Abstract
Electric motors are widely used to convert electrical power into mechanical power, with applications ranging from robots and transportation to drilling equipment. However, powering these motors through cables poses several challenges. For example, cables may disconnect if robots move beyond expected distances, restricting the robots' mobility. Also, cables are prone to wear and twist issues, necessitating frequent maintenance and causing operational disruption.To address these problems, wireless motor drive systems have been developed, applying wireless power transfer (WPT) techniques to electric motor drives. By transmitting power through magnetic fields, wireless motor drive systems can eliminate cables and alleviate their associated limitations. Besides, these systems inherently provide electrical isolation and thereby enhance operational safety.
Nevertheless, wireless motor drive systems face several extra challenges. First, the large air gap in the magnetic coupler introduces significant leakage inductance, degrading power transfer capability and system efficiency. Also, misalignment between the transmitter and receiver coils can cause variations in coupler parameters, further affecting the stability and performance of the system. Second, wireless motor drive systems typically involve multiple stages of power conversions. These redundant conversion stages not only increase the system complexity but also introduce cumulative losses. Moreover, the cascaded design relies on bulky and less durable electrolytic capacitors, which negatively impact the system's power density and reliability.
This thesis addresses the challenges in current wireless motor drive systems in two research directions. First, optimization design methods are investigated for the cascaded structure to improve performance. Second, integrated solutions are explored to enhance both power density and reliability. Meanwhile, the study conducts a separate analysis of DC and AC motors in both cascaded and integrated structures.
The thesis starts with a cascaded wireless DC motor drive system. The system employs a dual-bridge structure that simultaneously supports the power supply and motor control, eliminating the need for a separate motor drive converter. A control strategy is developed, featuring fixed frequency and duty cycle control on the primary side, combined with a fixed off-time control on the secondary side. This method effectively saves the need for wireless communication between the primary and secondary sides while ensuring soft switching of active rectifiers across various load conditions. In addition, the influence of frequency deviation on the inverter's soft switching behavior is analyzed, and a parameter boundary is established to ensure soft switching on the primary side. Moreover, by considering the speed-torque relationship for different mechanical loads, design guidelines are provided for implementing the cascaded wireless DC motor drive systems. Experimental results show that the system can achieve a maximum efficiency of 96.56% at around 200 W output.
Subsequently, the thesis studies a cascaded wireless AC motor drive system employing a series/capacitor-parallel (S/CP) compensation. While structurally similar to conventional series/series-parallel (S/SP) compensation, the proposed S/CP topology offers superior voltage gain flexibility through a different design concept. Soft switching is prioritized in the design. Based on the derived soft switching boundaries, design points can be readily identified. Meanwhile, virtual parameters are introduced to mitigate the terminal effect on soft switching. Furthermore, noting the motor load's substantial tolerance to bus voltage variations, an optimization design method is proposed. The method can facilitate soft switching, ensure bus voltage requirements, and maintain high efficiency over a broad range of loads and misalignments. Experimental results show that the system can achieve a maximum efficiency of 96.81% at approximately 600 W output.
Following the cascaded structures, the thesis advances to the integration architecture and initially focuses on wireless DC motor drive systems. We introduce a matrix converter on the grid side, combining two power conversion stages into a single step and eliminating the use of electrolytic capacitors. Simultaneously, the secondary-side integration is achieved by utilizing the filter property of the parallel compensation. Moreover, a three-phase delta-sigma modulation is used in the proposed system to minimize grid current harmonics and regulate motor speed. Compared to space vector pulse width modulation (SVPWM), delta-sigma modulation reduces device stresses and switching times, allowing the system to operate at a higher switching frequency. Besides, we present the implementation of the delta-sigma modulation in the proposed system. Experimental results show that the prototype achieves a maximum efficiency of 89.56% with a total harmonic distortion (THD) of 3.12%.
Furthermore, the thesis extends the study to integrated wireless AC motor drive systems. We propose a dual matrix converter configuration to achieve full integration on both grid and load sides. Similar to the integrated wireless DC motor drive system, this approach facilitates direct AC-AC power conversion and eliminates the need for electrolytic capacitors, thereby enhancing power density and reliability. Furthermore, modulation strategies are studied for the proposed structure. Initial analysis reveals that delta-sigma modulation suffers from phase shift-induced THD degradation, prompting a shift to SVPWM. The study examines various SVPWM waveforms to mitigate phase shift effects, developing solutions that maintain stable THD performance across wide load variations. Besides, the implementation leverages symmetrical properties of modulation waveforms to introduce an improved dwell time calculation method, effectively reducing THD in both grid-side and load-side currents. Experimental results demonstrate that the proposed system can achieve a maximum efficiency of 93.3% and a minimum THD of 1.11%.
Finally, we conclude the thesis and suggest potential challenges for future research on wireless motor drive systems.
| Date of Award | 15 Jul 2025 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Chi Kong TSE (Supervisor) |