Evaluating the Practical Low-Speed Limits for Back-EMF Tracking-Based Sensorless Speed Control Using Drive Stiffness as a Key Metric

Although it is widely known that back-electromotive-force tracking has a low-speed limit, there is very little published literature evaluating this limit. This paper shows that the low-speed limit is mainly a function of the amplitude of the inverter harmonics and that theory and simulation will provide erroneous results when inverter harmonics are not incorporated. This paper will also provide a method for evaluating this low-speed limit using drive stiffness as the metric. The experimental results show quantitatively how the drive stiffness suffers as speed decreases, which ultimately sets the practical low-speed limit.