A method of controlling a multiple stage compressor, while maintaining compressor stability, comprising: - providing a compressor having a first compressor stage and a second compressor stage disposed on opposite ends of a motor shaft, each of the first and second compressor stages (42, 44) having a respective flow reduction device for controlling flow of refrigerant through the respective compressor stage (42, 44), a condenser and an evaporator connected in a closed refrigerant loop, and a control system; - defining a Mach ratio and impeller outside diameter for each compressor stage (step 600), wherein the Mach ratio is defined based upon a Mach number and a maximum head factor, up to which the compressor operates free from surge conditions, the Mach number being proportional to the impeller outside diameter; - calculating, for each stage (42, 44) of the compressor, a minimum required motor drive frequency (step 602) to operate free from surge conditions for the actual head factor and flow reduction device position based on the defined Mach ratio; - adjusting, for each stage (42, 44) of the compressor, the flow reduction device position (step 604), while maintaining the minimum required motor drive frequency to achieve a leaving chilled water temperature set point; - if the flow reduction device position for each stage (42, 44) of the compressor reaches a fully open position and the leaving chilled water temperature set point is not met, and a maximum motor drive frequency has not been exceeded, increasing the actual motor drive frequency to increase motor speed (step 606); - measuring a suction pressure and a discharge pressure for each stage (42, 44) of the compressor; - calculating, for each stage (42, 44) of the compressor, a saturated suction temperature (step 608) corresponding to the measured suction pressure and a saturated discharge temperature corresponding to the measured discharge pressure for calculating the actual head factor and speed of sound; - calculating an actual minimum motor drive frequency (step 610), wherein the actual minimum motor drive frequency is the greater of a first actual minimum motor drive frequency associated with the first compressor stage and a second actual minimum motor drive frequency associated with the second compressor stage; and - selecting a higher Mach ratio (step 612) which provides a desired safety margin free from surge conditions. - providing a compressor having a first compressor stage and a second compressor stage disposed on opposite ends of a motor shaft, each of the first and second compressor stages (42, 44) having a respective flow reduction device for controlling flow of refrigerant through the respective compressor stage (42, 44), a condenser and an evaporator connected in a closed refrigerant loop, and a control system; - defining a Mach ratio and impeller outside diameter for each compressor stage (step 600), wherein the Mach ratio is defined based upon a Mach number and a maximum head factor, up to which the compressor operates free from surge conditions, the Mach number being proportional to the impeller outside diameter; - calculating, for each stage (42, 44) of the compressor, a minimum required motor drive frequency (step 602) to operate free from surge conditions for the actual head factor and flow reduction device position based on the defined Mach ratio; - adjusting, for each stage (42, 44) of the compressor, the flow reduction device position (step 604), while maintaining the minimum required motor drive frequency to achieve a leaving chilled water temperature set point; - if the flow reduction device position for each stage (42, 44) of the compressor reaches a fully open position and the leaving chilled water temperature set point is not met, and a maximum motor drive frequency has not been exceeded, increasing the actual motor drive frequency to increase motor speed (step 606); - measuring a suction pressure and a discharge pressure for each stage (42, 44) of the compressor; - calculating, for each stage (42, 44) of the compressor, a saturated suction temperature (step 608) corresponding to the measured suction pressure and a saturated discharge temperature corresponding to the measured discharge pressure for calculating the actual head factor and speed of sound; - calculating an actual minimum motor drive frequency (step 610), wherein the actual minimum motor drive frequency is the greater of a first actual minimum motor drive frequency associated with the first compressor stage and a second actual minimum motor drive frequency associated with the second compressor stage; and - selecting a higher Mach ratio (step 612) which provides a desired safety margin free from surge conditions.