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RE: 3 phase converting

Original poster: "David Thomson" <dwt@xxxxxxxxxxxx>

Hi Steve,

>  >You can run a three phase
>  >motor okay, but you can't generate a true rotating magnetic field

> That's kind of a contradiction in terms. If you didn't have a
> rotating magnetic field then by definition you would not be
> able to start a three phase motor.

Most of the time you can't start a three phase motor without a true
rotating magnetic field.  That's why a starter cap, hand crank, or
switch is used.  The rotor sets up and maintains the rotating magnetic
field once the motor starts running.

The Egg of Columbus demonstration has no rotor, which is why the
phased current is needed.

> I once visited the EE department at Glasgow university and
> they demonstrated a spinning "Beer Can Of Columbus" that
> worked off single phase current. It used two coils 90 degrees
> apart with one fed directly from the line and the other fed
> via a capacitor. I think they used low voltage AC like 24v or

Tesla used two phase for his EOC demonstration, too.  As for using
capacitor phased power for a rotating magnetic field, I've researched
this and talked to experts over the past couple years.  Nobody seemed
to think this would work.  If you have a link to an actual
implementation of capacitor phased current generating a rotating
magnetic field in a stator with no rotor, I would be very interested
in reading up on it.

> My conclusion- You don't need three phases to generate a
> rotating magnetic field. It just happens to be the most
> efficient and economical way of doing it.

In my case, since I have an actual application for a rotating magnetic
field, I can draw no conclusions until I have a working unit in front
of me.

> As an aside- Even with only one phase and no added capacitors
> (so no rotating magnetic field at all) a three phase
> induction motor will run. But pretty poorly and the starting
> torque is zero.

Once again, there is a rotor involved.  My application has no rotor.
And also, once again, not all three phases will run.  You can verify
this by checking each leg of the wiring.  I've tried it already.  My
thought was that if I could get a three phase motor to run with single
phase, that all three legs would be hot and I could tap these three
legs for real three phase power.  But it doesn't work.  I could never
get more than two legs hot at once.