High power density electric drive for an hybrid electric vehicle

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-φ, 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° 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