Liquid level protection system for refrigeration compressor

Claim 2

A control circuit for protecting a refrigeration system including an electric motor, which system includes a housing for retaining liquid including at least some refrigerant at a given level in the lower portion of the housing, said control circuit comprising: a voltage divider circuit, including an impedance component coupled over a common terminal to an element having a negative temperature coefficient of resistivity and mounted in said housing above said given level; a pair of conductors, for applying an a-c voltage across said voltage divider circuit; a rectifier coupled to one of said conductors; a d-c relay, having a winding coupled between said rectifier and the other of said conductors, and having a contact set connected to regulate power transfer to the electric motor; a first semiconductor switch having input, output, and gate connections, coupled in parallel with said relay winding; a filter capacitor, coupled between said rectifier and said other conductor, for maintaining a d-c energizing voltage across the first semiconductor switch; and circuit means, including a second semiconductor switch, coupled between the gate connection of the first semiconductor switch and the common terminal in the voltage divider circuit, to provide a turn-on signal for the first semiconductor switch as a function of an impedance change in said element signifying cooling as the refrigerant liquid contacts the element and boils off.

Claim 3

A control circuit as claimed in claim 1, further comprising a radiation emitting unit coupled in series with the input and output connections of the first semiconductor switch, to provide a visible indication of the first semiconductor switch conduction which interrupts the motor circuit.

Claim 4

A control circuit as claimed in claim 1, in which said second semiconductor switch has its anode coupled through a diode to the common terminal in the voltage divider circuit, and a Zener diode coupled between the gate and anode of the second semiconductor switch, to provide a d-c trigger signal for rapid energization of the second semiconductor switch and thus gate on the first semiconductor switch upon an increase in the effective resistance of said element as refrigerant contacts said element.

Claim 5

A control circuit for protecting a refrigeration compressor driven by an electric motor, the compressor having a housing which retains refrigerant, and also retains some oil at a given level in the lower portion of the compressor housing, comprising: a voltage divider circuit, including an impedance component coupled over a common terminal to a thermistor having a negative temperature coefficient of resistivity and mounted in the compressor housing above said given level; a pair of conductors for applying an a-c voltage across said voltage divider circuit; a rectifier coupled to one of said conductors; a d-c relay, having a winding coupled between said rectifier and the other of said conductors and having a contact set connected in the power transfer path for the electric motor; a first semiconductor switch having input, output and gate connection, coupled in parallel with said relay winding to effectively short the relay winding when the semi-conductor switch is conducting; a filter capacitor, coupled between said rectifier and said other conductor, for maintaining a d-c energizing voltage across the first semiconductor switch and the d-c relay winding; and circuit means, including a second semiconductor switch, coupled between said first semiconductor switch and the common terminal in the voltage divider circuit, to gate on the first semiconductor switch as the impedance of the thermistor increases to signify increased heat conduction by contact with the refrigerant.

Claim 6

A control circuit for protecting a refrigeration compressor driven by an electric motor, the compressor having a housing which retains refrigerant, and also retains some oil at a given level in the lower portion of the compressor housing, comprising: a voltage divider circuit, including an impedance component coupled over a common terminal to a thermistor having a negative temperature coefficient of resistivity and mounted in the compressor housing above said given level; a pair of conductors for applying an a-c voltage across said voltage divider circuit; a rectifier coupled to one of said conductors; a d-c relay, having a winding coupled between said rectifier and the other of said conductors and having a contact set connected in the power transfer path for the electric motor; a first semiconductor switch having input, output and gate connection, coupled in parallel with said relay winding to effectively short the relay winding when the semi-conductor switch is conducting; a filter capacitor, coupled between said rectifier and said other conductor, for maintaining a d-c energizing voltage across the first semiconductor switch and the d-c relay winding; and circuit means, including a second semiconductor switch, coupled between said first semiconductor switch and the common terminal in the voltage divider circuit, to gate on the first semiconductor switch as the impedance of the thermistor increases to signify increased heat conduction by contact with the refrigerant.

