[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

RE: 3 phase converting

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

>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.

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

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

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.

Motor textbooks explain this by saying that the single phase pulsating field
is the sum of two rotating fields spinning in opposite directions. When the
rotor is at rest they cancel giving zero torque. But set it spinning by hand
and the torques no longer cancel because the slip frequencies for the two
fields are different.

Steve C.