Low-cost sensorless control of four-switch, brushless DC motor drive with direct back-EMF detection

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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 from 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.