The AC motor also works by rotating the stator field, but it makes use of the natural alternating nature of the AC wave to turn the field coils on and off sequentially. The AC induction motor does not need brushes because the rotor is essentially a passive device that is continuously being pulled in one direction. To use an old analogy, the rotor is the “horse,” and the rotating stator field is the “carrot.”
To explain the principles of how the AC wave can be used to sequentially energize the field coils, we will examine the operation of a theoretical two-phase motor.
The action of two-phase AC on the motor is to cause the stator magnetic field to effectively rotate clockwise (called a rotating field), even though the coils themselves are stationary.
In image, at 0° phase A is at peak voltage while phase B is 0 V. At this point, phase A has all the voltage, and phase B has none; therefore, the windings connected to phase A (top and bottom) will be energized, and the windings connected to phase B (left and right) will be off. This situation is depicted in the coil drawing (upper left) of image. The polarity of the applied voltage causes the top winding to present a north (N) magnetic pole to the rotor and the bottom winding to present a south (S) magnetic pole to the rotor.
At 90° later in the power cycle (image), phase A voltage has gone to 0 V (deenergizing the top and bottom windings), and phase B has risen to peak voltage, energizing the left and right windings. Specifically, the positive phase B voltage will cause the right side winding to present a north magnetic pole to the rotor and the left winding to present a south magnetic pole (as indicated in the coil drawing of image).
At 180°, phase B voltage has gone back to 0 V (deenergizing the left and right windings), and phase A has descended to a negative peak voltage. Once again, the top and bottom windings are energized but this time with the opposite polarity from what they were at 0°, causing the magnetic poles to be reversed. Now the bottom winding presents a north magnetic pole to the rotor, and the top winding presents a south magnetic pole.
At 270°, phase A has ascended to 0 V (deenergizing the top and bottom windings), and phase B has gone to a negative peak. Once again, the left and right windings are energized but this time with the left winding presenting a north magnetic pole to the rotor and the right winding a south magnetic pole.
This analysis explains how two-phase AC* causes the magnetic field to act as if it were rotating in a clockwise (CW) direction. (You can see this in the coil drawings of image, where the north pole apparently rotates CW.) What was not apparent from the discussion is that the rotation of the field is smooth and continuous—it doesn’t jump from pole to pole as might be inferred from the discussion. For example, consider the situation at 45°. From Figure image, you can see that both sets of poles
are partially energized, causing the resultant N-S magnetic field to be halfway between the two poles.