I hope this message actually gets posted. My last two attempts failed.
I am hoping the problem has something to do with cookies and posting
through Google.

When I learned about motors, I was taught that a motor generates a back
emf because there is a rotating magnetic field cutting the conductors
in the winding. When a motor is running the back emf is close the
applied voltage and the amount of current drawn is proportional to the
net emf / the inductance and resistance of the winding. Increase the
load on the motor and the speed decreases, back emf goes down, net
voltage goes up, and current goes up. Decrease the load and the speed
increases, net voltage goes down and current goes down. Decrease the
load some more until it is negative ( mechanical power going into the
motor ) speed increases, net voltage goes negative ( back emf is
larger that applied voltage ) and the current goes negative ( power
goes into the mains ).

This works from locked rotor to induction generator for single phase
and three phase motors. Just don't try the locked rotor for very long
unless you have an AC servo motor.

Now when you have a three phase motor running on single phase power, it
still works. A back emf is generated that keeps the net voltage across
the terminal connected to the mains from being very large. But you
also have a back emf generated in the windings that connect to the
terminal that is not connected to the mains. Not quite as large as the
emf from the mains, but nearly as large. So you have single phase
power being consumed and three phase power being generated.

This may not be the only way to analyse a RPC, but it works for me, and
I think it works for Don Young and Pentagrid. Speak up if you
disagree.

A couple of aside issues. An induction generator will work without
being connected to the mains. Google enough and you will find some web
sites that talk about using an induction motor and a lawn mower type
engine to power field ham radio stations. Such a generator is load
sensitive.

Also a rather nice RPC can be made using a single phase motor to drive
a three phase motor using an adjustable belt drive. Both motors are
connected to the mains and the belt drive adjusted so the current to
the single phase motor is at or below the nameplate current when
supplying three phase power to the load. I think this type of RPC will
supply three phase power that is more balanced and therefore suitable
to run things as surface grinders that are sensitive to harmonics in
the power.

Dan


Robert Swinney wrote:


An induction motor is a consumer, not a generator. As you know true
induction generators (induction motors) have to be excited by overdrive from
the AC mains in order to generate. Tht is not done in any fashion in a RPC.
The RPC is a load on the mains, not a supplier to the mains. Again, I'll
say, we need lose the idea of a RPC being a generator. Think of it as more
of a converter; well, that's part of it's name now isn't it?

Bob Swinney