Efficient electric vehicles

BMW i3 cross-section view [Courtesy of BMW Group].

This project is conducted in collaboration with the BMW Group.

Problem statement: Low switching modulation for inverter control

Inverter-fed AC motors face significant current waveform quality deterioration due to their operation in switched mode. The current waveform quality can be improved by increasing the inverter switching frequency. However, this results in increased inverter switching losses. Consequently, the requirements of low motor and inverter power losses cannot be fulfilled simultaneously when carrier-based modulation is employed for inverter control. Thus, the concept of low switching modulation has been developed and extensively investigated over the last years. Various low switching frequency modulation techniques exist. The main objective of them is the limitation of the inverter switching frequency, without compromising the quality of the motor output current. To this end, the optimized inverter pulse patterns are calculated, which either eliminate specific low order voltage harmonics or minimize the harmful effects of the harmonics. One of the most notable low switching frequency modulation technique for inverter control is the Synchronous Optimal PWM (SOPWM). It has been developed to optimize the motor performance, while operating the inverter at low switching frequencies. It provides also potential for improvement in the torque ripple, acoustic and EMC performance.

Project description: Development of an optimal model-based control method for a 3-Level-Inverter feeding an electrical machine with high magnetic anisotropy

For an existing inverter, which feeds a Permanent Magnet Synchronous Motor (PMSM) with high operating-point-dependent magnetic anisotropy, A control strategy will be developed, which will optimally drive the system up to the magnetic saturation region. Electric motors with high operating-point-dependent magnetic anisotropy are used nowadays in the automotive industry. This feature has a considerable influence on the optimization of the inverter pulse patterns, when the SOPWM strategy is employed for inverter control. Thus, this project also aims at investigating the influence of the magnetic anisotropy of a PMSM on the optimization of the inverter pulse patterns. Both the static and dynamic optimal operation of the motor drive system in the whole torque/speed range will be examined. The requirements to be met are the following: 

  • Maximum drive system efficiency,
  • Maximum motor torque,
  • Minimum torque ripple,
  • Minimum current ripple on the DC-link, and 
  • Active balancing of the inverter neutral point voltage.

A suitable modulation strategy will be chosen for each motor operating point leading to an optimal drive system performance.