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RE: 3 phase converting

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 10:38 AM 1/20/2005, you wrote:
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.

This doesn't sound right. Motors such as split phase synchronous (Slo-Syn) use a PM rotor and capacitor phasing to generate the second phase. Obviously, there are tons of induction motors using run capacitors to generate the second phase. Garage door openers would be a good example, as are TV antenna rotators (anything where you want cheap, no brushes, and reversible).

However, say you have two coils and you want the phase of the current through one to be 90 degrees away from the phase of the other. Could you not just put an appropriate capacitor in series with the one coil. Say that the coils have an impedance of R+jX..

Make sure that R = X...

You'd put a capacitor of impedance -j2X in series, so one leg would have impedance R+jX and the other would have impedance R-jX. Since R=X, the current through one coil will be lagging the voltage by 45 deg, and the current through the coil with the capacitor will be leading by 45 deg, hence 90 degrees apart.

If the inductance is large compared to the resistance, then a capacitor of just -jX should work. If R<<X, then current through one coil is 90 degrees lagging, and the current through the other coil will be in phase.

If you want balanced currents, you might need to fiddle a bit with Ls and Cs to get the phases and amplitudes right.

> 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.

Three phase provides economic advantages over two phase, because the current in all conductors is the same.