Solving the problems of electric motors run from power converters

In this PhD thesis, Eugenio Gubía proposed a solution for the problems of electric motors run from power converters. Controlling motors and electric generators by means of power converters has negative effects with the presence of overtvoltages and high-frequency currents throughout the installation. These effects accelerate the ageing process of the motors and, moreover, can provoke faults in the correct operating of the control circuits.

Thus, in his PhD, Mr Gubia has developed a generic method that can be adapted to each one of the possible installations with the view to identifying the origin of the high-frequency problems and, moreover, to analyse possible solutions. Likewise, the proposed method can be adapted to the design stage of a new installation.
In order to design this method, it was necessary to set up simulation models to reproduce the behaviour of motors, transformers, cables and the power converter, using frequencies in the order of 10 megaHertz. These frequencies are much higher then the tens of kiloHertz used in classical models.

Concretely, in this work it has been possible to reproduce the overvoltages produced as a result of using long connection cables between the inverter and the motor given that, the longer the cable, the greater the probability of overvoltage. This type of cable can be found, for example, in wind-powered generators, which incorporate the generator (the motor) in the upper part of the structure and the power converter at the base.

Also in this thesis, the problem of earthing currents in the installation and the supply grid was investigated, a problem known as electromagnetic interferences or conducted EMI. These have become highly important of late as they can provoke a fault in other units connected to the same earthing system. In fact, the regulations which limit the amount of such currents are increasingly stricter.

Eugenio Gubía has put forward that solutions to both problems, overvoltages and interferences, require the use of filters, the study of which, in this work, has been approached from a perspective of wave theory in transmission lines.
If the electromagnetic interferences do not have a technique or device for their quantification at the heart of the operation, neither can the efficacy of the filters applied to eliminate said effects be measured.

To this end, the work developed in this PhD offers a precise methodology for the analysis of both overvoltages and electromagnetic interferences and that, moreover, enables an efficient filter design.

The author of the thesis is currently researching the possibility of applying this methodology to other fields of power electronics.

Contact :
Iñaki Casado Redin
Nafarroako Unibertsitate Publikoa
inaki.casado@unavarra.es
(+34) 948 16 97 82