Three-Phase Motors

Three-Phase Motors
Three phase AC power is comprised of three independent voltages. Each phase is displaced 120 degrees from the others.

When phase one (A) is at zero volts, phase two (B) is near its maximum voltage and flowing in the positive direction. The third phase (C) is near its maximum voltage as well, but flows in the negative direction.
These three phases will change from positive to negative as the AC power cycles. A rotating magnetic field is produced if each of the three phases is connected to an electrically independent winding in an AC motor stator.

In this example, using time 1 as our reference point, the current flow in the green phase A winding is positive and pole A1 is north. The opposite pole, A2 is magnetically south. The resultant magnetic field is shown moving from north to south.
The current flow in the blue phase B winding is negative, so pole B2 is north and B1 is south. The resultant magnetic field is shown flowing from B2 to B1.
There is no current flow in red phase C, so these poles are not magnetized. They are neutral. The result is that there is no magnetic field being produced in this winding.
These magnetic fields produce a rotating force in the direction shown by the arrow. This arrow represents the turning of the rotor.

Moving to time 2, the red phase C current is negative going, thus poles C1 and C2 are south and north respectively. Their blue phase B current is positive going and poles B1 and B2 are north and south, respectively. Because the green Phase A is at zero, the A poles are neutral. The arrow represents rotation in the direction of the magnetic field.

Finally, at time 3, we see that the green Phase A is positive going and the red phase C is negative going. Their respective poles are energized with the resultant magnetic fields producing a continuation of the rotating magnetic field. This force is what creates the motion of the rotor.

AC power cycles 60 times per second between positive and negative. In a fraction of a second, the phases have shifted 60 degrees causing the relationship of the north and south poles to change at the same rate. Because the motor has established an induced magnetic field, the opposite fields of the rotor and stator attract each other, causing the rotor to follow the stator's magnetic field change.

As the rotor continues to follow the stators magnetic field, the three phases will shift yet another 60 degrees. It is this continuous change in polarity that causes the rotation of a motor.