Re: Flywheel on a rotary phase convertor


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
>


I am sorry but thinking of RPC's the way I do just seems to make sense.
When I learned about motors, I learned that they generate a back emf
because they have a rotating magnetic field and conductors that cut the
magnetic field. And if you increase the load on a motor, it slows
slightly and the back emf drops and the current rises. And if you
decrease the load the back emf increases and the current decreases.
Now consider decreasing the load even more, so that the load is
negative. ( putting mechanical power into the motor trying to make it
run faster than synchronous speed ) The back emf increases and the
current goes negative. That is current is being supplied by the motor
to the mains. So as I see it a motor can work from locked rotor to
being driven. ( Don't try locked rotor for very long unless you have a
AC servo motor ) Same physics for all cases.

Now this happens whether the motor is a single phase motor or a three
phase motor.

Now what happens when you have a three phase motor and run it on just
one phase?
After you get it started, it will run on single phase power. You still
have a rotating magnetic field, and windings for three phases. So the
rotating magnetic field generates a back emf in all the windings. So
you consume power from the single phase, but generate three phase
power.

Incidently you do not have to have an induction motor connected to the
mains in order for it to work as an induction generator. It just will
not work with a large variety of loads.

Another by the way. One of the better ways to make a RPC is to connect
a single phase motor to a three phase motor using an adjustable belt
drive. You monitor the current going into the single phase motor and
adjust the drive so the current is at or below name plate current when
driving the three phase load. Recommended ( by me ) for driving three
phase machines as surface grinders that are sensitive to unbalanced
three phase power.
In that case it is pretty obvious that you have a three phase induction
generator being driven by a single phase motor.

You may find this way of analysing a RPC as weird, but it works for me.
I have problems understanding RPC's as transfomers that produce a
voltage that is not in phase with the input voltage. And it lets me
think about how a flywheel would affect a RPC.


Dan

.

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