After studying the development of BDCM sensorless control with six-step square-wave PWM (SWPWM) and six-step back-EMF detecting scheme (BEDS), the BDCM sensorless control with twelve-step SWPWM and twelve-step BEDS had been proposed and implemented in this paper. The proposed twelve-step BEDS generates the twelve commutation instants from six back-EMF zero-crossings during each cycle where the six-step BEDS generates the six commutation instants from six back-EMF zero-crossings. Since the analysis of six-step BEDS is helpful to develop the twelve-step BEDS, this paper studies the six-step SWPWM and six-step BEDS first and explains why some specific six-step SWPWM type is always used in the six-step BDCM sensorless control. Then, we obtain the specific twelve-step SWPWM types and develop the proposed twelve-step BEDS. From the simulated and experimental results, BDCM sensorless control with twelve-step SWPWM shows less current harmonics than that with six-step SWPWM.