Rotor power losses in planar radial magnetic bearings - Effects of number of stator poles, air gap thickness, and magnetic flux density

Rotor power losses in magnetic bearings cannot be accurately calculated at this time because of the complexity of the magnetic field distribution and several other effects. The losses are due to eddy currents, hysteresis, and windage. This paper presents measured results in radial magnetic bearing c onfigurations with eight pole and 16 pole stators and two laminated rotors. Two different air gaps were tested. The rotor power losses were determined by measuring the rundown speed of the rotor after the rotor was spun up to speeds of approximately 30,000 rpm, DN = 2,670,000 mm-rpm, in atmospheric air. The kinetic energy of the rotor is converted to heat by magnetic and a ir drag power loss mechanisms during the run down. Given past publications and the opinions of researchers in the field, the results were quite unexpe cted. The measured power losses were found to be nearly independent of the number of poles in the bearing. Also, the over-all measured rotor power los s increased significantly as the magnetic flux density increased and also i ncreased significantly as the air gap thickness decreased. A method of sepa rating the hysteresis, eddy current and windage losses is presented. Eddy c urrent effects were found to be the most important loss mechanism in the da ta analysis, for large clearance bearings. Hysteresis and windage effects d id not change much from one configuration to the other.