Innovative Predictive Current Control for Synchronous Reluctance Machines

In recent decades, the use of power converters has become very popular in the field of electric drives. Several control techniques have been proposed for power converters and every year, the ongoing research and the always more powerful microprocessors, lead to new high performance solutions. Despite this, since the output of the worldwide research often results in complex and hardly applicable solutions, other well-established techniques, such as linear and hysteresis control with pulsewidth modulation, are still the main choice in a great number of industrial applications. The reasons of their leadership can be found considering the characteristics of these methods: on one side simplicity of comprehension and implementation and, on the other, sufficiently good performance and robustness. Due to these relevant features, despite there is still extensive room for improvements, it is not painless to propose solutions that can be attractive for people working in industry to compete with, and possibly to replace, traditional methods. Desirably, a control algorithm for electric drives has to be simple and easily understandable. Besides, it has to be suitable for real-time applications. Robustness and reliability, beyond that performance, have to be guaranteed since the nature of the different applications, e.g. home appliances and automotive. In this perspective, Predictive Control could represent a candidate to introduce improvements and gains in the aforementioned industrial applications. Predictive control is a wide class of controllers that uses the model of the system for the prediction of the future behavior of the controlled variables. This information is used by the regulator in order to obtain the optimal actuation, according to a predefined optimization criterion represented by a cost function. This control techique is based on concepts that are extremely simple and intuitive and besides, depending on the type of predictive control, the implementation can also be simple. In parcticular, Finite Control Set allows considering the discrete nature of the power converter and results in an extremely simple implementation. Beyond simplicity, other advantages can be recognized. First, with predictive control it is possible to avoid the cascaded structure obtaining a very fast transient response. Besides, nonlinearities can be included in the model avoiding the need of linearizing the model for a given operating point and improving the operation of the system for all conditions. Finally, it is possible to include limitations of the variables when designing the regulator. The aim of this thesis is to study Predictive Control applied to the current control of synchronous reluctance machines, analysing and addressing some open research topics regarding this kind of control. In particular, two main aspects are studied, namely the need of a precise knowledge of the machine model and the possibility to drive a synchronous reluctance machine along the Maximum Torque per Ampere, the Flux Weakening and the Maximum Torque per Voltage operations. The performance are strictly related to the accuracy of the model used for the prediction. In case of parameters mismatch or variation, rather than other model inadequacies, the prediction could be affected causing a worsening of the overall behaviour of the drive. The first part of this work is commited to study this aspect, analysing the effects of mismatches and variations focusing in particular on the detrimental effects of iron saturation. A novel model-free solution is presented to overcome the limitations given by an inadequate model. This method allows achieving good reference tracking and limited current ripple in every working condition. Besides, it presents great advantages in terms of simplicity: no additional hardware and no complicated calculations are required. The design is effortless since there are no gains, thresholds and so on, that have to be tuned. This technique could be used to develop an universal drive, meaning that completely different machines could be controlled with exactely the same algorithm, without self commissioning or identification procedures. Thanks to the aforementioned features, this techique could allow the spread of predictive control in industrial applications. In order to fully exploit the characteristics of the drive while assuring the lowest power losses in every working condition, a proper control algorithm has to be used. In the second part of this work, a predictive regulator able to track the most suitable trajectory depending on the machine operation is presented. In particular, the Maximum Torque per Ampere, the Flux Weakening and the Maximum Torque per Voltage trajectories are considered. The proposal is a combination of predictive control and hysteresis control, since its aim is to keep the current error within a certain hysteresis band, and it allows combining the benefits of the two control techniques. This study is carried out considering Predictive Current Control for Synchronous Reluctance machines. This kind of machine has been considered since it is of great interest due to the fact that it features high power density, superior reliability, high efficiency and it is cost effective due to the absence of permanent magnets and circuits in the rotor. Besides, since its significant iron saturation, its control represents a challenge (in particular) for predictive control schemes and for this reason it is a perfect case study.

Negli ultimi anni, l'utilizzo di convertitori di potenza in applicazioni di azionamenti elettrici è diventato molto diffuso. Diverse tecniche di controllo per convertitori di potenza sono state proposte e ogni anno, i risultati della ricerca e gli sviluppi di microprocessori consentono di raggiungere performance sempre maggiori. Nonostante ciò, poichè gli output della ricerca sono spesso soluzioni complesse e di difficile implementazione, le soluzioni più usate in ambito industriale rimangono quelle ormai consolidate, come il controllo lineare ed il controllo ad isteresi. Un algoritmo di controllo per un azionamento elettrico dovrebbe essere semplice e di facile compresione. Inoltre dev'essere adatto ad applicazioni real-time. Robustezza ed affidabilità, oltre che alle performance, devono essere garantite, in particolare in applicazioni come gli elettrodomestici e l'automotive. Alla luce di ciò, il Controllo Predittivo rappresenta un valido candidato per introdurre vantaggi e miglioramenti in ambito industriale. Questa tecnica di controllo sfrutta un modello del sistema per predire il comportamento futuro delle variabili controllate. Questa informazione è utilizzata per scegliere l'azione di controllo migliore in base ad un criterio di ottimalità predefinito. Questo tipo di controllo è basato su idee che sono concettualmente semplici e intuitivi. Inoltre, l'implementazione della versione Finite Set risulta particolarmente facile. Oltre alla semplicitò gli altri vantaggi sono la possibilità di evitare la struttura in cascata (tipica del controllo lineare), le nonlinearità e le limitazioni possono essere direttamente incluse nel modello. Lo scopo di questa tesi è di studiare il controllo predittivo applicato al controllo di corrente di una macchina Sincrona a Riluttanza, analizzando ed affrontando alcune tematiche ancora aperte. In particolare, due aspetti sono considerati: la necessità di conoscere in modo preciso il modello della macchina e la possibilità di controllare la macchina lungo le traiettorie di MTPA, Flux-Weakening e MTPV.