Analysis and feedback control of magnetic bearings with reference to flywheel energy storage.

Abstract

In high speed applications magnetic bearings offer many potential advantages over mechanical bearings. The type of magnetic bearing most suitable for energy storage flywheels is selected and analysed for the purpose of designing feedback control loops. A nonlinear as well as a small signal linear model of the "current driven" magnetic bearing with unlaminated magnetic components is derived. Subsequently describing functions characterising the small- as well as large signal behaviour of the same bearing in the "voltage driven" mode, are obtained. It is shown that workable results are obtained for most practical situations by using linear systems theory, although the magnetic bearing is a nonlinear device. The describing function model enables the designer to identify the mechanisms leading to limit cycles under adverse operating conditions. Feedback control loops designed around the small signal characteristics produce practical results in the case of the "voltage driven" mode which are superior to that of the "current driven" case. An essential refinement, where energy losses and vibrations arising from rotor imbalance are eliminated, is described. A discrete time filtering technique is used. Two experimental models were built and fully tested in order to verify the above theoretical approaches.