Lecture 11: The mystery of non-Lorentz force

2024/1  
NIDEC Technical Adviser 
Takashi Kenjo

Nidec is recognized as a manufacturer of various types of motors. There are many types of motors, their sizes varying from small motors with a diameter of about 2mm to large motors used in hydroelectric plants.
Let me relate a scene from a session of in-house training. It was a training session on the technical aspects of motors aimed for office workers at the research institute. The instructor was an enthusiastic engineer who was in his early thirties, while the students included a competent manager who was in his early sixties. Let us call him M. br /> After attending the two-day program, M said “It’s curious that there are so many different types of motors, isn’t it?.” I was sitting in the same room, but to me this sounded like an expression of wonder rather than a straight question.
It has been some time since I last wrote an article for this series “Quest For Motors’ Hidden Abilities,” but it was M’s words that made me feel that I needed to write again. There are several ways I can answer this question. The answer will differ depending on what type of work the asker is engaged in. Here, I will mainly be addressing those working in motor design who have an interest in physics and math and engineers who work with motors used in automobiles. 
The topic is something that has not be discussed in any book in the world. The topic is『 “What is non-Lorentz force?”
Another reason why I thought of writing about this subject is a question raised by a young researcher who had done his thesis research on magnetic levitation. I felt that young people who are doing academic work on motors should also know what the Lorentz force is, what a non-Lorentz force is, and how they are related. This is not a basic problem but rather related to advanced mathematics and physical interpretation.

The linear principle of (a) is almost never used in motors. On the other hand, c an we say that the per pendicular principle o f (b) is often used ? The p rinted motor shown in Fig. Fig. 2 , and the coreless motor, shown in Fig. 3 , are examples. Another example is the moving-coil motor used to control the magnetic head in hard disk drives, shown in Fig. 4 .
As these examples suggest, the motors that employ this principle fall in the category of precision motors or specialized motors, while the larger industrial motors or traction motors used for railroad and electric vehicles do not employ the Lorentz force. Motor designs that make use of the non-Lorentz force have many more applications and a much greater impact on the economy.

Quick summary of above discussion

The motor is a device that uses electricity to generate force and produce rotary motion, but very few motors use the simple straight-line vector forces like that shown in Fig. 1(a). In comparison, there are many more motor designs in which a magnetic field is incorporated to create perpendicularly acting forces. Yet, industry is propelled by a principle that goes beyond this. This is the non-Lorentz force.

Case of permanent-magnet slotted motor

The magnetic Lorentz force appears in Fleming’s left-hand rule, known also as the BIL law, shown in Fig. 5 . Torque (rotary force) can also be produced based on a different principle, i.e., from the attracton between the N and S poles of a permanent magnet, or the repulsion between N and N poles, or S and S poles, as shown in Fig. 6 . Since no current is used, there is no Lorentz force in this case. Let us go a step further. Fig. 7 (a) shows rotors of a typical DC motor. A large number of them are used in automotive applications. Fig. 7 (b) is presented to examine whether such contructions make use of the Lorentz force. As the illustration shows, no Lorentz force should be at work here.

Use of hysteresis – self-starting synchronous motor

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