Performance Evaluation of Radial Electromagnetic Bearings using Measured Magnetic Characteristics of Core Materials over Wide Frequency Range

Radial electromagnetic bearings (EMBs) provide friction-free and lubrication-free support to high-speed rotors against various radial disturbance forces including their self-weights. The EMB performance critically depends on the high-frequency behaviour of the core material used. This paper investigates the high-frequency magnetic characteristics of an Si-Fe steel, namely 50C470, for an 8-pole radial EMB, used to support a 30,000-rpm rotor. The B-H loops of 50C470 are obtained experimentally at eleven different frequencies between 50 Hz and 1000 Hz, and for peak flux densities up to 1.5 T at each frequency. The experimental data are used as material inputs to perform Finite Element Analysis (FEA) for performance evaluation of certain radial EMB design. The influence of the magnetic core material on the air-gap flux-density distribution, radial-force-density distribution, and the radial force versus control-current characteristics along x- and y-directions are demonstrated. Significant reduction is observed in the peak sinusoidal force along any given direction due to finite core permeability. High-magnitude control currents are shown to produce significant third harmonic in the force along a given direction, and prominent fourth harmonic in the total radial force on the rotor. The findings are useful to improve the design, modelling, operation and control of radial EMB.