Multiphysics design of integrated modular motor drives

High power density and efficiency are fundamental requirements in many applications such as electric vehicles (EVs), hybrid electric vehicles (HEVs), and propulsion and aerospace applications. In the context of increasing the power density and efficiency of electric motor drives, integrated modular motor drives (IMMDs) emerge as a solution to obtain both benefits. IMMDs incorporate physical integration and modularization of the electric machine and the power converter. The physical integration brings the electric machine and the power converter into close proximity so that they can share the same cooling circuit and enclosure. The cables connecting the power converter and the electric machine can be completely eliminated or greatly reduced in length. The result of this physical integration is a great reduction of the volume, the weight and the power losses of the electric motor drive, which improves its power density and efficiency. Furthermore, the elimination of the cables reduces the electromagnetic interference (EMI) of the drive which means that the EMI filter can be eliminated or reduced in size, and once again this improves the power density and the efficiency of the drive. Many challenges have to be met in the design of integrated motor drives. A first challenge is to mount the power converter in a small space inside the machine in a mechanically stable way as it is exposed to the motor vibration. A second challenge is the thermal management of this power converter as it resides close to the electric machine windings with its relatively high heat generation. The design of a modular, small size and low losses power converter facilitates the design of integrated motor drives. The modularity divides the machine into a number of modules with a driving converter for each module. This means that the converter is split into several small low rating units with smaller space and cooling requirements. One more advantage of the drive modularity is the fault-tolerance. If a fault occurs in one module, the drive can continue working with the remaining healthy ones. Using wide bandgap (WBG) power devices in the converter implementation facilitates the design of a small size and low losses power converter module. The commercially available WBG devices are Gallium Nitride (GaN) and Silicon Carbide (SiC). These WBG devices are existing in small package size and they dissipate lower power compared to Silicon (Si) devices. The main contribution of this thesis is the design of two novel integrated modular motor drive topologies: a yokeless and segmented armature (YASA) axial flux permanent magnet synchronous machine and a switched reluctance machine (SRM). Both topologies are providing a stable mechanical mounting for the power converter modules and a shared cooling circuit for the electric machine and the power converter. For the first integration topology - the YASA machine - the physical integration is realized by designing a 3D aluminium part with a polygon-shaped outer surface for mounting the converter modules and an inner surface that well encloses the teeth of the machine. A cooling channel is introduced between the inner and the outer surfaces of the 3D aluminium part to decouple and evacuate the heat from the YASA tooth and the power converter module. This integration topology is named circumscribing polygon (CP) integration topology. This topology is thermally optimized using computational fluid dynamics (CFD) simulations. Each integrated module comprises the machine tooth, the converter module and a shared cooling for both of them. The full motor drive can be easily synthesized from the individual modules. A discrete GaN based half-bridge inverter module is designed for this integration topology. First, electromagnetic and thermal models of the YASA machine and its driving converter are needed to design the CP integrated YASA drive. In this PhD, analytical electromagnetic models developed by former PhD colleagues for the YASA machine are used. Finite element and lumped parameter thermal network (LPTN) models are built for the YASA machine and the power electronics. These models are used to find optimal geometrical parameters of the YASA machine and an optimal inverter model design. In addition, loss models for the GaN switches are developed as well as an electromagnetic finite element model for the parasitics in the inverter PCB to investigate their influence on the losses and temperature of the switches. Next, a three teeth CP integrated YASA setup is built to validate the integration concept and the introduced models. The measurements prove the validity of the integration concept and the introduced modelling. The power density of the designed CP integrated modular motor drive for the YASA machine is extensively investigated in this thesis and some power density enhancement techniques are proposed, simulated and validated by measurements on the setup. Another contribution of the thesis is the design of a DC-link structure that can be integrated with the CP YASA drive. An analytical design methodology for the DC-link capacitors is provided. Electromagnetic and thermal models are built to study the performance of the designed DC-link structure. The design of this DClink structure is validated by measurements. For the second integration topology - the SRM -, the integration is realized by designing a 3D part with a radial cross-sectional area with outer polygon shape and an inner circular shape. The power converter modules are mounted on the outer surface of this 3D part while the inner surface is retrofitted to tightly enclose the water jacket cooled SRM. By doing so, the water jacket cooling of the nonintegrated SRM is used to cool the converter modules as well. This integration concept is named polygon retrofitted (PR) integration concept. This integration topology has the advantage of plug and play of the converter modules without much modifications in the original non-integrated machine. A discrete SiC based asymmetric H-bridge is designed for this integration topology. Several models are built for the SRM integrated topology. A dynamic model for the closed loop torque controlled SRM is built to compute the exact current waveforms of the SRM windings and the converter power devices. The computed currents from this model are used as inputs for the electromagnetic model of the SRM and the loss model of the converter. Electromagnetic and thermal models of the SRM and its driving converter are developed: an electromagnetic finite element (FE) model, 3D FE thermal models and LPTN models are built for the SRM. Also 3D FE thermal models and thermal network models are built for the asymmetric H-bridge converter. These models are used to design the asymmetric H-bridge converter and to study the performance of the complete PR SRM drive. Furthermore, a PR integrated SRM setup is built to validate the integration concept and the modelling of the integrated drive. The measurements confirm the effectiveness of the integration concept and the introduced modelling. A last contribution of the thesis is the design of a novel converter structure for the modular integrated SRM that can be configured to drive the SRM in the conventional mode and the modular mode. Switching from conventional to modular mode is done at high operating speeds and in case of fault in one coil or one inverter modules. The performance of the converter is validated to be good by experimental measurements in conventional mode, modular mode and in case of fault. Finally, a conclusion of the research conducted in this thesis and some ideas for future research are provided.