Magnetic Bearings and Bearingless Drives
By Akira Chiba, Tadashi Fukao, Osamu Ichikawa and
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About this ebook
Aimed at practising electrical and mechanical engineers and advanced students, Magnetic Bearings and Bearingless Drives provides an essential guide to an area of engineering previously only fully covered by large numbers of academic papers.
· Unique and comprehensive coverage of a cutting-edge subject for electrical and mechanical engineers
· A reference text and survey for designers, manufacturers and users of high-speed motors, generators and electrical drive systems
· Examines the basic principles behind magnetic bearings, with key technologies and applications illustrated through examples and case studies
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Book preview
Magnetic Bearings and Bearingless Drives - Akira Chiba
2004
1
Introduction
Tadashi Fukao and Akira Chiba
In this chapter, an overview of bearingless drives and magnetic bearings is presented; the principle of radial force generation is discussed and a typical motor drive with magnetic bearings is introduced and compared with the bearingless drive. In addition, a definition of the bearingless motor is given and the related technologies and early developments are reviewed. Typical application structures, winding configurations, radial force and torque comparisons and applications are also included.
1.1 Magnetic bearing and motor drive
A new bearingless concept was introduced into ac drive technology in the late 1980s. Since then, the theory and background of the concept has been studied, with many test drives developed to gain experience of the operation and behaviour of a variety of bearingless ac drives.
Ac motor drive technology has been developed and applied in a wide range of applications since the 1970s because of their advantages over dc motor drives, such as high performance, compactness, lighter weight, use of low maintenance motors and lower motor cost. The increase in power and rotational speed of ac drives has widened the application area. One maintenance task that still remains with an ac drive is bearing lubrication and renewal. In some applications, bearing maintenance is still a significant problem. For example, the bearings can present a major problem in motor drive applications in outer space, and also in harsh environments with radiation and poisonous substances. In addition, lubrication oil cannot be used in high vacuum, ultra high and low temperature atmospheres and food and pharmacy processes. Hence motor drives with magnetic suspension can enlarge the possible application areas of motor drives.
Figure 1.1 shows the principles of rotor radial force generation in both the magnetic bearing and bearingless motors. A rotating shaft is surrounded by the stator core. The rotor and stator are magnetized with four poles in a north, south, north, south sequence. There are strong magnetic attractive forces under these magnetic poles between the rotor and the stator cores. For example, a magnetic force of 40 N is generated in 1 cm² with an airgap flux density of 1 T. In Figure 1.1(a), these four magnetic poles have equal flux density and hence equal attractive force magnitudes. Thus, a vector sum of the four radial forces is zero. However, in Figure 1.1(b), one north pole is stronger than the other three poles so that the net attractive force is strong. Hence the unbalanced airgap flux density distribution results in radial magnetic force acting on the rotor. In this case, rotor radial force on the rotor is generated in the right-hand direction. In both radial magnetic bearing and bearingless motors, rotor radial force is generated by an unbalanced magnetic field; i.e., the rotor radial force is generated by the difference of radial forces between the magnetic poles. The attractive force is an inherently unstable force as it is stronger if the rotor moves in the force direction. The zero-radial-force point at the centre of the stator bore is an unstable point so that negative feedback is