A novel starting method for sensorless brushless DC motors with current limitation

The paper presents a novel starting method for sensorless brushless DC motors. The proposed method is based on the identification of phase commutation instants using the PWM duty cycle deviation around its average maximum value under current control. These variations occur due to the back-electromotive force (BEMF) that act as a disturbance in the current control loop. The starting method is divided in two steps: rotor alignment and the rotor initial acceleration with DC-link current control. As a result, the phase current is limited in a safe level and a high starting torque can be produced. Even though the phase currents are measured by a shunt resistor at DC-link, the observability loss is prevented by the proper choice of PWM carrier frequency during the motor startup. Finally, simulation and experimental results are presented to evidence the good performance of the proposed method.