A PSC motor is usually a 2-phase asymmetrically wound motor. The main winding is designed to take the load current.The current flowing through the start winding is much less than the main winding. Therefore, the start winding will have a different electrical characteristic compared to the main winding. In order to produce the Magnetomotive Force (MMF) produced by the start winding very near to the main winding, the start winding has additional turns, higher resistance, and reduced current flowing through it. This makes the motor windings asymmetrical.
The motor is energized with a single-phase AC power supply, with a capacitor connected in series with the start winding (also called an auxiliary winding) as shown in Figure 1. The value of the capacitor is chosen so that the total impedance on the auxiliary winding produces sufficient phase shift in current to generate a rotating magnetic field in the air gap. Typically, the current flowing through the start winding leads the current flowing through the main winding by 90 degrees.
By adding a microcontroller-based control circuit to the motor, the voltage across the main winding and start winding can be maintained at 90 degrees to each other. The other advantage is that the starting capacitor can be removed from the circuit, thus reducing the total system cost.
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