A simple sensorless method for the detection of the mechanical rotor position of the wound-rotor induction machine in order to implement stator-flux orientation is described and evaluated in this paper. The method is based on the phase comparison of the actual and the estimated rotor currents using the classical model of the machine. It can be conceptually implemented in the rotor or in the stator reference frames. It has some similarity to the model reference adaptive system methodology, but uses a hysteresis comparator instead of a proportional integral (PI) controller. In this way, the method does not need parameter determination for the controllers and shows a considerable independence of parameter uncertainties. Simulation and experimental results show that the method is appropriate for the vector control of the doubly fed induction generator (DFIG) because it leads to the decoupling of active and reactive power chains. It is also appropriate for the control of the DFIG in transients like those when voltage dips occur.

A sensorless method for the direct estimation of the slip position of the wound-rotor induction machine is introduced, discussed, and experimentally validated in this paper. The slip position is necessary for the implementation of control by flux orientation. The method proposed is based on the phase comparison of an estimated air-gap power vector and the measured rotor currents in a common reference frame. The proposed method can be implemented in the rotor or in the field reference frames with a hysteresis or proportional-integral controller. The method is sensitive to the stator no-load active and reactive powers, but this dependence is not very important. Simulation and experimental results show that the method is appropriate for the vector control of the doubly fed induction machine. Unlike other methods, this method gives acceptable results when the load is small. In addition, it does not need a flux estimator for the implementation of flux orientation.

The doubly fed induction generator (DFIG) is normally controlled with stator-flux orientation. The DFIG is a poorly damped system with a natural frequency near the grid frequency. This leads to natural oscillations on the stator flux and on other variables. When voltage dips occur, there are oscillations on the stator flux that produces electromotive forces and consequently disturbances on the rotor currents. This paper presents a comprehensive study of the DFIG during voltage dips. The method presented in this paper, valid for symmetric and for asymmetric voltage dips, is based on the classical model of the induction machine with stator flux orientation and neglects only a small part of the cross-coupling terms. The response depends on the design methodology of the proportional-integral inner controllers. Analytical, simulation, and experimental results are shown. The analysis and results can be divided into two sets depending if the voltage dip magnitude is smaller or deeper than 0.5 p.u.

This paper presents a new sensorless method and its stability study for the estimation of the mechanical rotor position of the wound-rotor induction machine. The main purpose of this study is to implement the control of the doubly fed induction generator (DFIG). The method is based on the model reference adaptive system and uses the electromagnetic torque as the working error variable. The method does not need any information about the stator or rotor fluxes, and so, it is only indirectly dependent of the flux dynamics. As other methods proposed recently, this can also be implemented in the rotor or in the stator reference frames and with hysteresis or with proportional-integral controllers. The stability analysis gives an instability region on the rotor current dq plane described by a circle whose diameter is the no-load stator current. The method is robust to parameter variations depending only weakly on a single parameter. Simulation and experimental results show that the method is appropriate for the vector control of the DFIG although needing an additional approach to stabilize the system in the instability region.

We measured the proton spin-lattice relaxation times in the isotropic phase of liquid crystal $4^{\prime }\text{−}n\text{-pentyl-}4\text{-cyanobiphenyl}$ (5CB) confined into porous glass (CPG) with the average pore diameter $\sim 72\mathrm{nm}$. The analysis of $T_1^{-1}$ frequency dispersions, spanning over four decades, shows that the main relaxation mechanism induced by the ordered surface layer are molecular reorientations mediated by translational displacements (RMTD). The RMTD contribution to $T_1^{-1}$ is proportional to the inverse square root of Larmor frequency, a consequence of the equipartition of diffusion modes along the surface. Low and high frequency cutoffs of the RMTD mechanism clearly reveal that the surface alignment of liquid crystal is random planar with the size of uniformly oriented patches $\sim 5\mathrm{nm}$, depending on the treatment of the CPG matrix. According to the size of the uniformly oriented patches varies also the thickness of the ordered surface layer and its temperature behavior. The surface-induced order parameter is found to be temperature independent and determined by the local short range surface interactions.