The compressor (1) delivers air to a network system (2). The flow (4) is measured and compared with a desired value (5) and the error (Xd1) fed to a controller (R1) which varies the compressor speed. The discharge pressure (10) and intake volume (12) generate a second error signal (Xd2) which controls a blow off valve (8) to prevent surging at low throughputs. The second error signal (Xd2) is fed through two non-linear amplifiers (13, 16) into the first control circuit (R1). This modifies changes in speed to prevent surging as a result of over rapid changes. The output of one amplifier (13) modifies the error signal and the output of the second (16) reduces it.
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