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Start from coast protective circuit

  • US 4,751,449 A
  • Filed: 09/24/1986
  • Issued: 06/14/1988
  • Est. Priority Date: 09/24/1986
  • Status: Expired due to Fees
First Claim
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1. In a single phase AC induction motor having a main winding and an auxiliary winding both connectable to an AC power source, and having a start capacitor for providing a phase shifted field for starting torque, and having start switch means for automatically connecting and disconnecting said start capacitor to and from said AC source in starting and running modes, respectively, a control circuit comprising:

  • main voltage detector means for sensing main winding voltage;

    auxiliary voltage detector means for sensing auxiliary winding voltage;

    cut-out voltage comparator means responsive to said main and auxiliary voltage detector means and generating a turn-off signal when auxiliary winding voltage rises to a given cut-out value relative to main winding voltage as a function of motor speed corresponding to a given cut-out speed;

    logic means responsive to said turn-off signal from said cut-out voltage comparator means to turn off said start switch means to disconnect said start capacitor from said AC source;

    cut-in voltage comparator means responsive to said main and auxiliary voltage detector means and generating a turn-on signal to said logic means when said auxiliary winding voltage decreases to a given cut-in value relative to main winding voltage as a function of motor speed corresponding to a given cut-in speed, said logic means being responsive to said turn-on signal to turn-on said start switch means to connect said start capacitor to said AC source;

    power supply means coupled to said AC source;

    blanking interval means comprising disabling switch means coupled to said logic means, and RC timing means including a capacitor charged by voltage from said power supply means upon energization of said motor and actuating said disabling switch means from a first to a second condition after a given time delay blanking interval during which said capacitor is charged to a given switching value of said disabling switch means, said first condition of said disabling switch means disabling turn-on of said start switch means, said second condition of said disabling switch means enabling turn-on of said start switch means by said cut-in voltage comparator means if said auxiliary winding voltage is at or below said given cut-in value corresponding to said cut-in speed, such that said blanking interval means prevents immediate connection of said start capacitor to said AC source upon energization of said motor to protect against an over-current condition if said motor is de-energized while in said running mode and then re-energized while still coasting above said cut-in speed, and such that said start capacitor is disconnected from said AC source during said time delay blanking interval regardless of whether said start auxiliary winding voltage is above or below said given cut-in value corresponding to said cut-in speed, and such that after said time delay blanking interval said cut-in comparator means actuates said start switch means to connect said start capacitor to said AC source only if said auxiliary winding voltage has dropped to said given cut-in value corresponding to said cut-in speed.

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