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In this study, a sensorless vector control for the rotor-tied doubly fed induction generator (RDFIG) is proposed in the grid-connected mode. The proposed sensorless vector control method includes a slip speed/angle estimator which is based on the association of the high-order sliding mode observer (HOSMO) with a phase-locked loop (PLL). In addition, an extensive comparison between the PLL-based HOSMO estimator and the PLL-based second-order sliding mode observer (SMO) estimator is also presented. The Lyapunov stability criterion is used to determine both observer gains to allow their convergence in finite time. Both the proposed HOSMO and the SMO use the three-phase stator current and back-electromotive force (EMF) as state variables which enable the start of the estimation even before the machine is connected to the grid. The proposed HOSMO takes into account the dynamics of the back-EMF space vector. The PLL is used to extract the estimated slip speed/ angle from the estimated back-EMF. The performance of the proposed sensorless vector control strategy is validated experimentally with a 5.5-kW custom-designed RDFIG on a test bench based on the National Instrument (NI) PXIe-8115 realtime controller.

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In a brush dc equivalent (BDCE) controlled machine, quasi-square like constant air gap flux density and rotor induced voltage waveforms are utilized. Analytical expressions for calculating the developed power, flux density, stator and rotor copper losses for the BDCE controlled cage rotor multiphase induction machine are developed in this paper. From analysing these expressions, it is found that, for a BDCE controlled machine with a certain torque and a certain flux density, there is a best (optimal) combination ratio of field to torque phases. Also, in this paper, measurement results in a prototype cage rotor multiphase induction machine drive are presented.