Preface
As we progress into the twenty-first century global warming has become an important issue. CO2 gas emissions must be reduced to preserve the correct air content. Modern man, who accepts the fruits of recent energy-based technologies (such as mass transport using cars, trains and aeroplanes, and climate-controlled work and home environments via air-conditioners) as necessities, cannot put up with the inconveniences of even the not-too-distant past. To maintain and develop these energy-consuming technologies, alternative energy resources and efficiency improvements are necessary.
Effective efficiency improvements have been, and are being, actively investigated in Japan and Europe. For example, the integration of an electric motor and generator system into a gasoline-powered vehicle (hybrid vehicle) provides excellent fuel economy and harmful gas reduction. Air-conditioners are another example of an application that has seen substantial efficiency improvement. These are now driven by variable-speed motor drives with rare-earth permanentmagnet motors and power electronics inverters. The cost of these advanced motor drives is decreasing thanks to mass production. As can be seen in these examples, the integration of high-efficiency drives into power mechatronics systems is one of the keys for improving system efficiency and hence reducing CO2 gas emission.
In recent years, magnetic levitation and suspension systems have become a realistic proposition. For example, the Transrapid Train system developed by German engineers has been installed in Shanghai, China. In this train system, the bogies are levitated by electro-magnets. Therefore the operating speed is considerably higher than that for conventional high-speed trains such as the Shinkansen, TGV and ICE. The fastest passenger train in the world runs on the Yamanashi Maglev test line which has been developed in Japan. The bogies are again magnetically levitated by the interaction between superconducting coils and ambient-temperature coils installed along the track side walls. At a speed of more than 200 km/h, the wheels are lifted in a similar manner to an aeroplane so that the train is magnetically suspended and near-silence is experienced in the cabin. A top speed of more than 580 km/h has been recorded thanks to the magnetic levitation.
In addition to train systems, magnetic levitation and suspension has also been applied in other industries. Some applications, such as compressors, refrigerators, spindles and generators, require high rotational speed to minimize the weight, dimensions and cost, and to maximize the efficiency of the whole system. Low loss and maintenance-free operation are also required by the shaft-support bearings. However, chemical pumps, turbo-molecular pumps, blood pumps, bio-reactors, semiconductor processes, blowers and refrigerators operate under harsh environmental conditions, such as vacuum and extremely low and high temperatures, and in the presence of explosive, poisonous and bio-chemical fluids. Conventional mechanical bearings cannot be installed so that one solution is to suspend the machine shaft by magnetic levitation.
Magnetic suspension was not common even at the end of the twentieth century for several reasons: (a) real-time calculation speeds and peripheral functions in low cost digital processors were limited; (b) current regulators, i.e., power electronic inverters, were quite expensive; (c) controller structures required specialist design knowledge; (d) sensor devices for shaft displacement detection require space and are costly; (e) a large bearing space, inherent instability and specialist knowledge and experience are required to design, operate and repair such suspension systems; and (f) magnetic suspension needs both an electrical and a mechanical engineering background.
As we enter the twenty-first century, most of the problems described above have been solved. Fast A/D converters, 3-phase PWM functions and multipliers, as well as fast processing are available in the latest digital processors and field programmable gate arrays. Inverter costs have significantly reduced, thanks to package integration and mass production of power devices. Bearingless drives and generators integrate the magnetic bearings with the motor or generator, producing a compact size; while experiences in developing these systems are increasingly being reported in books and international conference proceedings.
The bearingless system is a key technology in the following power mechatronic applications: (a) high efficiency and compact systems with integrated magnetic suspension and high rotational shaft speed; (b) devices suspended by magnetic forces and operating in harsh environments; and (c) ultra-high-speed motor drives and generators with long flexible shafts requiring integrated vibration damping along the axial length.
The purpose of this book is to provide a fundamental understanding of bearingless drives for both mechanical and electrical engineers. The first part describes the basics of magnetic bearings including aspects of electromagnetics, controllers, practical problems, mechanical dynamics and power electronics with plenty of examples. During the first part, the reader will absorb the basic principles of magnetic suspension. The basic idea of 4-pole and 2-pole flux-wave combination machines is developed. Suspension force and current relationships are derived, and then the general controller design method is introduced. In the second part, the different bearingless machines are introduced; individual bearingless motors, i.e., permanent magnet, synchronous reluctance, induction, homopolar, consequent-pole permanent magnet and switched reluctance, are described and discussed. Some basic aspects of displacement sensors, controllers and power electronic circuits are also described. In the last chapter, some test machines and applications are reviewed. The book includes all the necessary material so that people will find that they have a thorough understanding of the different types of bearingless drives.
