Electric machine control for energy efficient electric drive systems

Licentiatavhandling, 2015

Pure electric vehicles and hybrid electric vehicles are of increasing interest in the automotive industry, much due to challenges such as emission level legislations and the environmental impact of the transportation sector. When continuously striving for better performing and more energy efficient electric powertrains, with high drivability and redundancy, there is an ever increasing need for further research and development in the areas of controls, design and system level optimization. Over the past decade, noise and vibrations from electric drives has increasingly become an area of attention for both academia and industry. The absence of the broad band noise from the internal combustion engine brings new noise, vibration and harshness (NVH) challenges for electric propulsion applications. Magnetic noise from electrical machines is of particular interest in automotive applications. It is not only related to the physical design and mounting of the electrical machine but also to the choice of control approach and voltage modulation strategy. This thesis is focused on energy efficiency enhancements in the electric drive system, primarily on the control of the three phase inverter. In addition to the energy efficiency perspective, also the appearance of electromagnetic forces and NVH-perspectives are considered. Alternative modulation techniques are investigated, where the so called discontinuous pulse width modulation is is proven to decrease the inverter losses, substantially. The appearance of electromagnetic forces is investigated extensively, with focus on radial forces acting as attractive or repulsive forces between the rotor and the stator. Its influence on stator deformations, and consequently noise and vibrations, is investigated where also influence of machine design, modulation techniques and perceived annoyance of the radiated sound are included. The scientific contribution can be summarized with two parts. Firstly, the interdisciplinary research where energy efficiency enhancements are coupled to NVHperformance. Secondly, the cause and effect of electromagnetic forces as the link between machine design, controls and NVH-perspectives. It is proven that when using discontinuous PWM(DPWM) instead of synchronous PWM (SPWM), the inverter losses can be decreased with up to -17 percent. While complementing the modulationwith randomization of the switching frequency the increase in perceived annoyance, due to increased radiated noise when using DPWM, is significantly decreased. At rotational speeds above base speed, no difference in perceived annoyance between DPWM and SPWM could statistically be ensured when randomization is used. At a randomization level with a variance of 1000 Hz the noise is comparable with the magnitude of the harmonics themselves, hence further randomization in terms of increased variance is without any gain.