Wow, Dan! Nine posts. I am flattered (Really !) you went to such trouble
to share your opinion with us. I am very sorry you apparently were
responding to my first reply to you, which was in error. Maybe your server
didn't deliver my second reply, the one in which I attempted to correct the
error in the first. I am sorry to have thrown such confusion into this
already overlong thread. Anyway FWIW, my 2nd reply in an attempt to correct
the
errorious one went like this:

{"Sorry, Dan - Make that overdrive via overspeed from the prime mover to
make
an induction generator. The induction generator (one made from a common
induction motor) will generate when excited by the mains and when its rotor
is driven by external means to a speed exceeding that of the motor's
synchronous speed. Slip is said to be negative under these conditions."}

Please note, the operative change made here was to insert "rotor driven by
external means to a speed exceeding that of the motor's synchronous speed"
in place of "overdrive from the AC mains". Again, I am very sorry for that
mistake and for you to have gone to the trouble of posting 9 times in order
to straighten me out on the matter.

As someone once said, "I'm afraid I've already told you more than I know".
Please understand, the following quote from McGraw-Hill EE Handbook, 10
Edition, Sec. 18-116 is the source of all my knowledge re. induction
generators; it goes:

"""Induction Generators. Any induction motor, if driven above its
synchronous speed when connected to an a-c source, will deliver power to the
external circuit. This generator action is easily visualized from the
motor-circle diagram. (Fig. 18-17)corresponding to the lower half of the
circle in which the current vector is directed below the OV line. [pls.
adivse and I can scan the figure to your direct e-mail addy, if you like] A
unique feature is that the power factor of the output is fixed in value by
the generator characteristics and is always leading, independent of the
external circuit. The explanation is that the generator draws all its
excitation from the system and so must receive a definite amount of lagging
kilovoltamperes for a given voltage and load current. For this reason,
induction generators alone cannot supply a power system but must always
operate in parallel with synchronous machines or with capacitors. They are
therefore, no more helpful in system stability than the addition of parallel
reactors with a rating equal to the generator magnetizing reactance.

An induction generator delivers an instantaneous 3-phase short-circuit
current equal to the terminal voltage divided by its standstill reactance,
but its rate of decay is much faster than that of a synchronous generator
of the same rating, and its sustained short-circuit current is zero.

Since an induction generator must have a laminated rotor, to provide for the
slip-frequency rotor magnetic field, its construction is not adapted to as
high speeds as synchronous machines employing solid steel rotors. For these
various reasons, induction generators have found few practical applications,
their chief use being perhaps, in variable-ratio frequency converter sets,
where the induction end of the set operates as a motor or a generator
depending on the direction of power flow through the set."""

Dan, I'm confused over the paragraph where you said:

"To further confuse you, you can build a very nice RPC by using a single
phase motor to drive a three phase motor via a belt drive adjusted so
mechanical power is going into the three phase motor. Now if you apply
single phase power to the three phase motor, it will act as a three phase
induction generator. If you do this use an adjustable pulley on one of the
motors and measure the current drawn by the single phase motor. Adjust the
pulleys so the current drawn by the single phase motor is close to but below
rated nameplate current when the RPC is driving whatever load you are going
to drive. As you might suspect such a RPC produces voltages that are very
closely balanced."

It seems to me you may be describing an idler driven by a pony motor,
similar to systems in which the pony is disconnected after the idler has
come up to speed. Could this be the case you're describing, except that the
pony motor is not disconnected after the starting interval?

Bob (easily confused) Swinney