We propose a position sensorless control scheme for a four-switch, three-phase brushless DC motor drive, based on

the zero crossing point detection of phase back-EMF voltages using newly defined error functions (EFs). The commutation instants

are 30° after detected zero crossing points of the EFs. Developed EFs have greater magnitude rather than phase or line

voltages so that the sensorless control can work at a lower speed range. Moreover, EFs have smooth transitions around zero

voltage level that reduces the commutation errors. EFs are derived the filtered terminal voltages vao and vbo of two low-pass

filters, which are used to eliminate high frequency noises for calculation of the average terminal voltages. The feasibility of the

proposed sensorless control is demonstrated by simulation and experimental results.-We propose a position sensorless control scheme for a four-switch, three-phase brushless DC motor drive, based on

the zero crossing point detection of phase back-EMF voltages using newly defined error functions (EFs). The commutation instants

are 30° after detected zero crossing points of the EFs. Developed EFs have greater magnitude rather than phase or line

voltages so that the sensorless control can work at a lower speed range. Moreover, EFs have smooth transitions around zero

voltage level that reduces the commutation errors. EFs are derived　the filtered terminal voltages vao and vbo of two low-pass

filters, which are used to eliminate high frequency noises for calculation of the average terminal voltages. The feasibility of the

proposed sensorless control is demonstrated by simulation and experimental results.