High power density electric drive for an hybrid electric vehicle

Abstract

This paper describes the electric drive system for an experimental hybrid electric vehicle 'Freedom'. The drive system is based on 4, PWM-VSI fed in-wheel PM brushless motors, using a novel PWM control strategy and a digital current phase angle control scheme that does not require a high resolution absolute position sensor. Each wheel of the vehicle is fitted with a 12-pole, 3-/spl phi/, PM motor with surface mounted Magnaquench magnets and an integral speed reducer packaged within the hollow rotor. Each PM motor is rated at 9 kW continuous (@3560 rpm) and 29 kW peak. The PM brushless motors had a trapezoidal EMF and were supplied with 120/spl deg/ quasi-square wave currents from the IGBT inverters operating at 18 kHz PWM frequency. The paper focuses on the development of the PWM-VSI inverters, the PWM strategy and the microcontroller system that implements the phase angle control scheme. Test results were provided on the performance of the electric drive. A new control strategy is outlined for controlling the braking torque when the battery is fully charged or when the motor speed exceeds the no-load speed. Two types of high power density inverter packages were developed for the Freedom vehicle. The Phase I system utilized forced air cooling to provide 32 kW peak power per inverter (@160 A pk) and Phase II system utilized direct liquid cooling to provide 45 kW peak power per inverter (@300 A pk). The liquid cooled system achieved an improvement of 2.5 times power density by weight and 4 times the power density by volume over the air cooled system.