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-EMF 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, which means 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 shows quantitatively how the drive stiffness suffers as speed decreases, which ultimately sets the practical low speed limit.