Control circuit for shutting off the electrical power to a liquid well pump

A circuit for shutting off the electrical power to the drive motor of a liquid well pump when the pump has exhausted the supply of liquid comprising,

• means for sensing the instantaneous alternating current to the motor,
• a bridge circuit connected to the sensing means for converting the alternating current to direct current voltage,
• an RC circuit connected to the output of the bridge circuit, the RC circuit having a time constant greater than the time required for the pump to complete one full pumping cycle, and the impedance of the charging path and the impedance of the discharging path of the condenser in the RC circuit being substantially the same whereby the RC circuit senses the average current to the motor,
• a comparator connected to the RC circuit,
• a reference voltage connected to the comparator, the reference voltage is obtained from an amplifier connected to the electrical power supply and in which the output of the amplifier varies inversely to variations of voltage of the electrical power, and
• switching means connected to the output of the comparator and to the electrical power supply to the motor, said switching means actuated by the comparator when the average current to the motor decreases to a predetermined amount for shutting off power to the motor.

A circuit for shutting off the electrical power to the drive motor of a liquid well pump when the pump has exhausted the supply of liquid comprising,

• means for sensing the current supplied to the motor,
• a comparator having two inputs one of which is connected to the output of the sensing means,
• an amplifier connected to the electrical power and having an ouput connected to the second input to the comparator, the output of the amplifier varying inversely to variations of the voltage of the electrical power supply, and
• switching means connected to the output from the comparator and controlling the electrical power, said switching means actuated by the comparator when the current to the motor decreases to a predetermined amount for shutting off power to the motor.