7 Conclusions

This technical brief provides a method to measure the BEMF from the phases of a BLDC or PMSM motor driven in Sinusoidal Sensorless mode. Enlarging one of the dead times added for transistor switching creates a big enough acquisition window to measure clean BEMF signals. This method can be implemented using 8-bit MCUs that don’t have all the capabilities of 16-bit and 32-bit cores.

When using this method, information about the rotor’s position can be obtained every 60° when the BEMF's Zero-Cross signal is detected, and the drive is updated to keep synchronization with the motor. The highest efficiency is achieved when the BEMF is synchronized with the driving voltage because the voltage drop across the coils is minimum.

Compared to the method presented in this document, the FOC algorithm is still better because the rotor position information is obtained much more frequently (every PWM cycle). This new method, using dead time to measure BEMF, can be applied for low-end applications, where the control doesn’t have to be very complicated, and the dynamic load variation is not too high.

Applications like fans and pumps can use this measurement method for BEMF detection and have a Sinusoidal Sensorless Drive. From a noise and efficiency aspect, the control obtained with this newly proposed method is still superior to the classical Trapezoidal Drive.