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This book is written for students and practicing engineers involved in the design of magnetic and electromechanical devices. The material presented is a compilation of the practical approaches used over the author's 37-year career at Eaton Research Labs and is intended to help the reader gain a "feel" for locations and strengths of magnetic fields and an intuitive insight into what magnetic fields do and how to use them. This book makes magnetics easy to understand and practical to apply in magnetic research, experimentation, and analysis of magnetic fields encountered in engineering challenges.
Accurate and reliable methods are presented for the design of magnetic sensors, actuators, controls, and other electromechanical devices with the notable exclusion of rotating machines that are well covered by various authors and courses in university Electrical Engineering departments. Actuators, solenoids, and magnetic sensors have been around in various forms for over a century, and they are critical components of control and protection systems including relays and circuit breakers.
This book has a strong foundation in the methods developed by H. C. Roters with additional topics in the areas of permanent magnet materials and permanent magnet performance in particular. The methodologies also take full advantage of complex spreadsheet capabilities, as well as finite element analysis as a counterpart to the calculations. Design examples include calculations for losses and temperature rise, which are critical for all electromagnetic systems. The smallest design usually has the highest temperature rise. The best design usually considers the trade-off between size and temperature rise.
The design calculations presented are practical in the sense that they can be quickly and accurately applied in a spreadsheet model using the permeance method (also known as reluctance method or magnetic circuit method). The permeance method evaluates the magnetic field from the perspective of a magnetic circuit, analogous to an electric circuit. Chapter 1 describes this in detail and aims to provide an understanding of magnetic flux paths based on the simple question, "If I were a magnetic flux line, where would I go?" The accuracy of the permeance method is demonstrated with comparisons to measurements and finite element simulations.
Practical methods also address the issues of time and effort. Some ideas need only "feasibility" level accuracy, while other design-specific challenges require high-level accuracy. This relates directly to budget and schedule issues on engineering projects. Increased effort (model detail, complexity, size, time) is needed to achieve increased accuracy. The best strategic approach is to use a method that is quick and provides enough accuracy to make a valid design decision. A spectrum of calculation methods can be considered 1) a hand calculation, 2) a simple spreadsheet model, 3) a complex spreadsheet model, 4) a 2D or axisymmetric finite element model, 5) a 3D finite element model. A spreadsheet model can also be used to quickly determine the starting size for a finite element model.
A critical step for gaining confidence in the validity of any analysis is to check the results against those of a simple calculation. In general, when doing a complex analysis (such as a finite element simulation), the first step should be a simple calculation (such as a spreadsheet calculation) and a visualization of the magnetic field. The finite element simulation results can then be quickly reviewed for the shape of the magnetic field and the magnitude of the flux density, current and force, to judge if the results are reasonable. Finite element models have many input values and boundary conditions that are prone to typographical errors (such as a decimal point error, or a dimensional units error). Errors can be quickly detected when compared to a simple calculation and magnetic field visualization.
