On 25 Mar 2004 07:06:57 -0800, (gradstdnt) wrote:
Gary Coffman wrote in message . ..
On Wed, 24 Mar 2004 20:03:28 -0800, "Robert Swinney" wrote:
gradstdnt sez:

". . .currents for each leg are not as nicely
balanced. . . . "

You should not be measuring current in the 3 legs when attempting voltage
balance. It will totally confuse you! The current flowing in a 'rotary
phase converter / parallel load' is very complex. Some of the current looks
like it should be flowing in 2 directions, against itself. Of course, this
doesn't actually happen, but the reactive currents combine, add, and
subtract in the 3 legs in ways that make them an extremely unreliable
indicator of balance. Stick with voltage balancing techniques and leave the
clamp-on ammeter for the final power factor adjustments.

Bob's right. Balance the phase voltages, not the currents. The only time
you want to measure currents is when tuning out power factor on the
primary.

Gary

I would like to clarify my three phase amp measurements. I was not
measuring currents to/from the converter. I was masuring currents in
each leg to the load motor while under power. I have measured both and
they are different. My initial thought was to better understand how
much current I was providing in the generated leg. I measured the
other legs for reference and found the results interesting. When the
voltages were blanced to within 1%, the currents were also nicely
balanced. I thought this was the goal of balancing but others have
pointed out issues with idling currents.By removing capacitance, thus
unbalancing the loaded system, the currents in each leg to the load
motor were also thrown out of balance. One let got hotter, as two
others got weaker.
In general, any further inbalance I have in the converter decreases
the current provided in the generated leg.

Thanks again for the input. I will work with balancing the converter
for my min and max loads. This should provide me with the boundary of
my operating conditions. I can then better evaluate the tradeoff
between loaded and unloaded voltage balance.

The load motor is a reactive load, the rotary converter is a reactive
source. Amp clamp meter readings won't be very helpful in such a
situation. The meter can't differentiate between reactive currents
and non-reactive currents.

To really know what's going on, you need to use a dual trace scope
setup to sum vector voltage and vector current in each circuit mesh.
That'll tell you how much actual power is being supplied by each phase.
You want those to balance. It would be a surprising coincidence if that
occurs at the point where the amp clamp meter readings are equal.

The main thing that you want to worry about when tuning a rotary
with ordinary test equipment is to achieve rough voltage balance
phase to phase over the operating range of the converter. To do
that, you want the voltage deviations from equality to swing roughly
symmetrically about nominal as you go from no load to fully loaded,
while at the same time deviating no more than 10% of nominal in
either direction.

Note that fully loaded means the load motor is mechanically fully
loaded to its nameplate hp. Under light loads, a load motor will
be a very reactive load, which can lead to some wildly misleading
amp clamp readings when driven by a reactive source like a rotary
converter.

Gary