Towards Low-Voltage, High-Current : A pioneering drive concept for battery electric vehicles

Abstract: The first electric low-voltage vehicles were constructed in the mid-19th century, but by the early 20th century they were progressively replacedby successors with internal combustion engines. As the consequences ofusing fossil fuels are better understood, our society is now transitioning back. The strong driving force towards electric transportation can be traced to several events and trends. The foremost of these is perhaps the rising awareness of climate change and the necessary reduction of the environmental footprint, as well associated political will for change. Alongside this, the pioneering automotive company Tesla, Inc. showed what electric cars are capable of and how to easily charge them along the road. The diesel gate unearthed in 2015, also played a major role. This transition is not without challenges, however. An electric car is expected to be reasonable priced, sustainable, environmentally friendly and electrically safe, even in case of an accident. Overnight charging at home should be possible, as well as the ability to quickly charge while in transit. While the industry has long experience with high-voltage electrical machines, the required battery technology is quite new and low-voltage in nature. Currently, the battery is the most costly part of an electric drivetrain and it has the highest environmental impact. Efficient battery use is therefore key for sustainability and a responsible consumption of the resources available. Nonetheless, most electric vehicles today use lethal high-voltage traction drives which require a considerable isolation effort and complex battery pack. Previous research results showed that a 48 V drivetrain compared to a high-voltage one, increases the drive-cycle efficiency. Hence, similar driving range can be reached with a smaller battery. This thesis provides an introduction to low-voltage, high-current, battery-powered traction drives. With the aim of increasing efficiency, safety and redundancy while reducing cost, a solution that breaks with century-old electric machine design principles is proposed and investigated. An overview and motivation to further investigate 48 V drivetrains with intrinsically safe and redundant machines is provided. The main focus of this work is the practical implementation of multi-phase low-voltage but high-current machines with integrated power electronics as well as components for a 48 V drivetrain. With this work, it is confirmed that today’s MOSFETs are not the limiting factor towards low-voltage, high-current drives. In the first part of this work, two small-scale prototype machines were constructed and tested. The air-cooled, small-scale 1.2 kW proto-type reached a copper fill-factor of 0.84. The machine’s low terminal-to-terminal resistance of 0.23 mΩ, including the MOSFET-based power electronics, allowed continuous driving currents up to 600 A. The resistive MOSFET losses stayed below 21 W. The second part focuses on the key components for a 48 V high-power drivetrain. A W-shaped coil for a multiphase 48 V machine with direct in-conductor cooling was designed and tested. With glycolwater, it reached a current density of 49.5 A/mm2 with 0.312 l/min flowrate. Furthermore, a reconfigurable battery pack for 48 V driving andhigh-voltage charging was investigated.

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