Reduced DC-link capacitor drives for more-electric aircraft applications

Abstract

As the race towards More Electric Aircraft continues in the aerospace industry, the demand is rising for reliable yet cheap flights. The increasingly power hungry aircraft system puts even greater pressure on designers to replicate the performance of conventional hydraulic aircraft systems by all electrical means. At present one of the heaviest and most size dominating components of aircraft electrical actuator drives is the de-link capacitor. Today the most advanced passenger aircraft such as the Airbus A-380 employ Fly-by-Wire techniques but still rely on electro-hydraulic actuators (ERA) for the critical primary control surface actuation. Non-hydraulic alternatives using electro-mechanical actuators are the subject of much current research for applications in primary control surface actuation but as yet are not able to meet their critical safety standards. The research work presented here follows previous work in the authors Research Group on secondary flight control surfaces in which fault tolerant drive arrangements were used to meet the required reliability figures mostly because of the lack of reliability of the de-link capacitors. This research shows that much smaller de-link capacitors are possible by modifying the control methods. Smaller capacitance requirements make much more reliable capacitor technology viable and hence may allow the required mean time between failures to be met from much simpler non-fault-tolerant conventional three phase drives. A new drive with a switch estimation based control is proposed here with the potential to operate at much lower de-link capacitor values. As part of the analysis a complete simulation of a three-phase 3.6 kW permanent-magnet synchronous machine (PMSM) with a PWM rectifier-inverter drive for a full electrically powered flap for a mid-sized aircraft like an Airbus A-320 is presented. The effect of the size of the capacitor is examined and ground rules to establish minimum acceptable size are derived exploiting the knowledge of the exact switching states in the rectifier and inverter. Measurements on an experimental rig are used to validate the simulation.