Patent ID: 7095207

Claim:
In an AC motor having a main winding and an auxiliary winding both connectable to an AC power source, and having a switch for disconnecting said auxiliary winding from said AC source, an improved control circuit for said switch comprising main voltage detector means for sensing the magnitude of the AC main winding voltage, main voltage zero crossing detector means for sensing the points at which main winding voltage polarity is instantaneously reversed, window pulse generating means for generating a pulse at the points at which main winding voltage polarity is reverse, auxiliary voltage detector means for sensing the magnitude of the AC auxiliary winding voltage, voltage comparator means for sensing the difference in voltage magnitude between the main winding and the auxiliary winding, auxiliary current zero crossing detector means for sensing the points at which auxiliary current flow changes direction, and phase comparator means for sensing the phase shift between the main voltage zero crossing points and the auxiliary current zero crossing points, wherein said phase comparator means operates to disconnect said auxiliary winding when the phase shift of the auxiliary winding current zero crossing falls within the main voltage pulse as a function of motor speed.