Unfortunately when I learned about electric motors, I was taught that
an electric motor GENERATES a back emf because there is a magnetic
field which is cutting conductors.
So my view of say a motor is somewhat different from yours. If you
have an electric motor and you increase the load, the speed drops
slightly, the back emf drops, and the current drawn goes up. If you
decrease the load, the speed increases, the back emf goes up, and the
current drawn goes down. If you decrease the load until it is
negative, ( ie mechanical power is being applied to the motor ) , the
back emf goes up until it is more than the applied emf, and the current
drawn goes negative. That is current goes into the mains. That is an
induction generator. Works whether the motor is a single phase motor
or a three phase motor.

If you have three phase motor and get it running on single phase power,
things are a bit more complicated. But you still get a back emf
generated, and on the terminal that is unconnected to the power there
is a back emf, but there is no forward emf. So you can draw current
from that terminal.

Consider this. If you get a three phase motor running on single phase,
you can use it to produce mechanical power. So the way I think of a
RPC is as a three phase motor running on single phase, with some of the
mechanical power being used to drive an induction generator.

I am a bit confused by your statement about overdrive from the AC
mains. Mostly by the word "overdrive". As I see it a RPC is connected
to the AC mains, so I think it would be excited by overdrive from the
AC mains. However an induction generator does not have to be connected
to the AC mains in order to work. You can use a gasolene engine to
drive an induction motor and generate electric power with no connection
to the mains. It just is just sensitive to the amount of power you
draw and does not regulate the voltage at all well.

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.

So I still analyse a RPC as an induction generator. Trying to analyse
it as some sort of transformer, I have a lot of problems figuring out
one ever gets anything that is not still in phase with the original
single phase mains. And how one calculates what the phase angle is
going to be.

I amy not be able to convince you that this is a valid way to analyse
RPC's, but it works for me.


Dan




Robert Swinney wrote:
Dan sez:
" In my opinion you need to realize that a RPC is an induction generator."

Dan, I know you have some experience with induction generators so I'll ask
you to respectfully consider that:

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

wrote in message
oups.com...

Robert Swinney wrote:
IMO, you need to lose the thinking of a RPC as being a form of generator.

Bob Swinney



As far as flywheels are concerned, a flywheel will keep the slip angle
from changing as quickly. So a RPC without a flywheel will draw power
from the mains more quickly when the load is increased. Score points
for that side. On the other hand, a RPC with a flywheel will draw
power from the flywheel when the load is increased as well as from the
mains. So score points for the other side.

In the real world, it does not make much difference as the change in
speed of the RPC should be slight, and therefore only a small amount of
power can be drawn from the flywheel. Having a flywheel would help
with an undersized RPC when the load motor is plugged.


Dan