Effects of magnetic flux leakage of axial electromagnetic bearing on bearing system

Abstract

Magnetic flux leakage inevitably occurs between an axial electromagnetic bearing and a rotor because of the struc- tural arrangement of the former. The effects of magnetic flux leakage of the axial electromagnetic bearing on an active electro- magnetic bearing system are studied in this research. Despite the occurrence of a magnetic flux leakage, the axial electromag- netic force produced by the axial electromagnetic bearing is greater than that without magnetic flux leakage at a smaller air gap ratio K (K = 1, 2, 3; where K is the ratio of the air gap between the stator and the rotor to the air gap between the stator and the thrust plate), and the axial electromagnetic force decreases continuously with the increase in air gap ratio K. When air gap ratio K increases to 4, the axial electromagnetic force with magnetic flux leakage is less than that without magnetic flux leakage. In addition, a deviation is noted between the centers of the rotor and the axial bearing because of machining precision, installation error, or change of running state, and the axial electromagnetic bearing produces additional radial forces on the rotor. The resultant radial force produced by axial electromagnetic bearing increases from 2.1 N to 9.18 N as the radial deviation of the rotor increases from 0.1 mm to 0.4 mm in our simulation conditions. The direction of radial force is the same as that of the radial deviation, which increases the control current of the radial electromagnetic bearing control system and the joule heat of the radial electromagnetic bearing coil. Hence, the additional radial forces produced by axial electromagnetic bearing must be considered in the optimal design of a radial electromagnetic bearing control system. The joule heat of the radial electromagnetic bearing increases from 0.38% to 17.1% because of the additional radial force produced by the axial electromagnetic bearing, which increases the control current of the radial electromagnetic bearing control system. Such increase has an adverse effect on the refrigeration expander, particularly on the cryogenic expander systems in which the bearing is used.