Commutation is a fundamental feature of all DC machines. In conventional DC machines the commutation function is performed by the commutator and brushes. These act as both position sensors and switches. The mechanical commutator has obvious disadvantages. Overcoming those disadvantages has been a major reason behind the development of brushless DC (BLDC) machines. In brushless DC machines commutation is performed by power electronic devices forming part of an inverter bridge. However, switching of the power electronic devices has to be synchronised with rotor position. Position sensing is therefore an essential requirement. This can be done by using sensors such as Hall Effect devices or a sensorless approach may be adopted. Advantages of sensorless techniques include reduced cost and wiring. The most common sensorless method is based on detection of the zero crossing of back EMF signals. But this technique works only above a certain speed since back EMF is directly proportional to speed. As a result BLDC systems which rely solely on back EMF signals for commutation suffer from relatively poor starting performance characterised by back rotation of up to one hundred and eigthty electrical degrees and large fluctuations in electromagnetic torque resulting from non-ideal commutation instants. This may not be acceptable for some applications and many researchers have attempted to overcome those problems. The aim of this project has been to investigate the possibility of a sensorless technique which does not cost more than the back EMF method but with a performance at start-up comparable with that obtained when Hall sensors are used. Initial investigations led to a saliency based method. Detailed theoretical analysis is presented which shows that the method is insensitive to variations in operational parameters such as load current and circuit parameters such as power device voltage drops and winding resistances. There is a close parallel between it and the back EMF method and this makes it easy to swap to the latter method at high speed if necessary. A starting strategy, relying on saliency related measurements, is proposed which offers starting performance much better than the back EMF method and almost as good as Hall sensor based techniques. Experimental evidence is provided to confirm that commutation instants determined by the proposed method are practically coincident with those obtained when Hall sensors are used.

This project is made for a deeper understanding ofhow frequency and amplitude of the waves that create the controlwave in a Pulse Width Modulated 2-level inverter affect the powerquality and power losses. The results were that a high frequencyreduces the Total Harmonic Distortion but increases the powerloss. The amplitude, however, reduces both the Total HarmonicDistortion and the power loss as it increases. All the analyseswere done in a simulation program called Simulink. The resultscan be applied when improving High Voltage Direct Currentinverters to develop a functional High Voltage Direct Currentgrid that enables wider use of renewable energy sources. / Projektet syftar till att få en djupare förståelse för hur frekvensen och amplituden på de vågor som skapar kontollvågen i en pulsbreddsmodulerad likspänningsomvandlare med två nivåer påverkar effektkvalitén och effektförlusterna. Resultatet av studien var att en hög frekvens minskar ”Total Harmonic Distortion” men ökar effektförlusterna. Amplituden å andra sidan reducerar både ”Total Harmonic Distortion” och effektförlusterna när den ökar. Alla analyserna är gjorda i simuleringsprogrammet Simulink. Resultaten kan appliceras när högspända likspänningsomvandlare vidareutvecklas för att skapa ett fungerande högspänt-likströms elnät som öppnar upp för en bredare användning av förnyelsebara energikällor. / Kandidatexjobb i elektroteknik 2020, KTH, Stockholm