Foreword
This book is primarily about bearingless machines, where the magnetic levitation of the rotor is provided from within the main stator bore (in addition to torque production if motoring, or electrical power production if generating). The bearingless type of motor is still, to some extent, in the development stage and therefore still at an early point of evolution. The basic theory, design and control rules are being researched and investigated and there is still much work to be done. The authors have had many years of experience in the design and operation of bearingless machines and this book represents their combined experience as pioneers in this area. Therefore this book describes the leading edge of the technology as it stands today.
However, this is not an advanced textbook suitable only for engineers with a good working knowledge of the subject. It is also aimed at both mechanical and electrical engineers with little prior knowledge and who wish to get an understanding of the subject. Therefore the first few chapters describe the basic operation of the bearingless machine, which will help strip away the initial thought that a self-levitating rotor is a very difficult problem that may be too difficult to understand and construct. The book then goes on to describe the operation of different types of bearingless machine, the control techniques required and then gives the specification and performance figures for actual machines that the authors have developed and studied. The last chapter describes examples of bearingless machines that others have developed. The only theory that is necessary for good understanding is control, which spans both electrical and mechanical engineering, though a basic knowledge of magnetic circuits and the operation of standard brushless permanent magnet machines, synchronous- and switched-reluctance machines and induction motors would be highly advantageous.
The types of bearingless machine covered in this book are brushless permanent magnet ac machines, synchronous- and switched-reluctance machines and induction motors. These represent the main motor topologies that can be used in bearingless machines. In addition, homopolar motors can also be used (which are really a derivative of stepping motors). There are also design compromises to consider. In a permanent magnet machine, thick magnets give good torque production and robust operation; however, thin magnets lead to reduced levitation winding current. Some of the solutions that are implemented to obtain stable and controllable levitation in these machines are quite ingenious; for instance, the use of a consequent-pole rotor in a permanent magnet machine to modulate the suspension winding MMF in order to generate airgap flux waves of the correct pole number, phase and frequency. A second example is the use of multiple rotor conductor paths in the cage induction motor to decouple the suspension winding from the rotor. These subtleties are all described in this book. The control strategies for each machine are also described. These can be quite complex since the position and magnitude of the airgap flux has to be carefully controlled so that torque is produced and the rotor is magnetically suspended in the centre of the stator bore. The aim is often to use a standard inverter for the main motor winding with additional power-electronic and control circuitry for the suspension winding. In all cases it is necessary to have radial position sensors to control the levitation.
The applications of bearingless motors are many and varied. As an engineer interested in drive motors and electric power applications, I can immediately see the relevance of this book to very high speed drives (in order to greatly reduce the motor volume) and also to flywheel energy storage. Currently, it is common to take a standard motor and replace the mechanical bearings with magnetic bearings to obtain very high speed operation; or in a flywheel, to have separate magnetic suspension and motor/generator components – this book will give engineers the basic knowledge to go one step further and consider the motor and bearings as an integral unit.
I was first asked to referee the initial proposal for this book as a specialist in the design of electric machines (primarily induction and permanent magnet machines) and then requested to help check the technical descriptions as a native English-speaking engineer. The authors have written the book in a second language, rather than writing it in Japanese and having it translated, in an attempt to reach a wider audience – they are to be congratulated in undertaking such a task. I have found the book enlightening and it has opened up a new sphere of interest and research.