Motor protection apparatus using fusible links

Claim 1

In a refrigerant compressor of the type having a compression mechanism for receiving a refrigerant gas from a suction line, compressing the refrigerant gas and discharging the compressed refrigerant gas, a motor for operating the compression mechanism and conductor means comprising a first, second and third conductor for applying three-phase electrical power to the motor, improved apparatus for protecting the compressor motor comprising in combination: first switch means comprising a magnetic contactor connected in series with the conductor means, said switch means being movable between an open position and a closed position; fusible link means connected in series with the first switch means, said fusible link means being normally in a closed circuit condition and being switchable to an open circuit condition in response to a range of temperatures greater than a predetermined temperature, said fusible line means comprising a first link connected in series with the first conductor, a second link connected in series with the second conductor and a third link connected in series with the third conductor; heater means comprising a first and second resistor connected in series between the first and second conductors and a third resistor operatively connected between the junction of the first and second resistors and the third conductor and being in heat transfer relationship with the fusible link means; first operating means comprising first coil means for generating a magnetic field capable of operating the magnetic contactor for moving the first switch means to the closed position in response to a pilot signal and for opening the first switch means in the absence of the pilot signal; generating means for generating the pilot signal in response to a demand for operation of the compressor; and second operating means responsive to the closed position of the first switch means and the absense of the pilot signal and comprising a relay responsive to the pilot signal for generating a magnetic field capable of opening normally closed relay contacts connected to the heater means for enabling the heating means to receive electrical power so that the fusible link means is heated to the predetermined temperature, whereby the motor is disconnected from the electrical power, said relay contacts comprising a first contact connected between the first and second resistors and a second contact connected between the second and third resistors.