Author ORCID Identifier

https://orcid.org/0000-0001-6189-3218

Date Available

12-3-2024

Year of Publication

2024

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Engineering

Department/School/Program

Electrical and Computer Engineering

Advisor

Dr. Dan M. Ionel

Abstract

Permanent magnet synchronous machines (PMSMs), particularly those of the axial flux type, are being researched and developed for various applications such as HVAC systems, aviation propulsion, and electric vehicles. The coreless (air-cored) stator axial flux permanent magnet (AFPM) machine topology offers notable advantages over conventional designs by eliminating magnetic cores and their associated losses. These advantages include potentially higher efficiency, zero cogging torque, and reduced audible noise and vibration. Eliminating the magnetic core also allows for more effective cooling systems, as coolants can be in direct contact with the stator windings, potentially improving power density and specific torque.

The absence of a magnetic core presents an opportunity to incorporate special windings, including printed circuit board (PCB) coils, in coreless AFPM machines. This work proposes a systematic multi-step design procedure for highly efficient PCB stator coreless AFPM machines with minimal eddy and circulating current losses. PCB stators have gained popularity due to their reliable and highly repeatable fabrication process, high modularity, and lightweight nature. In this type of machine, the copper conductors are directly exposed to airgap flux density fluctuations, which can lead to considerable losses due to eddy currents. Additionally, in these machines with a wide magnetic airgap, parallel conductors experience different induced voltages, resulting in circulating current losses. Designers face challenges in mitigating these stator winding losses, which are a primary source of loss in coreless machines. The significant flexibility in PCB coil shape designs and their interconnections provides an excellent opportunity to enhance the efficiency of coreless machines through optimized stator coil designs.

The proposed design procedure includes initial sizing, optimization of the machine envelope design using an evolutionary algorithm and computationally efficient 3D finite element analysis (FEA) models, and detailed design of a PCB stator layout to minimize eddy and circulating current losses. Several open-circuit loss mitigation techniques, including a novel layer transposition method, are proposed in this dissertation based on analytical equations and 3D FEA, while considering PCB manufacturing limitations and standards. Experimental results indicate significantly reduced eddy current losses and virtually zero circulating currents, achieving ultra-high efficiency of 96% under rated conditions.

To leverage the full potential of coreless axial flux PM machines, a high-performance control system tailored to this type of machine, considering their intrinsic features, is proposed. This dissertation introduces a fault-tolerant control system for both threephase and two-phase configurations. Initially, the performance and fault tolerance of a two-phase variant of a coreless AFPM machine are compared with its three-phase counterpart, indicating that both configurations have comparable specific power and efficiency, while the two-phase variant features a high level of fault tolerance due to electrically and magnetically isolated phases.

This dissertation proposes a dual-mode controller for coreless AFPM machines with independent phase modules across a wide range of speeds. Coreless AFPM machines exhibit ultra-low phase inductance due to a wide effective airgap. This low phase inductance can lead to high current ripple, resulting in additional power losses and limiting the flux-weakening capability. The proposed approach includes a combination of sine-wave field-oriented control (FOC) and square-wave control schemes to address these control challenges. The computational burden of the square-wave control mode is considerably lower than that of conventional FOC and eliminates the need for high-precision encoders, facilitating ultra-high speeds and improving reliability. The square-wave control mode also extends the machine’s speed range while fully utilizing the DC-link voltage. Inverters based on wide bandgap devices are used in the drive system, enabling high switching frequencies and significantly reducing current ripple due to low phase inductance and its negative impacts in inverter-fed coreless AFPM machines. Several approaches to improve the fault tolerance of the motor-drive system are introduced in this dissertation. These include a modular design for coreless AFPM machines with PCB stators, which provides electric and magnetic insulation, and encoderless operation with a flux observer-based sensorless control. The performance of the prototype machine, with both two-phase and three-phase configurations operating in different control modes and under normal and post-fault conditions, is experimentally investigated.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2024.428

Funding Information

This dissertation is based upon work supported by the National Science Foundation (NSF) under Award No. #1809876. Any opinions, findings, and conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF. The support of Regal Rexnord Corp., Ansys Inc., and University of Kentucky, the L. Stanley Pigman Chair in Power Endowment is also gratefully acknowledged.

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