Optimal controller design for the active magnetic bearings of a flywheel energy storage system

Abstract

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 thesis considers the problem of feedback controller design for the active magnetic bearings (AMBs) in a small-scale FESS by using optimal control approach. The study is based on a prototype flywheel system incorporating a 6 kg steel rotor with potential storage capacity up to 70 Wh. H-infinity optimal controllers are designed that incorporate 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, stand-by energy consumption of the system can be reduced, and overall efficiency improved. The controller design is divided into four major controllers, based on practical design requirements and experimentation. The first controller is a basic H-infinity that can effectively control the rotor levitation at zero speed. Secondly, a notch filter characteristic was added to this controller to reduce AMB current components caused by rotor vibration excitation from the motor poles. In the experiment, this case has a stability problem that causes it to become unstable at low speeds. To improve stability at low speeds the design concept that includes a PD controller in the system's plant is considered to assist in maintaining precession mode stability at low speeds. According to test findings, the system can operate with stable rotor and also reduce AMB power usage by 10 % (from 2.40 Watts to 2.16 Watts). The fourth controller design included an additional notch filter to reduce the effect of rotor unbalance excitation on the AMB coil currents. Experimental tests confirmed that the H-infinity controller provides stable suspension and can reduce power consumption of the AMBs by 40 % (from 2.40 Watts to 1.44 Watts) in comparison with a conventional PD 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 rotational frequency above 30 Hz.