Design, analysis and application of novel flux-controllable flux-modulated machines

Abstract

Direct-drive machines have drawn much attention in applications like electric vehicles (EVs), wind power generation systems and industrial cooling fans, etc, which usually operate at low speeds and require high torque density. Compared with gear assist driving systems, direct-drive systems can reduce the size, simplify the structure and operate with higher efficiency. Permanent magnet (PM) machines are good candidates for direct-drive applications due to their inherent high torque density and high efficiency, especially when high magnetic energy magnets are used. In recent decades, flux-modulated machines have become very popular which have the potential to further improve the torque density of PM machines due to the incorporated gear effect. The air-gap permeance is artfully modulated and significant harmonics will be induced, which can contribute to torque improvement when the pole-pair combination is specifically chosen. Another important feature for direct-drive machines is good flux controllability. Therefore, the machines can achieve wide constant power speed range when worked as motors, and maintain constant output voltage when worked as generators. The purpose of this thesis is to give a deep investigation on the flux-controllable flux-modulated (FCFM) machines, which can achieve both high torque density and good flux controllability. Bi-directional flux modulating theory is proposed and its analytical model is established. The pole-pair number (PPN) and rotating speed of air-gap harmonics are made clear, which gives an insight of the working principle of FCFM machines. Hybrid PM memory machines are firstly proposed, in which both AlNiCo and NdFeB are employed. Through apply DC current pulses, the magnetization state of AlNiCo can be flexibly changed and the air-gap flux is regulated accordingly. NdFeB is used to ensure high output torque. When the armature windings are designed with five phases, good fault-tolerant capability can be realized. The major drawback is the doule air-gap structure, which is relatively complicated and difficult to manufacture. Based on the double air-gap structures, improved FCFM machine concepts with single air-gap are proposed. The PMs can be employed on both the stator and the rotor. Hybrid excitation can be achieved through two approaches, one is employing DC field windings in the stator slots, the other is injecting DC bias current into the armature winding, which results in DC coil free hybrid excitation. The electromagnetic performances are firstly analyzed using finite element method (FEM), and verified by experimental tests.