Long term compressor control apparatus

Claim 1

In a system for controlling a compressor, driven by a constant speed electrical motor, for admitting fluid into a tank under pressure therein, for use by a pressure responsive tool, including: an inlet valve for admitting fluid through the compressor, a bypass valve for relieving fluid pressure of said tank at an outlet thereof; a master-controller responsive to a signal representative of a tank pressure differential relative to a pressure setpoint signal representative of an assigned reference tank pressure, the master-controller being operative in relation to said tank pressure differential in one of two selected modes, for applying a valve positioning signal for modulating the inlet valve in a first of said modes and for modulating the bypass valve in a second of said modes; means being provided for deriving a signal representative of the motor operative current; and means being provided for comparing said motor current signal with a reference signal characterizing a minimum motor current corresponding to an assigned minimum compressor fluid flow for generating an error signal and for selecting one of said first and second modes according to the sign of said error signal; the combination of: means for deriving an IV valve positioning signal representative of the IV valve present position; a subcontroller means responsive to said error signal for generating a corrective signal and for applying said corrective signal to said IV valve positioning signal for modulating the IV valve; said subcontroller means being enabled when the master-controller is in the second mode; whereby said subcontroller means maintains said assigned minimum compressor fluid flow when the master-controller is operating in the second mode.