The Role of Filters in Sensorless Vector Control Systems for Induction Motors Using Rotor-Flux-Induced Voltage Method

One method of speed-sensorless vector control for induction motors involves estimating the stator angular velocity using the rotor-flux-induced voltage. This is known as the E2d method. We investigated how a first-order lag filter, applied to the q-axis component of the estimated rotor-flux-induced voltage stabilizes the system when employing the E2d method in automatic speed regulator (ASR). The filter plays a role in suppressing oscillations and extracting speed information. Traditionally, the oscillations that should be filtered are considered to be the lower-order harmonics. However, simulations have shown that a longer time constant is required, making it insufficient to simply remove the harmonics. We further conducted a root locus analysis using linear models, finding that the filter time constants capable of stabilization are dependent on the speed. Additionally, this paper presents design guidelines for filter time constants based on root locus analysis and reports on the experimental validation of the proposed criteria.