| Due to the advantage of high efficiency and power density, high-speed permanent magnet brushless DC (BLDC) machines are emerging as a key technology for applications such as turbocharger, spindle drives, flywheel energy storage system, machine tools, compressors, molecule pumps. Researchers all around the world are getting more and more attention on high-speed machines. However, their high rotation speed demands that careful consideration should be given to both electromagnetic and mechanical design issues, such as e.g. rotor loss, bearing technology, rotor dynamics, and aerodynamic loss.Since high-speed machines have high power density and small volume, their loss density is also high. In result, the temperature rise inside the motor is usually high which can cause Hall-effect position sensors unworkable if they are installed inside the motor. Meanwhile, the small volume may also limit the use of the position sensors. And more importantly, in high power rating motors, the position sensor signals are easy to be disturbed by the noise. Moreover, BLDC machines require variable advanced-firing control during high-speed operation. However, if the firing sequence is directly determined by the position sensor signals, advance-firing commutation cannot be implemented.In order to solve the drawbacks of using position sensors, lots of researchers have been working on sensorless controls. The main idea of sensorless control is to use electrical signals derived from the motor windings, with suitable methods, to calculate the rotor position and speed. The most common sensorless control is based on the detection of zero crossings of the phase back-EMF or the third harmonic back-EMF. In high-speed motors, however, the phase currents can flow more or less continuously due to the effect of winding inductances, thus preventing the detection of zero-crossings of the phase back-EMF. On the other hand, the method of detecting the third harmonic back-EMF zero-crossings is not affected by the diode conduction angle or the inverter PWM on-off noise. And, this method has a lower requirement on the filters, and is suitable for a wide speed range.This thesis focuses on the third harmonic back-EMF-based sensorless control of a 15kW, 80,000rpm high-speed PM BLDC motor, with consideration of major problems in the motor design and sensorless control.Firstly, the influence of different stator structures on third harmonic back-EMF is studied. The 3-slot/2-pole and 6-slot/2-pole non-overlapping-winding structures have been comparatively studied, showing that the 3-slot/2-pole structure is not suitable for the 3rd harmonic EMF-based sensorless operation, because the 3rd harmonic winding factor is 0. The 6-slot/2-pole structure in which the fundamental winding factor is 0.5 and the third harmonic winding factor is 1, is suitable for the 3rd harmonic EMF-based sensorless operation. However, its fundamental winding factor is low, the winding is thus not fully utilized. In order to combine the advantage of both structures, another topology is proposed by employing unequal teeth, where the small teeth have no winding wound around. The proposed 6-slot/2-pole unequal-teeth structure improves the utility of the winding and is also suitable for the3rd harmonic EMF-based sensorless operation. Meanwhile, the inductance and the unbalanced magnetic force in these three different stator structures are also comparatively studied with different structure, showing that the 3-slot/2-pole structure and the 6-slot/2-pole unequal-teeth structure exhibit significant unbalanced magnetic force due to diametrically asymmetric, which will reduce the bearing service life and increase the motor vibration and acoustic noise. The 6-slot/2-pole structure with all teeth wound gives highest third harmonic component of back-EMF and lowerest unbalanced magnetic force, hence, is eventually employed in this thesis.Secondly, influence of the rotor structures with different magnet assemblies on the third harmonic airgap field and third harmonic back-EMF is studied. The airgap field is analyzed with both FEM and analytical model. In surface-mounted permanent magnet motors, when the pole-arc to pole-pitch ratio decreases, the airgap field distribution waveform becomes farther away from sinusoidal waveform, containing more higher 3rd harmonic component. This will produce higher third harmonic back-EMF which can be used for the sensorless control. However, the fundamental component decreases as pole-arc to pole-pitch ratio decreases, which will deteriorate the motor performance. Therefore, in order to enhance the third harmonic airgap field but meanwhile hardly decrease the fundamental component, it is not sufficiently good to reduce the pole-arc to pole-pitch ratio solely. Instead, magnet-segmenting technique can be employed. Influence of the number of magnet segments on the airgap field distribution is also investigated with FEM, showing that the number of segments has a significant effect on the airgap field waveform. It is also shown that the structure with two magnet segments per pole exhibits the highest third harmonic in the airgap field, and even the fundamental component is higher than that with one segment per pole. This is beneficial to improve the motor performance. Therefore, the structure with two magnet segments per pole is employed in the designed high-speed BLDC motor. Three motor configurations, viz., (1) parallel magnetization with pole-arc less than 1, (2) parallel magnetization with magnet segmenting and (3) radial magnetization, are comparatively studied. It is shown that the first and second configurations can give sufficient high third harmonic back-EMF in the motor windings. On the other hand, the rotor loss is negligible in low or moderate-speed PM BLDC motors, but becomes significant in high-speed motors in which case the magnets may be over-heated and permanently demagnetized. Therefore, the influence of the three motor configurations on the rotor eddy-current loss is also comparatively studied, showing that the third configuration is superior to the first and the second. However, the third configuration has difficulty in manufacturing, thus the first and the second configurations are usually employed.Thirdly, the advanced-firing commutation based on the third-harmonic back-EMF sensorless control is achieved. In this sensorless control, the commutation signal has a relationship with the comparator output of the zero-crossing signal. In a practical system, the zero-crossing signal may contain noise, which could cause an erroneous update of the estimated rotor position. Hence, this will cause the whole system unworkable. It is necessary to filter out the noise signal using software. For high speed motor, the digital filter will make the motor commutation retarded. Consequently, the commutation retarding will cause lower power factor, lower efficiency, lower power output power capability and high temperature rise, and the cooling system must be enhanced. Therefore, it is necessary to use the advanced-firing commutation based on the third-harmonic back-EMF sensorless control. Due to the rotor inertia during the motor operation, the electric cycle will not abruptly change. Hence, the electric cycle in the successive two commutations is almost the same. Hence, if the former commutation cycle and the digital filter time-constant are known, the next-step advanced-firing commutation can be achieved by using the timer delay. It is shown that the advanced-firing commutation can largely improve the motor power factor, efficiency, power output power capability, and at the same time reduce the loss.The above-mentioned three major problems have been solved during the research, whilst a prototype has been manufactured. Experiments have verified the related theoretical analysis, design method and sensorless control strategy.
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