Abstract: Electromagnets have the advantages of low cost, simple construction and ruggedness. In some applications, an electromagnet with the constant (or quasi-constant) traction characteristic is desirable for getting good controllability. In order to design a derect current electromagnet with constant traction characteristic, two basic questions including what kinds of magnetic circuit configuration are adoptable and what conditions should be satisfied must be considered. Focusing on solving the two problems, two kinds of cylindrical electromagnets are proposed and the corresponding conditions in the analytical function expressed by the related geometric dimensions of the electromagnet are also derived, based on the simplified equivalent magnetic circuit method as well as the analysis of the total permeance. Besides, the effects of the main parameters on the traction are discussed. The conclusions obtained by the analytical formulas may be used to provide a primary design, which then may be further improved by electromagnetic field numerical methods and/or by experiments. The comparison between the results calculated by the analytical formulas and by ANSOFT software is fairly good and it validates that the proposed method and the conclusions are simply suitable for the developing and designing of direct current electromagnets with constant traction.

Key words: Constant traction characteristic, Electromagnets, Equivalent magnetic circuit, Finite element method