Design, Modelling and Implementation of High Power Density Drive for Electric Vehicles

Doctoral thesis, 2024

The world is facing an unprecedented ecological crisis due to our ever increasing
demand for energy. Today roughly 37% the total CO2 emissions are generated by
transport. In the last decade transportation electrification has seen a large push. This
way it is possible to eliminate local emissions and if the electricity generation is CO2
neutral also the global emissions.
Some of the main challenges of electric vehicles are range and cost. Apart from
declining component cost, especially of the battery packs, important factors of the
electric vehicle drive-train are efficiency and size. With increased efficiency the cooling
system of the drive-train can be smaller and thus the system size and mass can be
further decreased. Further drive-train downsizing can be achieved by integrating the
inverter and electric motor and utilizing new wide band-gap semiconductors such as
SiC.
To understand the design process first the vehicle requirements and dynamics need
to be understood. Electric vehicle system architecture is therefore first presented in
this thesis. Once the global requirements are taken into account, the inverter design
can be performed. Further, an easy to use estimation tool of the inverter losses is required
to understand the cooling requirements and feasibility of the design. Therefore,
the second part of this work focuses on loss modelling of inverters and semiconductors.
Next, the DC link capacitors are discussed and the dimensioning processes are
presented. This work investigates the possibility of utilization of a double-three-phase
topology (DTP) to reduce the DC capacitor bank of the inverter. This is achieved by
means of carrier wave interleaving which makes it possible to reduce the capacitor
current stress. This modulation technique makes it also possible to optimize for the
capacitance rating of the capacitor bank. The DTP topology is then utilized when
designing a low C-rate inverter for an 800V traction system. The adopted DC link
capacitor bank is 23.5 μF.
Aluminium substrate printed circuit boards with high thermal dissipation characteristics
are investigated as an alternative to SiC power modules. The Al-PCB concept
utilizes surface mount devices which are generally cheaper and more available
as well as inhibit a higher degree of flexibility for the designer. This makes it possible
to mount temperature sensors and current sensors on the Al-PCB increasing the
power density further. The flexibility offered by Al-PCB can be exploited to adapt the
inverter to the machine geometry for better integration. A prototype with the volume
of 2.17 L is then experimentally verified in a prototype capable of delivering 250 kW
for a DTP drive.