BRUSHLESS ELECTRIC MACHINES WITHOUT PERMANENT MAGNETS

Prof. Tim Miller

Essen,  28+29 November 2023

The main theme of this two-day seminar is to develop a number of alternatives to an interior permanent-magnet motor which uses high-energy rare-earth magnets.  One reason for this exercise is to address concerns about the price and availability of this class of magnets: what are the options if they become too expensive or difficult to obtain?  Another reason is to work through a design exercise for several types of AC machine with a unified approach that includes the essential theory and applies the finite-element method in an efficient process.  All the alternative machines are designed around the same specification, 10 kW at 2800 rev/min with a 600 V DC inverter voltage.

The seminar should help in two directions.  For engineers working on PM machines with high power density, we show the effects of weakening and then completely removing the magnets, leading in several stages eventually to a pure synchronous reluctance motor; and we will consider how to restore some of the lost performance by geometry improvement in the rotor.  We will then substitute a wound-field rotor to explore the limits of torque production with an electrically excited field winding.  Finally we will make an assessment of the performance of a switched reluctance motor.  For engineers working on machines without magnets, the seminar will help to understand certain important approaches to their design, and the relationship between all the different alternative machines.

All the examples use the same frame size, but there are important differences in their operating principles, their controls, their mechanical design and handling, and their efficiency and cooling requirements. These differences are examined along the way, but the seminar begins with a review of all the main machine types (illustrated with real examples), and discusses the methods of torque production, the effects of variable speed, and the essential control requirements.  While the scope is broad, it is still realistic in terms of the types of machine that can be used under modern conditions.  We will also set the machines in a suitable historic context because this is important in understanding the trends that have led the industry to its current state.

While the seminar is based on numerical examples, it is rich in theoretical principles and it follows a highly unified method of using the finite-element method for efficient sizing and performance estimation.  

The seminar closely follows the book Design Studies in Electric Machines published in 2022 [430 pages] by J.R. Hendershot Jr. and T.J.E. Miller.  This book can be used as a companion text to the seminar.

Your background — The seminar should be meaningful to a wide range of engineers of different levels of experience, no matter what software they use from day to day.  A basic first degree in electric power engineering would be a suitable academic background, because the theme of the seminar is quite electrical;  even so, many experienced mechanical engineers and some postgraduate students may find themselves comfortable with the material.  In previous seminars, the class members have typically had quite varied backgrounds, and the seminars have evolved in such a way as to benefit from this.

Prof. Miller has been studying electric machine design for more than 50 years, in a very international context.   His training seminars have been developed intensively especially over the last 10 years or so, sometimes in partnership with JSOL/Powersys, the UK Magnetics Society, the FEMAG community, and many private companies (both in person and on-line). 

References :  Design Studies in Electric Machines, (ISBN 978-0-9840687-4-6, October 2022), and some of the theory can be found in the previous book Design of Brushless Permanent-Magnet Machines, (ISBN 978-0-9840687-0-8,  2010).