| Abstract |
Classically, the brushless DC motors are self-commutated by means of sensors as Hall devices, resolvers or optical sensors. Some indirect methods are also known, as the back EMF or the third harmonic detection. But these solutions cannot give any information at standstill. The present paper describes a possibility of position detection at standstill, without direct sensor. The position is determined by short current pulses, enabling to detect indirectly the saturation level of the different phases. In order to avoid the temperature effect on the time constants, the messurement is converted in a current difference, with positive and negative current pulses on each phase. So, it is possible to define a logical state corresponding to the actual position with an angular accuracy of one electrical period divided by 2m (m = phase number). Such a method is necessary in order to assure a starting in the good direction. An hazardous first switch on can let start the motor in the wrong direction. So, the proposed system offers the possibility to start surely, without back oscillation. The method is presented through a principle analysis, a process description and experimental measurements. |
