PSC motors

Although not suitable for distribution of energy, the principle of creating a rotating magnetic field with two coils at 90° is widely used when only a single-phase supply is available, as in most households. The arrangement is following:


Due to the inductive nature of coils, the current in Coil 1 (red) will be at 90° delay vs voltage.
The same would apply for Coil 2 (blue), without capacitor. The presence of capacitor in series changes the phase of the current in coil 2 which can be shifted with reasonable approximation at 90 ° vs Coil 1. So, a rotating magnetic field is produced.

Motors working under such arrangement are called PSC motors: Permanent Split Capacitor motors.


In order to achieve highest possible torque, coils are inserted in an electromagnetic steel stator, and are uniformly ditributed. Here is a typical layout for a 2 pole motor, in a 24 slot stator.


to achieve a sinusoidal field it is convenient to have an high number of slots, compatibly with the additional compexity of construction.
To understand the contribution of winding layout in the generation of magnetic field, in following representation the stator has been "cut" and coils strightened. The contribution of the two winding groups, red and blue, is visible as well as their sum, in gray. Here again the two pulsating fields originate the rotating field.
Interestingly, in this lay out - called concentric winding - the magnetic field, besides rotating, also oscillates between a triangular and a trapezoidal shape, in first approximation.
Ideally, to reduce vibrations and enhance efficiency the field should be sinusoidal.