In a recent long and wandering thread, (phase converter questions 8/24) it
was written:

"A pair of good posts. It really is refreshing to see
some solid input on 3 phase phase conversions which is both
soundly based and backed up by direct measurement. I hope it
will dispel some of the myths on minimum sizes of idlers and
the practical usefulness of precision "tuning"."

This, of course, refers mainly to the excellent report from Jerry Martes and
tests made with his dynamometer. Perhaps, I know a bit more than others
about Jerry and his dynamometer. I have knowledge of some of the design
particulars of Jerry's dyno and I know that it is a first class machine both
with regards to the way it loads and the way it measures HP. Therefore, I
would be the first to say Jerry's test were soundly based and backed up by
direct measurement. Jerry's tests proved that well designed 3-phase motors
have a built in power reserve. The amount of "reserve" varies among
manufacturers, and probably the quality of their products; it is known as
"service factor" and would account for a 3-phase motor running on
single-phase being able to deliver full rated 3-phase power out for a
limited time. Jerry explained it very well.

To those which may be "new" to rotary phase converters, I would caution not
to be mislead by erroneous conclusions that may be drawn from the RCM
thread, above. Jerry reported the load vs. current characteristics of
3-phase motors quite accurately and showed they can be driven beyond full
rated output for short periods, whether running on single phase or 3-phase.
Note that Jerry did not recommend this practice - he only reported on it.

Having a bit of experience with rotary phase converters, frankly I cannot
say where the "1.5 X" minimum size of idler came from. Perhaps Fitch threw
it out several years ago when researching the subject, I just don't know. I
do know that to successfully start a 3-phase load from an idler motor,
whether "balanced" or not the idler must have a certain minimum size in
order to take the load. How large, I'm not sure and have never experimented
in this area. Perhaps, for starting duty only, the 1.5 figure is a bit high
in some cases. I do know, though, that 1.5 is a good "platform" to work
from toward voltage balancing a rotary and load system. The case for
balancing can be made from a starting current standpoint. As various
respondents have reported, no ordinary residential circuit breaker could
stand the surge of starting a 15 or 20 HP non-balanced motor on single phase
current.

Precision "tuning", like magic, lies in the mind of the beholder. How much
precision lies somewhere between none and much too much. Myths? I think
not. No, a proper RPC system is merely an example of a serious metalworking
hobbyist adhering to good engineering procedure.

Bob Swinney