I went quiet while I researched the issue of how RPCs work and if a RPC
supplies some of its own kinetic energy to the three phase motors
connected to it. (It does.)

The purpose of this posting is to report my findings, and is a
continuation of the February 2009 thread "VFD as 3Ph shop supply".

I was looking for an academic analysis of how phase converters work, and
hit pay dirt by searching US patents. The pattern is that when an
academic patents something, the patent is most often a little paper
complete with theory and references, and can be a very useful starting
point. And a professor is likely to get the theory right.

This approach worked yet again, leading me to Berkeley Professor Otto J.
M. Smith, who spent a major fraction of his career on rural
electrification.

http://en.wikipedia.org/wiki/Otto_J._M._Smith

http://phaseable.com/

Dr. Smith comments (in 4,792,740, col 14, lines 11-15) that: "It is well
known to persons skilled in the art that induction machines can be used
either as induction generators or induction motors. All of the circuits
disclosed herein can be used for induction generators feeding power into
the public utility power supply."

Dr. Smith's patents listed below all pointed to a report written by
Claude Hertz. The report, "Current Techniques in Phase Conversion
Systems", Claude M. Hertz, presented to the IEEE 1978 Rural Electric
Power Conference in Minneapolis, Minnesota, on 1-2 May 1978, was
invited. Dr. Hertz was the Vice President - Engineering of Ronk
Electrical Industries, and his now obscure paper is still the best
summary available of the theory of static and rotary phase convertors.

The design equations for phase converters generally treat the motor as a
black box, one having a power factor that varies with load, but do not
delve into motor theory.

Use of measurements of currents and/or voltages to control capacitor
switching to achieve balanced drive to the three phases is much
discussed in both the above report and in the various patents and
articles listed below. The key observation is that the three currents
vary differently as the motor load varies. Likewise voltages across the
three leads. These variations can be used to implement very reliable
switching of capacitors (or autotransformer taps) for starting , and
also to maintain phase balance over the load range of the motor. The
schemes shown in the later patents are easily implemented by anyone with
some electrical knowledge, especially the use of a voltage relay across
one motor winding.


Of interest are the following US Patents:

4,484,125 Hertz, Claude "Three-phase delta-wound motor operated from
single-phase current", granted 20 November 1984. Rotary converter and
motor.

4,792,740 Smith, Otto J.M. "Three-phase induction motor with
single-phase power supply", granted 20 December 1988. Rotary converter
and motor. Allows full power. Appears to require 440 volts for a
220/440 motor.

5,545,965 Smith, Otto J.M. "Three-phase motor operated from a single
phase power supply and phase converter", granted 13 August 1996. Static
converter.

3,673,480 Johnstone, Colin "Circuit for operating polyphase induction
motors from single-phase supply", granted 27 June 1972. Static
converter. Cited by Phase-A-Matic.


And the following published articles:

"Automatic balance of three-phase motor currents for variable motor
loading using a static phase converter"; Leo Soderholm and Claude Hertz;
IEEE Trans. on Industry Applications; v.26, n.4, July/August 1990, pages
679-682.

"Determination of autotransformer-capacitor phase converter parameters";
Roshan Chhabra, Leo Soderholm, and Leon Charity; IEEE Trans. on Industry
Applications; vol IA-9, No 1, January/February 1973, pages 41-45.

"Induction Generator Theory and Application", J.E. Barkle and R.W.
Ferguson, AIEE Transactions (Power Apparatus and Systems), February
1954, pages 12-19.


Joe Gwinn