Variable Capacity Screw Compressor and Method

A variable capacity screw compressor comprising:

• a rotor housing comprising a suction port,
• a working chamber,
• a discharge port, and
• at least two screw rotors that include a female screw rotor and a male screw rotor being positioned within the working chamber for cooperatively compressing a fluid;
• wherein the suction port, the at least two screw rotors and the discharge port being configured in relation to a selected rotational speed;
• the selected rotational speed operates at least one screw rotor at an optimum peripheral velocity that is independent of a peripheral velocity of the at least one screw rotor at a synchronous motor rotational speed for a rated screw compressor capacity;
• a motor operable to drive the at least one screw rotor at a rotational speed at a full-load capacity that is substantially greater than the synchronous motor rotational speed at the rated screw compressor capacity;
• and
• a variable speed drive to receive a command signal from a controller and to generate a control signal that drives the motor at the rotational speed.