Reducing the Low-Speed Limit for Back-EMF Based Self-Sensing Speed Control by Using an FPGA-Operated Dead-Time Compensation

As the back electromotive force (back-EMF) scales with the rotor speed of the permanent magnet synchronous machine (PMSM), this generally results in a low-speed limit for back-EMF based rotor position estimation. As shown in [1], this low-speed limit is primarily determined by the inverter's nonlinearity effects which are significantly caused by the inverter's dead-time [2]. With decreasing rotor speed, the modulation index of the inverter is reduced and the relative voltage error due to the inverter's dead-time effect increases compared to the machine's back-EMF. Therefore, the aim of this paper is to show that the low-speed limit can be significantly reduced when a back-EMF based rotor position estimation method is combined with a highly accurate and dynamic dead-time compensation using FPGA-operated current oversampling. For the experimental results the rotor position estimation [3] is combined with the predictive dead-time compensation [4]. The results demonstrate a significant reduction of the low-speed limit to a rotor speed that is about 0.5% of the rated speed.