Research field: Electrical Drives for Induction Machines

DIRECT TORQUE CONTROL

STATOR FLUX VECTOR CONTROL

BIBLIOGRAPHY

In recent years, control techniques for high-performance induction motor drives have been largely developed as alternative solutions to high-performance four-quadrant DC servo drives. Many schemes have been proposed, all based on the decoupling of flux and torque control variables. The flux and torque are estimated using various combinations of stator voltage, stator current, and shaft position or speed. The control variables are usually the stator current components and then current regulators are used to generate the appropriate gate signals to the inverter switches.

 Direct Torque Control (DTC)

MAIN CONTRIBUTIONS

Ø     Improvement of the dynamic performance all over the speed range by means of a new voltage vector selection criterion, which is based on the value of the mechanical speed or stator flux angular frequency.

Ø     Development of a new DTC scheme that utilizes as reference the rotor flux instead of the stator flux. In this way it is possible to combine the advantages of rotor flux orientation and stator flux control without the need of determining the instantaneous position of the rotor flux.

Ø     Presentation of a detailed analysis on the effects of both flux and torque hysteresis band amplitudes on drive performance. The attention has been mainly focused on harmonic current distortion, average inverter switching frequency, torque pulsation and drive losses.

Ø     Development of a DTC scheme for electric vehicle induction motor drives. The proposed control scheme allows the maximum torque capability to be achieved in the entire field weakening region for any load condition compatible with the inverter current rating.

Ø   Improvements of the low speed performance using a new sensorless closed loop flux estimator. The results are the magnetization of the machine at very low speed, even under no load conditions and the elimination of the system instability introduced by an overestimation of the stator resistance. 

Ø     A theoretical investigation on the effects produced by the inverter voltage vector on the stator flux and torque variations. The analysis, based on a small variation model, allows the flux and the torque ripple to be easily determined, and can be useful in defining new voltage vector selection criteria.

Ø     A detailed comparison between the Field-oriented control and the Direct Torque Control. The performance of the two control schemes have been evaluated in terms of torque and current ripple, switching frequency of the inverter and transient response to step variations of the torque command.

Ø     Improvement of the drive performance using an Artificial Neural Network to select the optimal voltage vector that has to be applied to the machine in each cycle period. A sensible reduction of current, flux and torque ripple can be obtained without increasing the mean switching frequency, and without the need of a PWM pulse generator block.

Ø    Development of a new voltage vector selection algorithm, based on a simplified discrete mathematical model of the induction machine, which allows a sensible reduction of the RMS value of the stator current ripple without increasing the average value of the inverter switching frequency and without the need of a PWM pulse generator block.

Ø    Development of a new induction motor drive scheme in which a matrix converter is employed in driving an induction motor using the DTC technique. The result is a compact high-performance induction motor drive system with intrinsic regenerative breaking and unity input power factor operation capability.

Ø    Development of a new SVM technique that uses prefixed time intervals within a cycle period. This new technique, named Discrete Space Vector Modulation (DSVM), allows to synthesise a higher number of voltage vectors with respect to basic DTC scheme, without the need of timers or PWM signal generator. It is then possible to define more accurate switching tables that allow a sensible reduction of torque and current ripple in the whole speed range.

 Ø    Employment of a predictive algorithm for selecting the most opportune voltage vector sequence in each cycle period in DTC scheme based on DSVM.

Stator Flux Vector Control (SFVC)

In principle the SFCV scheme is based on driving the stator flux vector towards the reference vector defined by the input commands that are the torque and the rotor flux. This action is carried out applying by the SVM technique a suitable voltage vector to the machine in order to compensate for the stator flux vector error. Using this scheme it is possible to achieve a constant switching frequency and a reduction of torque and current ripple with respect to basic DTC.

Block diagram of the scheme based on Stator Flux Vector Control.

Ø      Development of the SFVC scheme.

Ø      A comparison between the Stator Flux Vector Control and the Direct Torque Control. The performance of the two control schemes have been evaluated in terms of torque and current ripple, switching frequency of the inverter and transient response to step variations of the torque command.

Ø      Development of a SFVC scheme for electric vehicle induction motor drives.

Ø      Analysis of the sensitivity with respect to stator parameter deviations and current sensor offsets of the SFVC scheme.

Ø      Improvements of the low speed performance using a new sensorless closed loop flux estimator. The stability of the control system and the sensitivity to stator resistance deviations are examined.

Ø      Theoretical and experimental analysis of an improved SFVC scheme with extended speed range, with inverter dead-time compensation method based on a feedforward action.

DIRECT TORQUE CONTROL

STATOR FLUX VECTOR CONTROL

BIBLIOGRAPHY