Design and Assessment of Battery Electric Vehicle Powertrain, with Respect to Performance, Energy Consumption and Electric Motor Thermal Capability

University dissertation from Chalmers University of Technology

Abstract: In this thesis, various drive cycles, legislative, official real-world and measured, have been studied and characterized based on their speed and acceleration content. Three reference vehicles (a City car, a Highway car and a Sport car) were conceptualized after performance requirements, with data on existing battery electric cars as a frame of reference. The acceleration performance, energy consumption and efficiency of the powertrain, comprising a traction motor, a power electronic module and a battery, was determined and analyzed for the various drive cycles. Furthermore, the consequence on acceleration performance, drive cycle fulfillment and energy consumption during re-scaling of the electric drive system was studied. Moreover, the electromagnetic losses for four different slot areas were compared, along with the thermal steady state and transient over load as well as temperature development during drive cycles. Through comparison between official and measured drive cycles, it was found that even though the measured cycles reach higher peak acceleration levels for a certain speed level, on an average they still spend only slightly more time at higher levels of acceleration compared to the official cycles. The resulting cycle average powertrain efficiencies were fairly similar for both the official and measured cycles, and showed to be slightly higher for cycles that spend more time at higher speed levels. During the powertrain sizing regarding torque and power, the acceleration requirement turned out to dominate over other requirements such as top speed, and grade levels. It was found that a down scaling of the electric power train resulted in an energy consumption down to 94% of the original powertrain size. The small slot geometry had the highest peak losses during the drive cycles, however, on a cycle average it had the lowest losses for many cycles. This fact, in combination with the highest peak torque and lowest material cost, makes it a very interesting option as an electric vehicle traction motor.

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