3. With the values you have
available this would do more harm than good.


Hm, why is that so?


Anyway, my main question is: if I do all this (as I outlined in steps
1-6), will I get a Mercedes Benz of phase converters? Or am I wasting
my time for load up to and under 5 HP?

i


This is an extract from an earlier post which goes some way to
explaining the peculiarities of "balancing" capacitors.

Snip

A converter of this type is basically a capacitor/inductor phase
shift system which produces an open vee 3 phase system. This phase
shifter is a series resonant circuit and when it is set up to give the
60 deg phase shift it is working a long way below its natural resonant
frequency. 60 deg is of course the correct phase angle between the
two legs of an open vee system.

The motor(s) is the inductor in the system and unfortunately the
apparent inductance of the motor changes with rotor speed. For any
particular rotor speed greater than about 90% of synchronous speed
(the lower limit varies a bit with motor type) it is possible to
choose a capacitor combination which produces a pretty close
approximation to balanced 3 phase at the motor terminals.

For near the full load rated speed of the motor, large run
capacitance is needed with most or all of it as a single capacitor
feeding the phantom phase from supply live. At light load the speed of
the rotor rises and if the capacitor value is chosen to achieve the
right phase angle the phantom phase voltage will be excessive. This
could be corrected by feeding the capacitor from a lower voltage
single phase source but this would mean feeding it from an auto
transformer across the supply.

It is much simpler (and of course everybody does this) to use
two capacitors arranged as a voltage divider to simultaneously achieve
the correct phase angle and phase voltage.

The effective capacitanceof the two capacitors connected in
series across the supply is the sum of the capacitances because the
source impedance of the supply is zero and this effectively parallels
the two capacitors.

Because the they also act as a voltage divider, this sum
capacitance is effectively fed from a voltage of supply voltage times
C1/(C1+C2) where C1 is the top capacitor and C2 is connected phantom
phase to neutral.

Because it looks nicely symmetrical there seems to be a
tendency to believe that C1 and C2 should be equal and any inequality
in their optimum value must result from some strange second order
effect. This is NOT true. There is nothing magic about equal C1 and
C2. It simply results in a capacitor of value C1+C2 fed from half the
supply voltage. At this low effective supply voltage it is only
possible to get close to balanced operation at no load or light loads
which enable the rotor to operate close to synchronous speed.
As the load increases with consequent slowing of the rotor speed the
total capacitance needs to increase with both more in C1 and less in
C2. By the time full load is reached the optimum value for C2 is
usually zero.

These effects are very noticeable if you're using a single
motor on a variable load up to near rated full load power and some
compromise necessary. The saving grace is that industrial motors are
surprisingly tolerant of reasonable overvoltage when operating at
light loads so the trick is to size the capacitors for at or near full
loads and to accept some overvoltaqe at light loads. This increases
the motor losses at light load but the total motor losses still remain
below the losses at rated full load so temperature rise is acceptable.

SNIP

In your case, while a small additional capacitor across L2 L3
might yield better phase balance for some load conditions, the 92uF
you have available is far too large and would do more harm than good.

I fully support Jerry Martes comment to the effect that fine
tuning rarely achieves significant practical benefit.

If you've a nice big idler and have solved the starting problem,
careful choice of balancing capacitors may give you a nice warm
feeling but you're unlikely to notice much practical difference in the
overall performance!

Jim