Design of the PLL for the speed sensorless stator-flux-oriented control of induction motors

This paper presents a method of designing the phase locked loop (PLL) for rotor speed estimation. The transfer function from the torque current to the estimated rotor speed is derived and is compared with that from the torque current to the real rotor speed. The damping ratio of the PLL is selected to make the two transfer functions as close to each other as possible. A first-order filer is further added to compensate the differences between the two transfer functions at high frequencies. Simulation results show that the proposed design of the PLL method improves the accuracy of the rotor speed estimation.