H-Infinity Controller Design for Efficiency Optimization of Magnetic Bearings in a Flywheel Energy Storage System

Flywheel energy storage systems (FESS) can offer improvements in power density, cost per kW of power, maintenance needs, operating life, and use of hazardous materials, compared with alternative energy storage technologies. In high-speed flywheels, magnetic bearings are used to suspend the flywheel rotor in vacuum conditions, thereby reducing energy losses from friction. This paper considers the problem of feedback controller design for the active magnetic bearings (AMBs) in a small-scale FESS by using an optimal control approach. The study is based on a prototype flywheel system incorporating a 6 kg steel rotor with a potential storage capacity of up to 70 Wh. An H-infinity optimal controller is designed that incorporates frequency domain specifications for stability and energy consumption. The controller design weighting functions are chosen to produce notch-filter characteristics in the feedback controller that minimize the RMS current within the AMBs coils. In this way, the stand-by energy consumption of the system can be reduced, and overall efficiency improved. Experimental tests confirm that the H-infinity controller provides stable suspension and can reduce the power consumption of the AMBs by 42% (from 4.56 Watts to 2.64 Watts) in comparison with a conventional PD (proportional-derivative) feedback control method. Moreover, the H-infinity controller can prevent vibrational instability of the rotor nutation mode, which is prone to occur when operating with a rotational frequency above 30 Hz.