Salient Pole Permanent Magnet Axial-Gap Self-Bearing Motor

Abstract

Recently, active magnetic-bearing motors have been designed to overcome the limitations of the conventional mechanical-bearing motors. Magnetic-bearing motors can work in all environments without lubrication and do not cause contamination; further, they can run at very high speeds. Therefore, they are very valuable machines with a number of novel features, and with a vast range of diverse applications (Dussaux, 1990). The conventional magnetic-bearing motor usually has a rotary motor installed between two radial magnetic bearings, or a mechanical combination of a rotary motor and a radial magnetic bearing (The mechanically combined magnetic bearing motor usually has n-pole motor windings and n±2-pole suspension windings), as shown in Figs. 1 and 2 (Okada et al., 1996), (Oshima et al., 1996 a,b), (Zhaohui & Stephens, 2005), (Chiba et al., 2005). The radial magnetic bearings create radial levitation forces for rotor, while an axial magnetic bearing produces a thrust force to keep the rotor in the correct axial position relative to the stator. However, these magnetic-bearing motors are large, heavy, and complex in control and structure, which cause problems in applications that have limit space. Thus, a simpler and smaller construction and a less complex control system are desirable. An axial magnetic bearing is composed of a rotary disc fixed on a rotary shaft and electromagnets arranged on both sides of the disc at a proper minute distance. This structure is similar to that of an axial-flux AC motor (Aydin et al., 2006), (Marignetti et al., 2008). Based on this, Satoshi Ueno has introduced an electrically combined motor-bearing which is shown in Fig. 3, in which the stator has only three-phase windings; however it can simultaneously provide non-contact levitation and rotation (Ueno & Okada, 1999), (Ueno & Okada, 2000). This motor is then called an axial-gap self-bearing motor (AGBM) to imply that the motor has self levitation function. Obviously, it is simpler in structure and control since hardware components can be reduced. The AGBM can be realized as an induction motor (IM) (Ueno & Okada, 1999), or a permanent magnet (PM) motor (Ueno & Okada, 2000), (Okada et al., 2005), (Horz et al., 2006), (Nguyen & Ueno, 2009 a,b). The PM motor is given special attention, because of its high power factor, high efficiency, and simplicity in production. In this chapter, the mathematical model of the salient 2-pole AGBM with double stators is introduced and analyzed (sandwich type). A closed loop vector control method for the axial position and the speed is developed in the way of eliminating the influence of the reluctance 4