Band-constrained technique for direct torque control of induction motor

In this paper, a novel technique for the direct torque control (DTC) of an induction motor is proposed, which overcomes the trouble of high torque ripple afflicting the conventional DTC technique. With the novel technique, the inverter voltage vector selected from the switching table is applied for the time interval needed by the torque to reach the upper (or the lower) limit of the band, where the time interval is calculated from a suitable modeling of the torque dynamics. By this approach, the control system emulates the operation of a torque hysteresis controller of analog type since the application time of the inverter voltage vector is dictated by the allowed torque excursion and not by the sampling period. It is shown by experimental results that the technique yields a considerable reduction of the torque ripple. A further and ultimate reduction is obtained by compensating for the delay inherent in the discrete-time operation of the control system. The outcome is that the torque ripple of the motor is constrained within the hysteresis band of the torque controller, for a band of customary value. An ancillary merit of the technique is the almost full elimination of the average torque error inherent in the conventional technique. If the hysteresis band is shrunk, the torque ripple is bound to swing out the band limits. Under this circumstance, an extension of the technique is developed, which helps keep the torque ripple at minimum. To assess the characteristics of the proposed DTC technique, the following quantities: average torque error, rms value of the torque ripple, and inverter switching frequency are measured for different stator flux angular speeds and hysteresis bands of the torque and flux controllers. As a comparison, the same quantities are given for the conventional DTC techniqu

An assessment of the inverter switching characteristics in DTC induction motor drives

The switching characteristics of an inverter feeding an induction motor controlled with the direct torque control (DTC) technique are assessed in steady state. At first, the application share of the inverter voltage vectors for the stator flux covering half a sextant is defined and predicted. The prediction indicates that, under operation at fixed inverter dc link voltage and stator flux magnitude, the application share depends only on the supply frequency of the motor and, to a small extent, on the load. Afterwards, the inverter transitions and the corresponding phase commutations within a stator flux sextant are analyzed. The outcome of the analysis permits to compute the commutations of the inverter phases in one turn of the stator flux and, from them, the inverter switching frequency is obtained. Its value is influenced by the sampling interval and the control delay arising from the microprocessor implementation of DTC. For given sampling interval and control delay, it is shown that the inverter switching frequency depends on the same quantities as the application share of the inverter voltage vectors. A comparison with the switching characteristics of an inverter controlled with the space vector modulation technique is carried out. At last, the paper discusses the sensitivity of the switching frequency of an inverter for DTC to the following quantities: inverter dc link voltage, sampling interval and control delay. Throughout the paper simulation and experimental results are given to confirm the theoretical finding

An Assessment of the Inverter Switching Characteristics in DTC Induction Motor Driver

The switching characteristics of an inverter feeding an induction motor controlled with the direct torque control (DTC) technique are assessed in steady state. At first, the application share of the inverter voltage vectors for the stator flux covering half a sextant is defined and predicted. The prediction indicates that, under operation at fixed inverter dc link voltage and stator flux magnitude, the application share depends only on the supply frequency of the motor and, to a small extent, on the load. Afterwards, the inverter transitions and the corresponding phase commutations within a stator flux sextant are analyzed. The outcome of the analysis permits to compute the commutations of the inverter phases in one turn of the stator flux and, from them, the inverter switching frequency is obtained. Its value is influenced by the sampling interval and the control delay arising from the microprocessor implementation of DTC. For given sampling interval and control delay, it is shown that the inverter switching frequency depends on the same quantities as the application share of the inverter voltage vectors. Acomparison with the switching characteristics of an inverter controlled with the space vector modulation technique is carried out. At last, the paper discusses the sensitivity of the switching frequency of an inverter for DTC to the following quantities: inverter dc link voltage, sampling interval and control delay. Throughout the paper simulation and experimental results are given to confirm the theoretical findings

Nonlinear H-infinity control for hybrid excited synchronous generators

The model of a hybrid excited synchronous generator is analyzed and a nonlinear optimal (H-infinity) control method is proposed for it. This type of generator receives primary excitation at its stator's winding through an AC/DC and DC/AC converter, and auxiliary excitation at a secondary winding that is fed by an AC to DC converter. Through the hybrid excitation scheme more control inputs are applied to the generator, thus achieving better performance for the system's control loop. To implement the proposed control method the dynamic model of the generator undergoes approximate linearization around a temporary operating point which is recomputed at each time-step of the control algorithm. The linearization procedure relies on Taylor series expansion and on the computation of the associated Jacobian matrices. For the approximately linearized model of the hybrid excited synchronous generator a stabilizing H-infinity feedback controller is designed. To compute the controller's feedback gains an algebraic Riccati equation is repetitively solved at each iteration of the control method. The global stability properties of the control scheme are proven through Lyapunov stability analysis

Kinetics of Holographic Recording and Spontaneous Erasure Processes in Light-Sensitive Liquid Crystal Elastomers

The optical mechanism for imprinting one-dimensional grating structures into thin films of a light-sensitive monodomain liquid crystal elastomer is investigated by analyzing the time dependence of optical diffraction properties. The recording kinetics shows an irregular oscillatory behavior, which is most expressed at small grating spacings and at temperatures close to the nematic-isotropic phase transition. The oscillations are attributed to the opto-mechanical response of the film, i.e., to contraction of the film during the recording process. At temperatures far below the nematic-isotropic phase transition, the spontaneous erasure kinetics exhibits exponential relaxation with relaxation time following the Arrhenius activation law. However, at temperatures close to the nematic-isotropic phase transition, the erasure process shows an interesting nonmonotonic behavior that we attribute to the non-linear relation between the concentration of the photo-transformed chemical groups and the nematic order parameter