actuators Article Fractional-Order Surge Control of Active Magnetic Bearings Suspended Compressor Parinya Anantachaisilp 1, * ,† and Zongli Lin 2, * ,† 1 Department of Electrical Engineering, Royal Thai Air Force Academy, 171/1 Phaholyothin Rd., Saimai, Bangkok 10220, Thailand 2 Charles L. Brown Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA 22904-4743, USA * Correspondence: parinya_a@rtaf.mi.th (P.A.); zl5y@virginia.edu (Z.L.); Tel.: +6-687-004-7363 (P.A.); +1-434-924-6342 (Z.L.) † These authors contributed equally to this work. Received: 7 July 2020; Accepted: 20 August 2020; Published: 24 August 2020   Abstract: H ∞ surge control in centrifugal compressors by using active magnetic bearings (AMBs) has been successfully designed and implemented. However, the structure and design process of H ∞ surge control are quite complex. This paper reports on the design and implementation of fractional-order proportional-derivative control (FOPD) that results in the required specifications of surge control with a simple controller structure. To validate its effectiveness, the proposed FOPD surge controller has been implemented on a centrifugal compressor test rig equipped with AMBs. Simulation and experimental results show that the FOPD surge controller outperforms integer-order PD (IOPD) control in extending the surge limit in terms of the mass flow and provides similar performance as the H ∞ controller. Keywords: surge control; centrifugal compressor; active magnetic bearings; fractional-order control; evolutionary algorithms 1. Introduction Active magnetic bearings (AMBs) have increasingly been used in compressor applications because they provide higher performance and reliability. An example of a compressor equipped with magnetic bearings is shown in Figure 1. So far, AMBs have been applied to compressor applications mainly to improve performance of the system, with two approaches. The first approach is to use AMBs to support the rotor instead of the traditional bearings. This technology has been applied to compressors in industrial applications for more than two decades [1]. The second approach, which is in the research stage but shows the effectiveness of AMB usage in compressors, is to control the instability in the compression system, referred to as surge. The rotor of the compressor is levitated by magnetic forces and is allowed to rotate with no mechanical contact and friction losses. Consequently, the maintenance cost is low because there are almost no consumable components and no lubrication is required. Moreover, AMBs have an active control capability which keeps the rotor near the clearance center during operation. This capability of AMBs helps compressors to operate efficiently at high rotational speeds. The performance of a compressor is affected not only by the rotor suspension mechanism, but also the stability of the compression system. The information of compressor characteristics and efficiency is elucidated in the compressor characteristic curve, as shown in Figure 2. Actuators 2020, 9, 75; doi:10.3390/act9030075 www.mdpi.com/journal/actuators