Advancement on design and control of electric motors

Abstract

Electric motors are electromagnetic devices to convert electrical energy into mechanical energy. The first electric motor was invented by William Sturgeon in 1832. Since then, electric motors have been playing an increasing important role in human society. Various sizes and types of electric motors have been developed and implemented in diverse applications, such as automotive, subway systems, dishwashers, computer printers, fans, refrigerators, pumps, machine tools and even toys. It is no doubt that electric motors have been one of the foundations of human daily life. It is still promising to continuously improve existing electric motors, and propose new ones by employing novel operation principles, to obtain better performance on power density, power factor and efficiency, etc. As an effort to configure a new electric motor, this thesis firstly presents a novel magnetic resonant coupling motor (MRCM) constructed without any iron or permanent-magnet core, i.e. a novel coreless and magnetless electric motor. Different from the conventional operation principle of existing electric motors, the application of the wireless power transfer (WPT) system using magnetic resonant coupling (MRC) technology is the key feature of the proposed MRCM. By adjusting the excitation frequency in accordance with the trajectory of resonant frequency splitting, a large electromagnetic force in different direction can be developed. From the perspective of an elementary pole pair, the selection of resonant topology, the force magnitude and direction, and the frequency splitting phenomenon are analyzed elaborately, and the underlying operation principle is developed completely. Based on the proposed design procedures, computer simulations in MATLAB, PSIM and ANSYS Maxwell are presented to verify the feasibility of the proposed MRCM, and a control algorithm to regulate torque ripple for the proposed MRCM is designed. Simulation results obtained so far are promising, and the proposed new design could well be a promising start for a new generation of future electric motor. Meanwhile electric motors much rely on suitable and effective control algorithms to operate properly and robustly with satisfactory control performance. In recent decades, emerging faster and more sophisticated microprocessors, like TMS320F28335-based DSP controllers, enable the utilization of more precise and complicated control algorithms, to boost the control performance of electric motors. Among various electric motors, permanent magnet synchronous motors (PMSMs) are widely used due to its advantages of high efficiency, high torque density, and low volume. Better steady and dynamic responses on speed and torque control are always demanded for PMSMs, and various control algorithms for PMSMs have proposed over the years. With the chronological order, control algorithms for PMSMs have been developing from field oriented control (FOC), to direct torque control (DTC), and further to model predictive control (MPC). The existing control algorithms perform satisfactory steady and dynamic response on torque. However, the steady and dynamic response on speed are still requiring further enhancement. By combining the advantages of MPC and direct speed control, model predictive direct speed control (MPDSC) is of great interest to realize an excellent speed control performance with satisfactory torque response. A single-vector-based MPDSC with compensation factors is thus devised in this thesis. An excellent speed tracking capability with very small speed offset and ripple is achieved by using the proposed single-vector-based MPDSC algorithm. Moreover, some compensation factors, such as torque suppression factor and stability factor, are proposed and integrated into the cost function to suppress large torque oscillations, and to improve the steady and dynamic state response. The simulation and hardware-in-the-loop (HIL) results confirmed that, compared to conventional DTC, model predictive torque control (MPTC) and FOC, the proposed single-vector-based MPDSC strategy performs better control performance in terms of speed ripple, torque ripple, current THD. To further enhance the speed response of MPDSC, more than one voltage vector can be used in every single control period, and a duty-ratio-based MPDSC with two cost functions for PMSM drives is therefore devised. To reduce the speed and torque ripples, one active vector and one zero vector are applied within every control period. More specifically, eight duty ratios are firstly deduced to form eight combinations of voltage vectors. Two cost functions acting sequentially are then employed to refine the combinations of voltage vectors. Three combinations of voltage vectors, which result in better dynamic torque response, are preselected from the first cost function. After that, the optimal combination of voltage vector is finally determined by the second cost function to minimize the steady-state offset and the ripple of rotor speed. The proposed duty-ratio-based MPDSC performs an overall superior performance than the single-vector-based MPDSC. The effectiveness of the proposed duty-ratio-based MPDSC has been well validated by both Simulink simulations and HIL tests.