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


In school I was taught that a motor generates a back emf because there
is a rotating magnetic field whose flux cuts the windings in the motor.
When a motor is running and you increase the load, the motor slows
slightly and the back emf drops so the net voltage increases causing
the current to increase. If you decrease the load the speed increases,
the back emf rises, net voltage drops and current drops. If you
decrease the load until it is negative, the back emf increases until it
is above the input emf, and the current goes negative. That is current
flows from the motor to the mains.

Now this works for both single phase and three phase motors from locked
rotor to being an induction generator. ( Don't try locked rotor for
very long unless you have an AC servo motor ) It even works for a
three phase motor running on just one phase. In that case the motor
consumes single phase power, but still generates back emf in all
windings. Which results in generating three phase power. Because the
back emf is less than the mains voltage, the voltage is not balanced.
But this can be improved by either adding capacitors.

So now you can at least see how I analyse RPC's. It isn't the only
way, but it works for me and maybe Don Young and Pentagrid. ( Speak up
if you disagree ).

As an aside issue, an induction generator will work without being
connected to the mains.
You can google and find some references to this as regards using an
induction motor driven by a small gas engine for operating ham field
stations. It is load sensitve.

And as another aside, you can build a fine RPC using a single phase
motor connected to a three phase motor via a adjustable belt drive. Two
terminals of the three phase motor are also connected to the mains. You
adjust the belt drive so the current drawn by the single phase motor is
at or below name plate rated current while the RPC is supplying three
phase power to whatever needs three phase power. I happen to think
this approach is good for things as surface grinders where good three
phase power is needed.


Dan