Active magnetic bearing design and characterization for high temperature applications

This thesis is motivated by one of the largest markets: aviation. This market, in which competition and investments are extraordinary, requires constant technical improvements in order to increase its competitiveness. Jet engines have an important influence on fuel consumption and servicing of airplanes, thus on the transportation cost. The present work was supported by the European Community [9]. This thesis investigates the use of active magnetic bearings for jet engines. It is expected that magnetic bearings could considerably reduce losses and service intervals in jet engines. The present work concentrates on the design and characterization an active magnetic bearing for applications at high temperature. The report begins with an introduction locating the accomplished work into the current economic and technical context. The advantages of such system are given, as well as the scientific contribution of the research which has been undertaken. An introduction to magnetic bearings is given, so that the key elements of the dimensioning of such a system are comprehensible for readers of all horizons. The maximum force produced by magnetic bearings is mainly limited by the heating related to the losses in the coils. A thermal model for high temperature magnetic bearings has been implemented. An experimental part allowed the validation of this model for a wide temperature range. The construction of a magnetic bearing requires various types of materials; soft magnetic materials, electrical conductors, electrical insulators as well as several fixation materials. Tests have been carried out and a catalogue lists the materials available for high temperature magnetic bearings. Position sensors are usually used in magnetic bearings. Eddy current position sensors have been developed. They were realized with coils printed on a ceramics substrate by using thick-film technology. Measurements done at high temperature show the great characteristics of these sensors. Problems of silver migration between the wires welding have been encountered during tests carried out at high temperature. Various solutions have been tested with the aim of avoiding the silver migration. The materials exposed at high temperature have unfortunately sometimes a limited lifespan. Studies of failures related to an exposure at high temperature have been done. Some failures are detectable by the magnetic bearing. An example of detection of possible short-circuits in the actuator coils is presented. Finally a prototype of an active magnetic bearing system with five degrees of freedom has been built. A furnace especially developed, makes it possible to create environments at high temperature. The characteristics of the active magnetic bearing have been measured during levitation at ambient temperatures from 25°C up to 550°C.