A Back-EMF Threshold Self-Sensing Method to Detect the Commutation Instants in BLDC Drives

By predicting future phase-current commutation instants of brushless dc (BLDC) machines, self-sensing control is achieved. Detection of the back electromotive force (EMF) zero-crossing moments is one of the methods used to predict the phase-current commutation instants. Most of the techniques based on back-EMF zero-crossing detection have the inherit disadvantage of an indirect prediction of commutation instants. This is the result of the commutation instant occurring at the 30° electrical position after zero-crossing of the induced back-EMF in the unexcited phase. Often, the time difference between the zero-crossing of the back-EMF and the current commutation instant is assumed constant. This assumption can be valid for steady-state operation; however, a different time span should be taken into account during transient operation of the BLDC machine. To overcome this problem, an enhancement is proposed here, which improves the performance while keeping the simplicity of the back-EMF zero-crossing detection method.