Electromechanical properties of radial active magnetic bearings

Nonideal properties of the electromagnetic actuators in radial active magnetic bearings are studied. The two dimensional nonlinear stationary finite element method is used to determine the linearised parameters of a radial active magnetic bearing. The method is verified on two test machines. The accuracy is 10-15 % in the magnetic saturation region. The effect of magnetic saturation on the bearing dynamics is studied based on the root locus diagrams of the closed loop system. These diagrams show the possibility of extending the operation range into the magnetic saturation region. The magnetic cross coupling between the x- and y-coordinates is studied in detail. The formulation in which the cross coupling can be regarded as phase errors is presented. The magnetic cross coupling can produce phase errors up to ten degrees. The effect of the power amplifier saturation is studied based on the nonlinear simulations and describing function approach. A high frequency large amplitude disturbance can be an origin for a limit cycle oscillation in the neighborhood of the crossover frequency. The eddy currents in the laminations, magnetic hysteresis and unmodelled eddy current paths produce phase errors to the linearised parameters. These errors are studied by measurements and models based on the reluctance network. The dynamic force measurement is done by measuring the accelerations at the both ends of the rotors and solving the equation of the motion of the rotor. The eddy current model is derived from the one-dimensional magnetic field solution inside an electric steel sheet. The model is a linear model, which is linearised at one operation point of the radial bearing. This model neglects the excess losses and the unmodelled eddy current paths. The linearised parameters from the hysteresis model are calculated by the describing function. These phenomena can produce phase errors up to ten degrees.reviewe