motor
I know 3 phase has the advantage of being able to start from any position unlike 2 phase. But my question is about power density…
In a 6/4 three phase motor, 1/3 of the stator is active at once, and only 1/2 of the rotor. In a 8/4 two phase motor, 1/2 of the stator and all of the rotor is active at once. Does this mean two phase is more powerful, and if so, by how much?
A standard 3-phase, AC motor has the stator windings distributed in the slots to form poles that do not have much saliency. A switched reluctance motor has both salient rotor poles and salient stator poles. You would need to rewind the stator to have a motor like that.
A synchronous reluctance motor has a salient-pole rotor, but distributed windings in the stator to form non-salient poles. To convert an induction motor to a synchronous reluctance motor, you could use the stator as it is and machine the rotor to look something like this:
You can get some induction-motor starting torque from the remaining rotor bars, but the best way to operate the motor would be with a variable-frequency drive (VFD). The VFD will provide reluctance torque from stand-still. That may or may not suit your purpose.
Of course, you may have some difficulty with rotor balance.
The stator magnetic flux can be visualized and mathematically described as two flux waves rotating in opposite directions. Because of the shape of the torque vs. speed curves and the effects of the rotor magnetic flux, once a direction of motion has been established, the forward torque is higher than the reverse torque as shown below. The solid line is the sum of the forward and reverse torques.
The above illustration is taken from Fitzgerald, Kingsley Umans, Electric Machinery 4th ed and the paragraph above is a summary of a 1-1/2 page explanation in that text. Also in that text, reference is made to another text for "an extensive treatment of fractional-horsepower motors."
Forward vs. Reverse Torque
The torque produced by the forward-rotating magnetic fields is higher than for the reverse because of the shape of the curves. The torque vs. speed characteristics can be determined by analyzing the equivalent circuit of the motor. That circuit for a single phase motor that has been started is shown below. A more complex version of the circuit may be required for an accurate determination of the characteristics.
Fitzgerald, Kingsley Umans, Electric Machinery 4th ed
Split Phase Starting and Running
A single-phase motor has two stator windings with different resistances and inductances or with a series capacitor in series with one of them. Split-phase motors have a phase displacement between the currents main and auxiliary windings. That creates an approximation of a two-phase motor. An analysis of a split-phase motor still makes use of the theory of two revolving fields. Another concept that is used is the symmetrical-component concept. Each of the two concepts has advantages and disadvantages.
Best Answer
There is no such thing as 1/3 pole at time. All three phases a sine with a shift of 120 degrees. When you add all the currents you get a rotating stator current, which is DC. This rotating current generates a stator rotating magnetic field.
The same principle applies to two phase motor. The phase currents are shifted by 90 degrees and this forms a rotating field.
A two phase system requires 4 wires, two per phase, meanwhile a three phase system requires only 3 wires, since the sum of currents shifted by 120 degrees is zero.
The rotor of the machine won't notice the difference between two phase or three phase stator.