Roberto Menis

Variable structure control of an SRM drive

The applications of a variable-structure system (VSS) to the control of a switched reluctance motor (SRM) drive is presented. After reviewing the operation of an SRM drive, a VSS-based scheme is formulated to control the drive speed. The scheme is then designed and tested by simulation. The results show that the VSS control is effective in reducing the torque ripple of the motor, compensating for the nonlinear torque characteristics, and making the drive insensitive to parameter variations and disturbances. >

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 technique.

Direct torque control of an induction motor using a single current sensor

A novel scheme for the direct torque control (DTC) of an induction motor (IM) is proposed, which uses a single sensor of current inserted in the inverter dc link. The rationale behind the proposal is to develop a low-cost but high performance IM drive. The scheme exploits a simple and robust algorithm to reconstruct the stator currents needed to estimate the motor flux and torque. The algorithm operates in two stages: first, it predicts the stator currents from a model of the motor and then adjusts the prediction on the basis of the sensed dc-link current. Experimental results are given to demonstrate the ability of the scheme in reproducing the performance of a traditional DTC IM drive.

Control schemes for steer-by-wire systems

Steer-by-wire (SbW) systems are candidate to replace the conventional steering equipment in the new generation of vehicles. The task of a SbW system is twofold: turning the steered wheels by tracking the handwheel rotation and providing the driver with a feeling of the steering effort. In this paper, the design of the control scheme for SbW systems is faced. Two schemes are considered: one is derived from the model of conventional steering equipment whilst the other one exploits the features of a SbW system to cope with the interaction of the steered wheels to the road surface. Implementation of the control schemes on a test vehicle is described and experimental results are given.

Disturbance torque estimation in a sensorless DC drive

The estimation of the disturbance torque in a sensorless DC drive is carried out by extending the classical observer theory. Three estimation schemes are formulated according to the representation of the disturbance torque and processing of the observer states. In addition to the disturbance torque, all the schemes deliver an estimation of the motor speed. Steady-state accuracy and dynamics of the schemes are analyzed in nominal conditions, identifying the scheme with the best performance. The effects of variations in the motor parameters are also analyzed, finding that a proper modeling of the motor makes the steady-state estimation of the disturbance torque insensitive to any variation. As a test, the schemes are applied to a sensorless DC drive for both compensating for the disturbance torque and closing the speed loop. The responses obtained with the best-performance scheme are reported.<<ETX>>

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 findings.

MRAS identification of the induction motor parameters in PWM inverter drives at standstill

A model-reference adaptive control system (MRACS) approach is developed for identifying the parameters of an induction motor (IM) in PWM inverter-fed drives at standstill. The theoretical background of the approach is first presented. The case of known IM torque constant is then considered and an algorithm for identifying the IM parameters is derived. A sensitivity analysis to uncertainties in the torque constant is accomplished. Finally, the MRACS approach is applied to a commercial field-oriented IM drive and the motor parameters are identified. A test which validates the experimental results is also carried out.

Voltage stability analysis of all-electric cruise liners

The paper presents a small-signal stability analysis of voltage control systems employed onboard all-electric cruise liners. To realize the stability analysis, a model of shipboard power station voltage control system has been determined, moving from dynamic models of alternators, voltage regulators and loads. Reduced order models of alternators have been utilized, to simplify at best the analysis. Analysis results have been validated through numerical simulations, using a simulator of power station electromechanic quantities developed at the University of Trieste. Such simulator includes alternators, voltage and frequency regulators, breakers, protections and R-L loads. Numerical results obtained from simulations of both complete and reduced order models fully validate the analytical findings, while computation times are in the order of real-time simulations.

Inverter voltage drop-free recursive least-squares parameter identification of a PWM inverter-fed induction motor at standstill

The paper presents a procedure for identifying the parameters of a PWM voltage inverter-fed induction motor (M) at standstill, based on the recursive least-squares (RLS) technique. The procedure does not require the knowledge of the voltage drop on the inverter, thus simplifying the identification process and increasing the accuracy of the results. The paper starts by addressing the issue of the parameter identification in a PWM inverter-fed IM at standstill. Then the solution which makes the identification free from the inverter voltage illustrated and the procedure built upon it is formulated. Finally, the equipment arranged for executing the identification tests is described and the results obtained from the experiments are shown.

Analytical study of Torque vs. speed characteristics of PM brushless DC drives

PM brushless DC (BLDC) motors should be fed by square-wave currents synchronized with the back emfs to develop a constant torque with the required value. In practice, phase inductances and limited source voltage produce non ideal square-wave currents and unbalanced phase currents during commutations. Such phenomena affect the torque developed by the PM BLDC drives by deviating its value from the requested one and by giving rise to torque ripple. In this paper, an analytical study of the torque characteristics, namely motor torque and torque ripple, of PM BLDC drives is carried out as a function of the motor speed and is supported by experimental data. Torque characteristics are expressed in terms of a quantity distinctive of the motor, thus giving a generalized formulation to the results.