Claim 7

A control circuit as claimed in claim 4, in which said second semiconductor switch is coupled through a diode to the common terminal in the voltage divider circuit, and a Zener diode is coupled between the anode and the gate of the second semiconductor switch, to provide a turn-on signal for the second semiconductor switch responsive to cooling of the thermistor and an increase in the voltage at the common terminal.

Claim 8

A control circuit as claimed in claim 4, and in which said first semiconductor switch is a silicon controlled rectifier having anode, cathode and gate connections, with the anode and cathode connections being coupled in a parallel path with said relay winding.

Claim 9

A control circuit as claimed in claim 6, and further comprising a light-emitting diode, coupled in series with said silicon controlled rectifier, to provide a visible indication that the silicon controlled rectifier has been gated on.

Claim 10

A control circuit for protecting a compressor driven by an electric motor, the compressor having a housing for retaining refrigerant material, and for retaining oil at a reference level in the lower portion of the compressor housing, comprising: a pair of conductors for receiving an a-c voltage; a voltage divider circuit coupled between said pair of conductors, including a resistor coupled over a common terminal to a thermistor which has a negative temperature coefficient of resistivity, which thermistor is mounted in the compressor housing above said given level; a series circuit coupled between said pair of conductors, including a rectifier and the winding of a d-c relay, so that a-c voltage supplied over the pair of conductors is rectified by the rectifier to energize the relay winding, which relay has a contact set connected in the power transfer path for the electric motor; a first silicon controlled rectiifer, having its anode and cathode connections coupled in parallel with the d-c relay winding, and having a gate connection; a filter capacitor, coupled in parallel with the anode-cathode path of the first silicon controlled rectifier, and in parallel with the d-c relay winding, to maintain a d-c energizing voltage across these components; and a trigger circuit for said first silicon controlled rectifier coupled between its gate connection and the common terminal in the voltage divider circuit, comprising a diode and a second silicon controlled rectifier, to gate on the first silicon controlled rectifier as the impedance of the thermistor rises when refrigerant contacts the thermistor, boils off to cool the thermistor and thus increases its impedance.

Claim 11

A control circuit for protecting a compressor driven by an electric motor, the compressor having a housing for retaining refrigerant material, and for retaining oil at a reference level in the lower portion of the compressor housing, comprising: a pair of conductors for receiving an a-c voltage; a voltage divider circuit coupled between said pair of conductors, including a resistor coupled over a common terminal to a thermistor which has a negative temperature coefficient of resistivity, which thermistor is mounted in the compressor housing above said given level; a series circuit coupled between said pair of conductors, including a rectifier and the winding of a d-c relay, so that a-c voltage supplied over the pair of conductors is rectified by the rectifier to energize the relay winding, which relay has a contact set connected in the power transfer path for the electric motor; a first silicon controlled rectiifer, having its anode and cathode connections coupled in parallel with the d-c relay winding, and having a gate connection; a filter capacitor, coupled in parallel with the anode-cathode path of the first silicon controlled rectifier, and in parallel with the d-c relay winding, to maintain a d-c energizing voltage across these components; and a trigger circuit for said first silicon controlled rectifier coupled between its gate connection and the common terminal in the voltage divider circuit, comprising a diode and a second silicon controlled rectifier, to gate on the first silicon controlled rectifier as the impedance of the thermistor rises when refrigerant contacts the thermistor, boils off to cool the thermistor and thus increases its impedance.

Claim 12

A control circuit as claimed in claim 8, further comprising a thermostat switch connected in said series circuit, to interrupt the current path for the first silicon controlled rectifier and afford a reset function in the circuit.

Claim 13

A control circuit as claimed in claim 8, and further comprising a transient suppression circuit, including a resistor and a capacitor, coupled in series between one of the pair of conductors and the common terminal in the voltage divider circuit.

Claim 14

A control circuit as claimed in claim 8, further comprising a Zener diode coupled between the gate and anode of the second silicon controlled rectifier, to provide a turn-on signal for the second silicon controlled rectifier as the thermistor is cooled and provides an increased voltage at the common terminal.