Date of final exam: 29/02/2000

E-mail: atilli@deis.unibo.it

Tutor: Prof.  A. Tonielli, Università di Bologna

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Non linear control of induction machines   ___________________________________________________________________________________________________________

Advisor:

Prof.  A. Tonielli, Università di Bologna

Summary of the thesis

The relevance of electric machines is constantly growing in the field of industrial automation in order to realize a controlled conversion of electric energy in mechanical energy and vice-versa.

In this Ph.D. dissertation different control problems related to induction electric machines are treated.

In the first part of the work, Doubly-Fed Induction Machines (DFIM) are considered. These machines are characterized by open and accessible rotor windings. They are very attractive for high power applications (more than 20kW), for energy generation and motion control. In fact, these electric machines can be controlled using the voltages applied to the rotor windings as inputs. In this way the "control power" is just a small fraction of the power flowing in the "stator port" which is directly connected to the line-grid. Owing to these peculiarities, the dynamic behavior of the DFIM is quite different with respect to the classical induction machines with short-circuited rotor windings, even if the physical principles involved are the same. Also the control objectives become very different. In particular, instantaneous active and reactive powers at stator side are very important. Two different solutions are shown, based on an original choice of the reference frame in the state space for the machine model representation. The first solution is based on the typical "cascade architecture" used in the control of electric drives. The second solution, instead, is based on a different approach, two versions are presented: the first one is a full-state feedback controller, while the second one is an output feedback controller. In both versions the relative degree between inputs and outputs is equal to one. In addition a linear, time-invariant internal dynamics is present and it is stable, but not asymptotically. Hence, if a classical control based on dynamic cancellation via state feedback (feedback-linearization) was applied, the machine would have a "pathologic" behavior. In order to avoid this phenomena, a particular reference for the internal dynamics has been designed.

All of the proposed solutions has been tested by simulations both in ideal and non-ideal conditions. Also many experimental results are shown in order to verify the practical validity of the proposed solutions. Some aspects related to the real implementation of the control algorithms are discussed.

In the second part of this work a particular control algorithm is considered for induction machines with short-circuited rotor windings (usually realized with the so called "squirrel cage"). This solution is based on the indirect field oriented (IFO) approach, usually adopted in commercial drives. The main characteristics of the proposed controller are: theoretical "solidity" (not always present in the control of the electric drives); tuning simplicity of the controller parameters; good robustness of the rotor magnetic flux control. Simulation and experimental results are presented in order to compare the proposed solution with a standard IFO controller. It is particularly underlined the high energetic efficiency guaranteed with the presented controller even with large errors on the rotor parameters. The proposed algorithm is very interesting for industrial application since it has an implementation complexity similar to the case of a standard IFO controller, but it guarantees better performances in terms of dynamics and energy optimization.
 

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