Patent ID: 7202627

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 zero crossing detector means for sensing the points at which main winding voltage polarity is instantaneously reversed, window pulse generating means for generating a first pulse window and a second pulse window at the points at which main winding voltage polarity is reversed, auxiliary voltage detector means for sensing the points at which the AC auxiliary winding voltage polarity is instantaneously reversed, auxiliary current zero crossing detector means for sensing the points at which auxiliary current flow changes direction, and a first phase comparator means for sensing the phase shift between the main voltage zero crossing points and the auxiliary current zero crossing points, a second phase comparator means for sensing the phase shift between the main voltage zero crossing points and the auxiliary voltage zero crossing points, wherein the first phase comparator means operates to disconnect the auxiliary winding when the phase shift of the auxiliary winding current zero crossing falls outside the first main voltage window pulse as a function of motor speed, and wherein the second phase comparator means operates to connect or reconnect the auxiliary winding when the phase shift of the auxiliary winding voltage zero crossing falls within the second main voltage window pulse as a function of motor speed.