Slotless High-Performance Electric Drives

Harmonic Loss Analysis, Sensorless Control, and Inverter Design

Time: Fri 2020-03-20 10.00

Location: Kollegiesalen, Brinellvägen 8, Stockholm (English)

Subject area: Electrical Engineering

Doctoral student: Jonas Millinger , Elkraftteknik

Opponent: Professor Zi-Qiang Zhu,

Supervisor: Oskar Wallmark, Elkraftteknik; Docent Juliette Soulard, Warwick University

Abstract

Electric motor drives are a cornerstone for many fundamental functions in today’s society, and their expected dominance within the transport sector is considered one of the main keys to abate global warming. Subsequently, the incentives for improved performance of electric motor drives cannot be sufficiently emphasized. This thesis strives for improved performance in electric motor drives of industrial power tools, such as the nutrunner system. A typical nutrunner system comprises a wall-mount power converter and a cable-connected, lightweight (handheld) nutrunner. The converter feeds the electric motor with a pulse-width modulated signal, which, apart from the desired fundamental component, also contains undesired high-frequency harmonics. As a consequence, harmonic losses are generated in the motor which reduce the performance. Harmonic losses occur in every conductive part of the motor, but rotor losses are considered more challenging due to the poor heat transfer across the air gap. Excessive rotor temperatures can cause premature bearing failure and even irreversible demagnetization of the permanent-magnets. The conventional solution is to use a large inductive motor filter to suppress the harmonic currents fed to the motor. However, the recent emerge of wide-bandgap transistors enables significantlyhigher switching frequencies in electric motor drives compared to their conventional, silicon-based counterparts, which can reduce the size of the required motor filter. The aim of this work is to eliminate the wall-mountconverter and make it sufficiently small for integration inside the nutrunner unit. Optimization of electric motor drives using wide-bandgap technology requires accurate models for the prediction of harmonic phenomena in electric machinery. The main focus of the thesis is to develop an accurate model for the predictionof harmonic losses in slotless permanent-magnet motors when fed by a wide-bandgap inverter. As a first step, the small-signal magnetic behaviors (including power losses) of silicon and carbon steel are characterized. Next, a computationally efficient, three-dimensional finite-element model for simulation of harmonic motor losses, is developed. The model is experimentally validated using a broad range of rotor-magnet segment thicknesses. The developed models show that wide-bandgap technology can effectively contribute to the elimination of the inductive motor filter. In the second part of the thesis, a novel sensorless control-method (enabled by a filter-less motor drive topology) is developed, and experimentally evaluated. The results show an estimation error of the rotor position below 2 degrees, which, in contrast to conventional slotted machinery, is practically unaffected by the load (current) level. In the third part of the thesis, a compact, filter-less wide-bandgap inverter is developed and experimentally evaluated. Despite operation at significantly higher switching frequencies, inverter power losses can be halved, which further reduces the system weight (due to a smaller heat-sink). To summarize, the utilization of wide-bandgap transistors enables substantial improvements in terms of reduced system weight, complexity, and power losses in slotless permanent-magnet motor drives.

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