Method for driving a high speed compressor

A method for driving a high speed compressor from a commercial AC power source having a frequency of 60 Hz or less, the high speed compressor including a gear box having an outer surface and a center axis, a plurality of compressing units supported on the outer surface of the gear box, each compressing unit including a scroll and an impeller rotatably housed in the scroll, each impeller having an impeller shaft extending into the gear box, a bull gear placed in the gear box, a drive shaft extending from the bull gear out of the gear box along the center axis of the gear box, and a pinion gear drive between each impeller shaft including a pinion gear mounted on the impeller shaft and engaged with the bull gear, with a gear ratio between the pinion gear and the bull gear being about 1/10 to 1/30, said method comprising:

• providing an induction motor drivingly connected with the drive shaft for causing the impellers to rotate by means of the bull gear and the pinion gear drive so that each impeller is rotatable at a predetermined maximum speed if the induction motor is directly powered from the commercial AC power source;
• interposing an inverter between the induction motor and the commercial AC power source for converting the AC power from the commercial power source into an AC power having a desired frequency greater than 60 Hz and less than 200 Hz and feeding it to the induction motor for actuation of the induction motor, whereby each impeller is rotated at a speed greater than said predetermined maximum speed;
• determining an excessive load on the induction motor by detecting a current flowing in the inverter;
• and
• providing a frequency controller for controlling an output frequency of the inverter, whereby the frequency converter causes the inverter to drop its output frequency when the current detected exceeds a predetermined current value.