Don Sez:

"Yes, I do. Of course, that depends in part on the relative
horsepower ratings of the idler motor and the lathe's own motor. I have
read here postings by people who have experienced that with equal
horsepower on each. And *that* might be something which could be
improved somewhat by a flywheel on the idler motor -- but expect the
housing to jump around during the reversal. :-)

Tuning and balancing capacitors might change the threshold at
which this reversal happens in that they will be applying a bit of
rotational bias -- but with equal sized or smaller idlers, you still
will have the idler reverse at some set of parameters." " Indeed so."

That "set of parameters" would have to be a reversal of polarity on the idler's two input line
leads. I'm afraid you are caught up in the erroneous premise that an idler motor and load motor
constitute a generator and load. They do not. Irrespective of rotational mass and any series
capacitance present, current flow (and torque) in each motor depends on the polarity of applied
voltage. Swap the two input (line) leads in either idler motor or load motor and that motor will
change direction. The other motor will continue to spin in the same direction as before.

In a RPC, the idler motor and load motor are not in parallel with respect to current flow. This
because of the way current must flow in the "3rd" leg to return to either side of the single-phase
line. There is no "parallel" way to connect a single phase source to a 3-phase load. For that
reason, only the inut line leads of either motor can be considered for polarity swapping in order
to achieve plug reversal.

Bob Swinney