Magnetic Circuits and Magnetic Materials

Electromechanical devices which employ magnetic fields often use ferromagnetic materials for guiding and  oncentrating these fields. Because the magnetic permeability of ferromagnetic materials can be large (up to tens of thousands times that of the surrounding space), most of the magnetic flux is confined to fairly well-defined paths determined by the geometry of the magnetic material. In addition, often the frequencies of interest are low enough to permit the magnetic fields to be consideredquasi-static, and hence they can be  determined simply from a knowledge of the net mmf acting on the magnetic structure. As a result, the solution for the magnetic fields in these structures can be obtained in a straightforward fashion by using the techniques of magnetic-circuit analysis. These techniques can be used to reduce a complex three-dimensional magnetic field solution to what is essentially a one-dimensional problem. As in all engineering solutions, a certain amount of experience and judgment is required, but the technique gives useful results in many situations of practical engineering interest. Ferromagnetic materials are available with a wide variety of characteristics. In general, their behavior is nonlinear, and their B-H characteristics are often represented in the form of a family of hysteresis (B-H) loops. Losses, both hysteretic and eddycurrent, are functions of the flux level and frequency of operation as well as the material composition and the manufacturing process used. A basic understanding of the nature of these phenomena is extremely useful in the application of these materials in practical devices. Typically, important properties are available in the form of curves supplied by the material manufacturers.
Certain magnetic materials, commonly known as hard or permanent-magnet materials, are characterized by large values of remanent magnetization and coercivity.
These materials produce significant magnetic flux even in magnetic circuits with air gaps. With proper design they can be made to operate stably in situations which subject them to a wide range of destabilizing forces and mmf's. Permanent magnets find application in many small devices, including loudspeakers, ac and dc motors,
microphones, and analog electric meters.



1 Magnetic Circuits and Magnetic Materials
2 Transformers
3 Electromechanical-Energy-ConversionPrinciples
4 Introduction to Rotating Machines
5 Synchronous Machines
6 Polyphase Induction Machines
7 DC Machines
8 Variable-Reluctance Machines and Stepping Motors
9 Single- and Two-Phase Motors
10 Introduction to Power Electronics
11 Speed and Torque Control
Appendix A Three phase circuits
Appendix B Voltages, Magnetic Fields, and Inductances of Distributed AC Windings
Appendix C The dq0 Transformation
Appendix D Engineering Aspects of Practical Electric Machine Performance and Operation
Appendix E Table of Constants and Conversion