Control system for screw compressor

An electronic control system for sequential actuation of a four-way solenoid valve is used to selectively load and unload a slide valve control for a screw compressor. The electronic control system can be responsive to output or refrigerant pressure or evaporator pressure or inlet pressure and pulses a four-way solenoid valve to selectively load and unload the compressor so that it will maintain system pressure within a preselected deadband. Selective pulsing of the four-way solenoid valve is used to gate hydraulic fluid to load and unload chambers separated by a piston coupled to a slide valve which shifts longitudinally and changes the capacity of the screw compressor. The control system senses system pressure (reflecting load), and when a limit pressure is reached, a gas bypass solenoid valve (or dump valve) and fast-unload system is actuated to entirely unload the compressor. The control circuit also monitors current to the motor which drives the screw compressor. When the motor current is above a normal limit, further loading of the compressor is inhibited. If the current continues to rise, indicating a decrease in voltage available for power, the compressor is driven to an unloaded state until the current matches the preset normal limits. By use of electronic control for selective pulsing of the four-way solenoid valve, energy savings are realized by maintaining operation within a narrow deadband of operation.