Simple position sensorless starting method for brushless DC motor

Position sensorless methods for brushless DC motors based on back-EMF zero crossing suffer from a starting problem since there is no back-EMF at standstill. A simple method by which the motor is started from standstill up to a speed wherein sensorless methods will be able to detect the correct commutation instants is proposed. The proposed method relies on a difference of line voltages measured at the terminals of the motor. It is shown that this difference of line voltages provides an amplified version of an appropriate back-EMF at its zero crossings. It is further demonstrated that this information can be used to trigger devices so as to develop an accelerating torque from zero speed. This method is simple to implement and it can reliably start the motor even with load. The effectiveness of the proposed method is demonstrated through simulation and hardware results.