A method and apparatus for producing and controlling flow rate of a compressed gas produced by a compression system having one or more stages of compression formed by a centrifugal compressor (12) or compressors, inlet guide vanes (14) to control flow through the compression stage or stages and an electric motor (24) directly coupled to the stage or stages having a restricted speed zone within which vibrational modes can damage the electric motor. A desired flow rate is obtained by speed adjustment alone and with the inlet guide vanes positioned at 0° when the speed will lie above or below the restricted speed zone. Where a desired flow rate would require extended operation of the electric motor at a speed within the restricted speed zone, the speed is set to the uppermost level of the restricted speed zone and the desired flow rate is obtained through appropriate positioning of the inlet guide vanes.