Development of 1 MW Hybrid Electric Propulsion Drive System

Aircraft electric propulsion electrical machine drives have recently been studied and developed by academics and industry. As the weight penalty is much higher in aerospace applications compared to the automotive counterpart, optimizing power density is a critical part of electrical drive system design. A Permanent Magnet (PM) drive system rated for 1 MW has been developed for flight demonstrations of two different electric hybrid propulsion systems, Hybrid Electric Propulsion (HEP) and Electrically Assisted Propulsion (EAP), where challenges exist in close integration with aircraft engines. In the HEP system, the electrical machine is integrated with an internal combustion engine via a gearbox in a parallel hybrid configuration. In the EAP system, two electrical machines are integrated with a turbofan jet engine, one machine being installed on the high pressure spool and the other on the low pressure spool, enabling power transfer between the spools by electrical means. The electrical machine is driven and controlled by a T-Type three-level inverter which has 2 x 3-phase outputs with a minimized DC-link filter using phase shifted PWM switching. The machine is an oil cooled surface PM type with distributed windings with a fundamental frequency of 1.2 kHz. Litz conductors wrapped with Kapton tape are used to minimize AC losses and increase the partial discharge inception voltage. Both the controller and machine are designed for partial discharge free operation in all flight conditions validated by electric field analysis. This paper discusses the details of the drive system and presents test results from a dynamometer, which has demonstrated a machine peak power density of 10 kW/kg and drive system efficiency exceeding 95%.