Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 213-227).

Electric machines are indispensable part in modern industry and they are widely used in many applications such as power generation, vehicles, elevator, robot, and so on. Owing to the diversity of the application occasions, different requirements have been proposed. In the applications, such as wind power generation, low-speed, high-torque and high-efficiency electric machines are required to match with the low-speed load. Currently, this problem is handled by either adopting a speed-boost mechanical gear, or using low-speed machine design. The former one causes mechanical wear and tear, audible noise and low efficiency, whereas the latter one increases the generator size and weight as well as raw material cost. In recent years, another option, namely integrating a coaxial magnetic gear into a permanent-magnet (PM) machine has been proposed. This option allows for directly mounting the outer, low-speed shaft with the load while enables the electric machine coupled with the high-speed inner gear to operate at high speeds. However, this magnetic-geared electric machine desires a complex structure, involving two rotating bodies and three air-gaps, which increases manufacture difficulty and cost. The research work of this thesis attempts to solve the speed-matching problem by developing a new electric machine. The incorporation of vernier concept with PM machine gives birth to the permanent-magnet verier (PMV) machine. Different from traditional synchronous machine, the rotor of the PMV machine rotates at a definite fraction of the synchronous speed, as if it were geared down from the high rotating field set up by the stator. The PMV machine, therefore, can be regarded as a combination of a gear with fixed gear ratio and an electric machine. This kind of machine is attractive in applications which require low speed and high torque, and mechanical gearing is undesirable. The main objective of this thesis is to present the design, analysis and control of the proposed PMV machine. After the introduction on mechanical gears, magnetic gears, and low-speed machines, the design details of the proposed outer-rotor PMV machine are provided. Moreover, the working principle, stable torque generation mechanism and structural features are presented. Due to the introduced vernier effect, it is important to perform the finite element analysis (FEA) for the proposed PMV machine. The electric circuit equation and the motion equation are coupled with the Maxwell’s equation to calculate the key parameters of the proposed machine. The analysis results of the magnetic field distributions, air-gap flux density distributions, flux linkages, winding inductances, back electromotive forces (EMFs), cogging torque and static torque are presented in detail. The performances of PMV machine are compared with other machines to show its advantages and disadvantages. The control strategy of the PMV machine as a brushless DC machine is also presented. The conventional 120-degree conducting, dual-closed-loop control strategy is used for speed and torque control and the experimental setup are given. The results are obtained and compared with the simulation results, thus verify the validity of the design. Finally, the potential applications for the proposed PMV machine are suggested. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2005. / Also available in print.

The task in this project is to design a generator for a vertical axis wind turbine withpower rated to 20kW at a wind speed of 10m/s. The project is conducted at theDivision of Electricity at Uppsala University with collaboration from ElectricGeneration AB. The design has just a few moving parts, which decreases maintenancecosts and increases its toughness. The turbine absorbs wind from every direction butits rotation speed ratio is lower than horizontal axis wind turbines. It means that thegenerator must be bigger and therefore more expensive. Price is an importantcriterion for the generator. Neodymium magnets are expensive so the amount of thismaterial must be limited.Several designs have been simulated but one final design has proven the mostpromising. It fulfills all specifications such as efficiency above 95%, 20kW outputpower and it also has a relatively low amount of hard magnetic material.A design with a single row of cables per slot was decided upon to eliminate heatpockets between cable rows, which can occur in designs with two cable rows perslot. It would be interesting to study designs with two or more cable rows per slot, asit could lead to a smaller and more efficient machine.

This objective of this thesis is to establish the whole structure for optimal efficiency operations of a disc permanent magnet linear synchronous machine (DPMLSM) based on a digital signal processor (DSP). The thesis is to present the derivation algorithm and strategy of achieving optimal efficiency operations. To fulfill the operational requirements the theoretical basis and experimental database will first be developed, along with the constructions of adequate digital processor-base control and peripheral circuits. Hence the appropriate trigger angles of machine stator phase windings can be advised, and the control objectives of the DPMLSM under steady-state as well as dynamic conditions can be achieved.

Multi-phase motors have several advantages over the traditional three-phase motors. In this study, the additional degrees of freedom available in five-phase permanent magnet motors have been used for three purposes: 1) enhancing the torque producing capability of the motor, 2) improving the reliability of the system, and 3) better adjusting of the torque and flux linkages of the five-phase direct torque controlled system. 1) Due to the fact that space and time harmonics of the same orders will contribute positively to output torque, a five-phase permanent magnet motor with quasi-rectangular back-EMF waveform is supplied with combined fundamental and third harmonic of currents. For modeling and analysis of the motor a 0 3 3 1 1 q d q d frame of reference is defined where 1 1q d rotates at the synchronous speed and 3 3q d rotates at the three times synchronous speed. Based on the mathematical model in the 0 3 3 1 1 q d q d frame of reference, it is shown that this system while having a higher torque density with respect to a conventional permanent magnet synchronous machine, is also compatible with vector control algorithm. 2) A resilient current control of the five-phase permanent motor with both sinusoidal and trapezoidal back-EMF waveforms under asymmetrical fault condition is proposed. In this scheme, the stator MMF is kept unchanged during healthy and faulty condition. Therefore, the five-phase permanent magnet motor operates continuously and steadily without additional hardware and just by modifying the control algorithm in case of loss of up to two phases. The feature is of major importance in some specific applications where high reliability is required. 3) High torque and flux ripple are the major drawbacks of a three-phase direct torque controlled system. The number of space voltage vectors directly influences the performance of DTC system. A five-phase drive, while benefiting from other advantages of high order phase drives, has inherently 32 space voltage vectors which permits better flexibility in selecting the switching states and finer adjustment of flux and torque. A sensorless direct torque control of five-phase permanent magnet motor is implemented. Speed information is obtained based on the position of stator flux linkages and load angle. Experiments have been conducted on a 5kW five-phase surface mount permanent magnet motor and a 3kW five-phase interior permanent magnet motor by using TMS320C32 DSP. The results obtained are consistent with theoretical studies and simulation analysis, which further demonstrate the feasibility and practical significance of the five-phase permanent magnet motor drives.

With the growing interests and high requirements in low-speed linear drives, the linear machines possessing high force density, high power density and high efficiency feature become in great demands for the linear direct-drive applications. There are many available linear machine topologies, but their performances for exhibiting the high-force density capability dissatisfy the industrial requirements. In order to solve this problem, the new machine topologies emphasizing on high force density are explored and studied. The objective of this thesis is to present the design, analysis, and application of permanent magnet (PM) linear machines which can offer a higher force density at the same magnetic loading and electric loading than the conventional machines. Although in recent years there are many emerging advanced PM rotational machines for direct-drive rotational drives, the development of advanced PM linear machines for direct-drive linear drives is sparse. In spite of the motion type of electric machines, the inherent operating principle is the same. By studying and borrowing concepts of the high torque density rotational electric machines, the linear machine morphologies of the promising candidates are designed and analyzed. The problems and side effects resulting from the linearization are discussed and suppressed. Two main approaches for machine design and analysis are developed and applied, namely the analytical calculation and the finite element method (FEM). By analytically solving the magnetic field problem, the relationships between the field quantities and the machine geometry are unveiled. With the use of analytical calculation, the machine design and dimension optimization are conveniently achieved. With the use of FEM, the machine design objective and its electromagnetic performance are verified and evaluated. Finally, the proposed low-speed PM linear machine is applied for direct-drive wave power generation. By mathematically modeling the wave power, generation system and the generator, the conditions for maximum power harvesting are determined. By using the vector control, the generator output power is maximized which is verified by the simulation results. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2005.