Study of the perovskite-type manganites Ca1 - x Ce x MnO3 (0.00 ≤ x ≤ 0.25) has been carried out. The electrical resistivity values for these samples were measured in the temperature range from 300 up to 900 K and show that the doping of the phase CaMnO3 with cerium induces simultaneously a marked decrease in the electrical resistivity and a metal-to-insulator transition. This behavior is exceptional since such a transition has not been observed to date in these compounds for these experimental conditions and can be attributed to the formation of Mn3+ ions as a result of charge compensation. The values of metal-to-insulator transition temperature, T MI, change with Mn3+ amount. Further the carrier doping tends to suppress the semiconductor behavior of CaMnO3 and the T MI is attributed to charge ordering observed in these compounds around 255 K.

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We measured the proton spin-lattice relaxation times in the isotropic phase of liquid crystal 4-n-pentyl-4-cyanobiphenyl (5CB) confined into porous glass (CPG) with the average pore diameter approximately 72 nm. The analysis of T1(-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 T1(-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 approximately 5 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.

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.

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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.

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