On Sandwiched Magnetic Bearing Design

This paper presents an innovative design for magnetic poles used in active magnetic bearings (AMBs). The proposed hybrid sandwiched magnetic pole design not only practically reduces the bearing size, but also dramatically diminishes its cost. The magnetic force to counterbalance the gravity of the rotor is provided by the embedded permanent magnets (PMs), while the electromagnetic bearing is merely used to regulate the position deviation of the rotor. Therefore, the required electricity consumption to activate the coil wound on the poles is fairly limited. This implies that the required ampere-turns and size of the AMB can both be reduced efficiently. The numbers of associated amplifiers, sensing circuits, and magnetic poles are thus reduced as well. The coupling effect of magnetic forces is taken into account such that either poles for electromagnets (EMs) or the embedded PMs are located 90° apart. The merits of this type of design by orthogonal pole allocation include: simpler control synthesis and decoupled sensing signals for rotor position deviation. Since the variation of coil current is a function of flux density change between PMs and the silicon steel strips attached on the rotor, the PMs can also be used to provide necessary magnetic flux source for sensing rotor position deviation. More importantly, two perpendicular PMs can prevent the rotor from getting extremely close to either PM, such that the magnetic attraction force becomes too strong to be counterbalanced. On the other hand, the concave-like pole profile design enhances the average flux density by 11% and the required ampere-turns are further reduced by 18%, compared with the traditional rectangular pole design. One of the induced benefits of reduced size for AMB is to diversify applications where the operating space is limited. The proposed design has been assessed using intensive computer simulations and experiments.