A compressed air system and method of operating the system wherein the slip between the compressor motor and its powering alternator is used as an indication of whether the compressor is running in a loaded or unloaded mode. A slip parameter such as % Slip is determined and compared to an allowable value in real time. If the compressor is determined to be erroneously in a loaded mode, a corrective action may be taken to reduce the probability that the motor is later started with the compressor still in the loaded mode.