New sensorless commutation technique for brushless DC motors

In brushless DC (BLDC) drives commutation is performed by power electronic devices forming part of an inverter bridge. Switching of the power electronic devices has to be synchronised with rotor position. Determination of position, with or without sensors is an essential requirement. The most common sensorless method is based on detection of the zero crossings of back EMF signals. This technique works only above a certain speed. 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 eighty electrical degrees and fluctuations in electromagnetic torque. The aim of this project has been to investigate the possibility of a sensorless technique which is cost effective but with a performance at start-up comparable with that obtained when Hall sensors are used. Initial investigations led to a saliency based method. Theoretical analysis is presented which shows that the method is insensitive to variations in operational parameters such as load current and speed or circuit parameters such as power device voltage drops and winding resistances. Also a starting strategy, relying on saliency related measurements, is proposed which offers starting performance as good as Hall sensor based techniques.