The authors of this book work in different institutions and have different backgrounds. However, they have come together to produce this book.
Professor Tadashi Fukao was with the Tokyo Institute of Technology. He says, “power electronics has introduced a freedom of frequency in electrical power technology”. He has been studying high frequency power converters and high-speed motors and generators for many years. He has supervised research projects on synchronous reluctance, homopolar and disk-type bearingless drives. Since 2001 he has been with the Musashi Institute of Technology. He was the President of the Institute of Electrical Engineers of Japan (IEEJ) during 2003–2004. He is responsible for the introduction chapter of this book.
Dr Osamu Ichikawa began working in the field of bearingless machines in 1992 with the Tokyo Institute of Technology. He studied synchronous reluctance and disk-type bearingless drives for his PhD thesis and homopolar bearingless drives for his post-doctoral studies. He is now with the Polytechnic University. He is responsible for the chapters on the synchronous reluctance, homopolar and hybrid types of machine, and also the mechanical structure and position regulation chapters.
Mr Masahide Oshima, a PhD candidate, started his permanent magnet bearingless project in 1992 at the Tokyo University of Science, Suwa. He has been studying surface-mounted permanent magnet and buried permanent magnet bearingless drives. He is responsible for the cylindrical PM, salient-pole PM and buried PM bearingless motor chapters.
Mr Masatsugu Takemoto, a PhD candidate, started studying switched reluctance type bearingless motors in the Tokyo University of Science in 1996. He continues the project, and has extended into 2-pole permanent magnet bearingless motors, in the Tokyo Institute of Technology. He is responsible for the switched reluctance machine and the controllers and power electronics chapters.
Dr David Dorrell is a senior lecturer with the University of Glasgow, UK. He joined a consequent-pole bearingless project while he was on an International Invitation Program in the Tokyo University of Science in 2002. Since then he has been with the project and is responsible for editing all the chapters.
As the primary author, I started to work with high-speed synchronous reluctance drives in 1985 on a PhD program with the Tokyo Institute of Technology. In experiments, I tested some combinations of rotors and stators in the power range of 1–3 kW and up to a speed of 24 000 r/min. The changing of one rotor for another in the machine was not very easy. It usually took more than two weeks. The problem was associated with the mechanical ball bearings. In replacing the rotor, one end’s housing and a ball bearing must be removed. After replacing the rotor, much attention should be paid to the mechanical alignment while fixing the housing with bolts and nuts. Despite careful attention, the mechanical loss at 10 000 r/min could increase by about 40 W from a correctly-fitted and run-in value of 20 W. Therefore the motor needs to be driven for several days to reduce the mechanical loss. The rotational speed can be increased by 1000 r/min per day and after two weeks the test machine can be operated up to the rated speed of 24 000 r/min with a reasonably low mechanical loss. During this period I wondered if there was any other way to suspend the rotor.
Let me explain how I began to think about bearingless motors. One day a test machine was excited by a dc current to adjust the origin of rotor rotational position. After the dc power was switched on, the rotor was supposed to align to the stator magneto motive force (MMF) direction, though it did not rotate. The rotor was positioned in the stator by a magnetic attractive force because the housing bolts were loose. If the dc current was decreased the rotor could be rotated by hand. It was also found that there was a significant magnetic attractive force between the stator and the rotor at the rated excitation. This experience led me to think about useful applications of magnetic force. In spring 1987, the basic concept of the bearingless motor was proposed. In 1988, a prototype bearingless motor was constructed in the Tokyo University of Science. A patent, including the general idea of bearingless motors for various electrical machines, was submitted in January 1989. A short one-page paper was also presented in the IEEJ national convention. Since that time I have been studying several types of bearingless drives and generators. I am responsible for the magnetic bearing chapters, the primitive bearingless chapters and several other chapters.
All the chapters are carefully selected so that both mechanical and electrical engineers can have a thorough understanding. Basic theories are described in detail with practical examples. Most bearingless machine variations are covered; however, some recent developments such as 2-pole permanent magnet, coreless, short pitch and bridge windings are not included. Advanced readers may go to the proceedings of international conferences for further details.
The authors are based in universities. They not only analyse and simulate bearingless drives but also construct test machines to find practical problems. One of the purposes of this book is to help with a smooth transfer of technology into industry.