On Tue, 14 Sep 2004 09:11:24 +0000 (UTC), "Don Foreman"
wrote:

I think this is intriguing, might be worth a bit of head scratching. It's
intriguing because it offers the possibility of a "self-tuning" RPC that
adapts to varying loads. Electronics could be quite simple because they
need not produce significant AC power (as in a VFD) but only control DC
excitation current.

Assuming that a neutral point is not available, a neutral-equivalent voltage
reference could be synthesized with opamps. It would be half the
line-to-line excitation plus a quadrature component produced by an RC
phaseshift circuit. Perhaps the third-leg phase voltage relative to this
quasi-neutral could then be compared to the other phases relative to the
same point, and excitation then self-adjusted to minimize the difference
between generated third-leg magnitude and the other phase magnitudes
relative to the quasi-neutral. Generated third-leg would be whatever it
turns out to be, since we have only one controlled variable -- excitation.
However, if the magnitude is right then I think the phase would be pretty
close to right due to the geometry of the machine.

Such a controller could be accomplished digitally with a microcomputer of
course, but I'd find it easier to see and understand what's going on in
a controller realized with a few opamps.

Comments? I could be up for helping to design and even build such a gadget
just to see if and how it works.


Should be an interesting project.

A possible alternative method of synthesising the signal is
a couple of low power transformers in Scott connection. The
first is a centre tapped auto across the supply. The second
is of any convenient ratio with primary connected from
centre tap to phantom phase.

Transformers are a nuisance but they have the big
advantage that they fully isolate the control circuitry from
the high power bits.

If isolation isn't important, an opamp looking at
the difference between a three equal resistor artificial
neutral and the phantom phase is yet another way.

The really interesting measurement will be to see
what happens to the phantom phase angle when the regulator
corrects the voltage drop resulting from a heavy load. If
the model really describes the way it works we should see
the effect of a small shift in the angular relation between
the rotating field magnet and the rotating field component
of the supply.

There's a good chance that it will be a really
useful improvement on the basic rotary converter but it's
pity that 3 phase alternators are a lot rarer than motors.

Jim