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
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
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
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
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
"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.
Seems an appropriate thread to mention that Phase-A-Matic has info on their
website on making a rotary phase converter using their static phase
converter. They have this in PDF files that you can download. Just thought
this was a good resource.
Thanks for getting back to the group and letting us know what you
discovered. Nice report.
Unka' George [George McDuffee]
He that will not apply new remedies,
must expect new evils:
for Time is the greatest innovator: and
if Time, of course, alter things to the worse,
and wisdom and counsel shall not alter them to the better,
what shall be the end?
Francis Bacon (1561-1626), English philosopher, essayist, statesman.
Essays, "Of Innovations" (1597-1625).
I was told a long time ago, by an engineer working for Brook Crompton,
that a static phase convertor was sometimes known as a Steinmetz convertor.
I assume that this refers to Charles Steinmetz. Possibly it was him who
first figured out how to run a three phase motor on single phase power,
although I have never been able to verify this:
Steinmetz invented the classic capacitor-start single phase induction
motor. He also invented a kind of static phase converter in the
process, but he appears to have been interested only in starting. See
US patent 620,988, granted in 1899, which is cited in 5,454,965 to
Smith, so I would assume that this is the most applicable Steinmetz
So they do. Their original patent is in the list I provided. Don't
know if Phase-A-Matic still does it that way.
The original question was if having a big idler motor would protect a
VFD or inverter from such abuse as someone plug-reverse a machine.
Phase-A-Matic permits plug reversal so long as the idler is at the very
least twice the size of the motor being reversed, but static converters
are pretty robust, so one cannot conclude that a VFD would be so happy.
That said, the 2:1 motor size ratio is certainly the minimum.
Thanks. It's certainly an issue of wide interest in RCM.
One thing. I think I recall some debate and difficulty on starting
RPCs, given that most three phase motors lack centrifugal switches to
disconnect the start capacitor. I came upon a solution in my research.
In the report and patents, a simple approach is given: Use a voltage
relay to measure the voltage of the generated phase, the relay in turn
controlling a contactor that disconnects the start capacitor when at
least 80% of normal speed is achieved. This approach was praised in the
report for its simplicity and reliability, and appears in both relay and
its solid-state equivalent in more modern patents.
[ ... ]
Well ... I have a Nichols horizontal mill (older than I am) with
a 3/4 HP three phase motor, and it is powered from a vastly oversized
VFD on a distant wall. It is rated at 30A, so I figure that it can
handle 7-1/2 HP from single phase, and somewhat more from three phase.
Anyway -- I don't bother trying to vary the speed at that
distance, but I regularly plug reverse the spindle with this, in spite
of the advice which I regularly give to not have switches between the
motor and the VFD. This is so overpowered that the VFD doesn't even
notice the motor, and the 20' of three phase cable is sufficient to
capacitively bypass the worst of the inductive voltage spikes. So -- I
don't advise this -- but it can work under certain conditions.