An I-F starting method for smooth and fast transition to sensorless control of BLDC motors

This paper proposes an I-f starting method for smooth and fast transition from open-loop frequency control to sensorless field oriented control (FOC) of BLDC motors. The method is divided in three stages: (i) initially the rotor is aligned with a desired angular position; (ii) then the stator currents are controlled in an arbitrary reference frame to impose the angular frequency of the rotating magnetic field with a suitable profile; (iii) finally, a smooth transition between the arbitrary and the rotor reference frames is accomplished by reseting the current controller states and references. The estimated rotor position that is required for FOC as well as for the proposed starting method, is computed by using the estimated back-EMFs from a Luenberger observer. The proposed method ensures that the electromagnetic torque does not change abruptly during the transition avoiding undesired transients. In addition, a procedure based on mechanical model is described to determine a suitable ramp profile. The good performance and robustness under no-load and load conditions of the proposed starting method are demonstrated by simulation and experimental results.