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A compressor control system for a commercial refrigeration system includes a variable speed compressor and multiple fixed speed compressors. The control system calculates a variable which fluctuates according to the desired capacity of the system and is calculated as a function of the difference bet

A compressor control system for a commercial refrigeration system includes a variable speed compressor and multiple fixed speed compressors. The control system calculates a variable which fluctuates according to the desired capacity of the system and is calculated as a function of the difference between a variable indicative of the refrigeration load on the system, such as compressor suction pressure, and a target value. A “PID”proportional-integral-differential (PID) controller is used in which terms have been added which fluctuate as a function of the square of the difference between the sensed variable and the target value and as a function of the second derivative of the control variable with respect to time. Fixed speed compressors are turned on or off and the speed of the variable speed compressor is adjusted to bring the actual refrigeration capacity of the system in line with the system refrigeration requirements.

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Capacity control system for a commercial refrigeration system having at least one refrigerator case, said refrigeration system including a compressor system for compressing refrigerant, said compressor system including a variable speed compressor and fixed speed compressors whereby the capacity of s

Capacity control system for a commercial refrigeration system having at least one refrigerator case, said refrigeration system including a compressor system for compressing refrigerant, said compressor system including a variable speed compressor and fixed speed compressors whereby the capacity of said system can be controlled by varying the speed of the variable speed compressor and by turning said fixed speed compressors on or off, said control system including load sensing means for detecting a variable indicative of the refrigeration load on the system, means for comparing said variable with a target value and generating a difference signal varying in accordance with the difference between the variable and the target value, and calculating means for calculating a compressor control signal for controlling the capacity of said system as a function of the second derivative with respect to time of said difference signal.