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?


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.

  • \$\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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