On Sun, 21 Mar 2004 19:58:30 -0800, "Paul Baygents" wrote:
So I've got my converter running. However, I was told that by adding
run capacitors I could lower the needed Amps to drive the idler?

Yes.

Is this true? If so would I simply put them inline on the incoming 220
hot leads going to the idler? Or else where?

There are two different uses for run caps in a converter. You can
use them as balance caps to equalize the phase voltages, and you
can use them as power factor correction caps to lower reactive
current draw from your electrical service.

Balance caps connect between L1 and L3, and between L2 and L3.
(L3 is the "wild" or manufactured leg). Typical values are in the range
of 20 to 40 microfarads per horsepower. Note that the amount of
capacitance between L1 and L3 will probably be different from the
amount of capacitance between L2 and L3 at the point of best balance.
You have to experiment to find the exact values for your particular
motor. (Paralleling a number of lower value capacitors lets you form
any value you actually need.)

Note, you'll want to balance the converter so the wild leg voltage is
a bit high when unloaded, and sags a bit low under full load. You
can't get it to be *exactly* the same as the other legs under both
unloaded and fully loaded conditions. If you can balance L1 to L3
and L2 to L3 within 10% of L1 to L2 when loaded and unloaded,
call it good and quit.

Power factor correction caps connect between L1 and L2. The value
is whatever reduces the 1 ph line current from the service entrance
to the lowest value. Again experimentation will be required. Do this
*after* you balance the converter, if you're going to balance it. The
value won't be the same for a balanced converter as it will be for
an unbalanced converter. (Usually, it'll be smaller for a balanced
converter.)

What the power factor correction caps do is cancel the inductive
reactance due to the motor which is seen by the service entrance.
This reactance can set up high circulating currents in the primary
feed if it isn't canceled. These are wattless watts in the sense that
the current is out of phase with voltage and doesn't represent real
power being drawn from the utility, but they can cause the wires to
heat up (series R loss), and require you to use a larger than normal
breaker to prevent nuisance tripping.

I can't supply you with exact values since those will vary depending on
the particular motor you're using, ie different motors have different
impedances. What you're shooting for with the power factor correction
caps is to make the motor impedance R + j0. You probably can't hit
that exactly, but getting close can make a big difference in the current
draw.

Balance caps and power factor correction caps aren't strictly necessary
to make a working rotary converter. But they are nice to use because
they'll give you better phase to phase voltage balance, and will reduce
the converter's power factor, letting you run from a smaller breaker.

Gary