High-speed permanent magnet brushless DC motors

This thesis describes an investigation into some of the key design issues for high-speed permanent magnet brushless DC motors. The investigation encompasses the optimal design of motors for operation using a simple sensorless commutation strategy based on the detection of the zero-crossing of the back-emf waveforms, the optimal split ratio of the rotor diameter to the stator outer diameter, with due account of iron loss, and a systematic investigation into rotor resonances and the influence of key design parameters on the rotor resonant frequencies. The investigation was initiated by a design study on a high-speed, exterior rotor permanent magnet brushless DC motor which was undertaken on behalf of Urenco (Capenhurst) Ltd. Comprehensive electromagnetic design and analysis was performed using proprietary CAD software developed by the Electrical Machine and Drives Research Group at the University of Sheffield. However, due to fairly rigid constraints on the prototype motor, as specified by Urenco, the design options were limited, and, consequently, the resultant motor performance was not optimal. Nevertheless, despite the occurrence of resonance modes which prevented operation of the motor above -16krpm, useful results were obtained from a prototype motor and good agreement was obtained with dynamic simulations. This motor also served to highlight several key design issues pertaining to high-speed permanent magnet brushless DC motors, which are investigated in subsequent chapters. Previous work on high-speed permanent magnet brushless DC drives has focus sed primarily on the motor with little attention being given to the commutation strategy at the design stage. For operation using a simple sensorless commutation scheme based on the detection of the zero-crossing of the back-emf waveforms a low freewheel diode conduction angle is required if the zero-crossings are not to be obscured due to the current which flows through the freewheel diodes. It is shown that by employing a stator core design which differs somewhat from conventional designs, in terms of the width of the teeth and the back-iron radial thickness, high-speed motors which result in a low diode conduction angle are realisable without any significant degradation in machine performance. The design process has been validated by measurements on small prototype motors, and sensorless operation has been achieved at speeds in excess of 120krpm. It had been reported previously for low speed motors, that the optimal ratio of the rotor diameter to the stator outer diameter, or split ratio, is important in determining the optimal motor design for minimum copper loss and a simple expression had been derived for i determining this optimal ratio for a motor equipped with distributed windings. This work has been extended to cater for motors with concentrated windings, and comparisons have been made with results deduced from a comprehensive CAD package and, neglecting the effects of the end-windings, good agreement has been obtained. It has also been shown that as the motor speed is increased the optimal ratio of rotor diameter to stator diameter is determined increasingly by the iron loss, which significantly reduces the optimal split ratio. The influence of key design parameters on this optimal ratio has been investigated, and it has been shown that the optimal ratio of rotor to stator diameter is highly dependent on the torque density, the stator flux density and the pole number. Finally a systematic investigation into the resonant modes of a prototype high-speed rotor has been undertaken. Finite element predicted and measured natural frequencies have been compared at each step of the rotor assembly process, to enable an accurate model of the complete rotor to be constructed. This model has then been validated on another prototype high-speed rotor, and good agreement has been obtained. The bearings have also been incorporated into the finite element model, and, again, good agreement has been obtained with measured resonant frequencies. Utilising this model, the influence of design parameters such as the rotor active length, the bearing spacing, the shaft extension and the shaft diameter were then investigated. It was found that the shaft extension can significantly affect the rotor resonant modes, and this is illustrated by the damage caused by a resonant mode on the shaft extension. Thus, careful attention needs to be paid to rotor resonance modes during the design of high-speed motors to ensure that resonant frequencies occur outside the operating speed range.