Investigation of High-Efficiency Radial and Tapered Brushless PM Machines

Abstract

In recent decades, factors such as the rising price of energy and the introduction of new electric machine efficiency standards have been the driving force for extensive research towards higher efficiency machines. To achieve this, both new materials and design modifications should be considered. Brushless permanent magnet (PM) machines are an excellent substitute to replace conventional machines in many applications as they offer higher efficiency, reliability and power density. Therefore, this thesis addresses the design challenges in two separate brushless PM machines (radial and tapered types) and investigates these challenges to achieve higher efficiency. In the first part of the thesis, a 16-kW, 3-phase, 4-pole radial-flux spoke-type interior PM (IPM) machine with inherent voltage regulation capability has been presented. The machine topology was studied using analytical modelling, finite-element simulation and practical measurements. The measured parameters and performance of the machine were in close agreement with the calculated values. Since the target application was a portable AC generator, the results have been compared with the commonly used wound-field generator. It was found that this new radial IPM generator can offer 6% higher efficiency and 8.6% fuel saving. In addition, the generator presented low total harmonic distortion (THD) and good voltage regulation meeting the unity power-factor application requirements. Furthermore, as it is a common practice for 3-phase generators to run under single-phase loading, the single-phase performance of the machine was also studied. Three winding configurations were investigated including 1-phase star, 1-phase delta and a proposed new connection named “open delta”. It was concluded that the proposed open-delta connection can operate under single-phase loading effectively. Finally, the machine performance was also studied in motoring when driven by an inverter in open-loop in volts-per-Hertz control mode, which showed a high power-factor over a wide loading range. In the second part of the thesis, the primary goal was to study a new tapered brushless PM machine topology which used amorphous magnetic material (AMM) in the stator. A preliminary work was also presented in this part to compare the AMM material with the conventional lamination material (SiFe) and the soft magnetic composite (SMC) material using identical stator sizes in a flat axial field machine topology. Next a tapered PM machine was studied due to its manufacturing advantages, it had the following specifications: 2-kW, 12-slot, 10-pole, 7000 rpm. A detailed and accurate loss study was conducted using 3D simulations (due to the unique shape of the machine) and practical tests to identify the major loss components for further efficiency improvements. An improved mechanical loss separation technique has been proposed and demonstrated using the open-circuit test results. In addition, a novel locked rotor test was introduced as a reliable method to separate the stator and rotor losses which is applicable to other electrical machines. After the loss separation study, a design optimisation was performed which considered three design elements: air-gap length, PM shape, and rotor yoke design. Then, a new rotor design was constructed considered the optimization of the yoke only which resulted in a substantial 3.5% improvement in efficiency.