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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?

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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.

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  • \$\begingroup\$ I thought it only did 1/3 at a time like in this video m.youtube.com/watch?v=hDJnLt7cBTY \$\endgroup\$ – Guest Mar 25 '17 at 10:00
  • \$\begingroup\$ you do ... when you consider a 6/4 SR machine. This post appears to be considering a sinus excited stator WHICH you do not do with an SR machine \$\endgroup\$ – JonRB Mar 25 '17 at 13:21

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