Design, analyze and control of novel stator-PM electrical machines

Abstract

Stator-PM machines have advantages in long track linear motion and in harsh environment applications, since they apply the robust rotor without PMs or windings. Generally, there are four types of stator-PM machines, naming double salient machine, flux reversal machine, flux switching machine and stator-PM Vernier machine. It is found that there are some common characteristics existing in stator-PM machines. Firstly, it is found that the winding flux in some stator-PM machines is biased. Secondly, some staor-PM machines apply the short-circuit or open-circuit equivalent magnetic path. Thirdly, it is found that the torque density for most of the stator-PM machines is relatively lower than rotor-PM machines. So in this thesis, these three topics are addressed. The main contribution of this thesis is listed as below, 1) developing novel biased flux machines based on biased flux characteristic, 2) developing doubly complementary machines to avoid open-circuit or short-circuit magnetic path, 3) exploring new topologies with torque density higher than conventional rotor-PM machines for low speed applications. The biased flux actually is the unique characteristic for stator-PM machines. Since the exciting flux is biased, the biased current can be used to modulate the exciting field and generate the torque. Based on this characteristic, the research about biased flux machine design, analyze and control is chapter 2. Firstly, the definition of biased flux machine is given. Secondly, a set of biased flux machines have been developed. Two methods have been proposed to design and analyze the biased flux machines, which are magnetic path construction method and magnetic field modulation method. One NNSS 12/11 biased flux machine have been designed and built. The detailed performances such as back-EMF, electromagnetic torque, cogging torque and efficiency have been tested. One novel control based on zero-sequence current is also developed. It is verified that with this control, the speed range of the prototype can be expanded as 1.6 times. Meanwhile, the concept of transferring other machines such as electrical exciting and hybrid exciting machines into biased flux machines to remove the DC exciting windings is also proposed and demonstrated. To overcome the disadvantages of the machines with open-circuit or short-circuit magnetic path, a new type of stator-PM doubly complementary machines (DCMs) are proposed. These machines have two complementarities as one exists in complementary rotors and the other exists in the basic units belonging to same phases. The first one applies the complementary rotors to construct a consecutive switching path for permanent magnet exciting flux, so that shorted or open magnetic path is avoided to reduce cogging torque and improve PM utilization. The second one is designed to reduce back-EMF harmonics and improve the torque density. The topology, working principle, characteristics and design methodology for DCMs are presented. Simulation and experiments are conducted for a 12/11(stator teeth/rotor teeth) DCMs to evaluate its performance and verify this design methodology. It is revealed that this machine could obtain good sinusoidal back-EMF waveform, small torque ripple and high PM utilization factor due to the doubly complementary characteristic. By transferring the doubly complementary idea into disc machines, a novel disc machine is also proposed for direct drive application. It is shown that the torque density of machine is high and the efficiency is more than 90% at rated speed. To further improve the torque density of stator-PM machines, one novel kind of Vernier machines with consequent-pole PMs in slots is proposed in Chapter 4 and the influence of different combinations of rotor and winding poles is compared. These machines have the paralleled magnetic path, which means the armature field will not pass through PMs. So the risk of demagnetization is eliminated and the armature reaction is intensified to improve the torque. Meanwhile, the simple structure makes these machines mechanically reliable and endurable. The principle of the proposed machines is introduced and the combinations of different winding and rotor poles are discussed. One prototype with 24 slots, 2 winding pole-pairs and 22 rotor pole-pairs has been built and tested. The results show that the torque density for prototype could reach 3 Nm/kg under nature cooling condition and the efficiency could get higher than 90%. Meanwhile, the total harmonic distortion (THD) for back electromotive force (back-EMF) is only about 5%, cogging torque is only about 0.8% and electromagnetic torque ripple is merely 5%. The comparion of this prototype with industrial applied fractional slot concentrated winding permanent magnetic machine is presented. It is shown that this stator-PM machine has 10.5% higher torque density. In to further improve the torque density of stator-PM machines, one novel dual-PM excitation Vernier machine with short-pitch distributed-windings was studied in Chapter 5. The special parallel magnetic path constructed by the dual-PM structure could increase the equivalent permeance of the air-gap to better utilize the magnetomotive force (MMF) for torque improvement. The distributed-winding could increase the electrical gear ratio of the machine for speed scaling-down and torque turning up. Meanwhile, the short-pitch coils are applied to eliminate back-EMF harmonics to reduce torque ripple. The principle and characteristics of the proposed machine are introduced in detail. Some essential issues such as the torque percentages contributed by the excitation field harmonics, the effect of the shot-pitch coils and the torque comparison of dual-PM Vernier machine with conventional rotor-PM Vernier machines are introduced. The tested prototype has shown that this machine has good potential for direct-drive application as the torque density could reach 6 Nm/kg under nature ventilation, the total harmonic distortion (THD) for back-EMF is less than 4%, the torque ripple is less than 4% and the efficiency is about 90% under rated speed.