Analysis of Bus Voltage Sag during High Battery Power Operations on Hybrid Electric Urban Air Mobility Vehicles

Abstract

This paper analyzes test results performed on a hybrid-electric power generator where the effects of voltage sag caused the inverter to lose the ability to control the generator. The prototype generation system consisted of a four-cylinder Rotax 915iS turbocharged gasoline aviation engine driving a YASA P400RHC high performance generator controlled by a Cascadia PM150DZ inverter. The system was designed to be incorporated into an eVTOL type of vehicle to provide power to distributed electric rotor systems. The prototype power generation system was connected to an active DC electronic resistive load that pulled current from the prototype based on an arbitrary power versus time profile. When driven by the Rotax at 5,500 RPM and 100% throttle, the YASA generator is capable of producing about 85-90 kW. As more power is demanded from the system, the battery pack – which has a starting voltage of approximately 420 V – has to provide the remaining power. When this power demand from the battery is high, the battery pack voltage begins to drop. The Permanent Magnet Synchronous Machine (PMSM) generator has both voltage and current limits that limit the amount of torque the generator can produce to balance the torque input from the Rotax. The voltage limit is partially a function of the bus voltage, which is dictated primarily from the battery pack voltage. When these limits are reached, the system can no longer keep the speed controlled and the Rotax torque, being unbalanced by the YASA, causes the engine to over-speed. This paper analyzes test results where the system is intentionally loaded to produce this behavior and cause the over-speed. A simplified model of the PMSM current and voltage limits are developed to notionally simulate the phenomenon.