Improving compressor surge performance with advanced control

Single stage compressors supported in magnetic bearings are known to exhibit compromised capacity to handle loads associated with compressor surge and stall - generally in a frequency range from about 10 Hz to 100 Hz. The present work explores the premise that this reduced capacity is an artifact of the choice of PID control for the magnetic bearings, rather than something fundamental to magnetic bearing physics. A case study of a machine similar to a chiller compressor was used to compare performance achieved with two kinds of PID control (local control versus tilt/translate control) to that achieved with a generalized, multiple-input multiple-output control approach, such as H-infinity. It was found that H-infinity could readily achieve between two and three times the load rejection of PID in the frequency range associated with surge and stall, nearly reaching the theoretical capacity limit in that range. The more common approach of simply increasing the physical size of the bearings was also examined, and it was found (as expected) that a 40 percent increase in bearing size yielded roughly a 40 percent increase in capacity. More importantly, it was found that the smaller bearing with the better control algorithm substantially outperformed the larger bearing with conventional PID control. These observations provide motive to more strongly consider advanced MIMO control methods for active magnetic bearings.