The brushless DC motors are sunchronous motors, self-commutated through a transistor bridge. The aim is to generate a magnetic field always positioned so that the torque is maximum. This principle needs a rotor position detection. Sensors as Hall effect devices, resolvers or optical disks are generally used. For small motors as those used in computer peripherics, it becomes impossible to implement such devices. Thus, the goal is to realize indirect sensors. The present paper describes a methodology to control a motor from standstill to full speed without direct position and speed sensors.
At standstill, 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 measurement is converted into a current difference, with positive and negative current pulses in each phase. So, a logical state is defined, corresponding to the actual position with an angular accurancy of one electric period devided into 2m (m = number of phases). Such a method is necessary in order to assure a starting in the correct direction. A hazardous first switch on can let start the motor in the wrong direction. Consequently, the proposed system offers the possibility to start surely, without back oscillation.
The proposed system of position detection is used to start the motor from standstill to a medium speed, where the well-known method of the phase back E.M.F. position detection is applied. From medium speed to the nominal speed the motor is accelerated using the phase back E.M.F. position detection. Consequently, the motor is fully controlled in a closed loop sensorless mode from standstill to the nominal speed. The method is presented through a principle analysis, a process description and experimental measurements.
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