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Special Issue on Electric Machines and Systems: Design, Analysis, and Control
Edited by Dr. Eng. Ahmed Masmoudi
1 Introduction
Permanent magnet machines are inherently efficient and hence suitable candidates to electric and hybrid propulsion systems. However, with their armature distributed windings, PM machines have generally been considered to be poor candidates for achieving wide ranges of constant power operation. Currently and referring to the works of [1] El-Refaie et al. (2006), this problem has been by-passed thanks to the substitution of the distributed windings by concentrated ones, leading to the so-called "fractional slot permanent magnet machines".
Beyond the extension of the flux weakening range, fractional slot PM machines exhibit remarkable features, especially:
- reduced copper losses thanks to short end-windings ([2] Zhu, 2009);
- high power densities ([2] Zhu, 2009);
- low cogging torque ([2] Zhu, 2009); and
- improved fault-tolerance capability ([3] Bianchi et al. , 2005).
Generally speaking, torque production capability of AC machines, including fractional slots PM ones, grows up with the increase of the winding factor. Of particular interest are machines having a number of slots Ns which differs from the number of poles 2p by unity. Nevertheless, these machines are penalized by an asymmetrical distribution of the magnetic forces along the air gap. Such a problem results in non-uniformly radial forces leading to high acoustic noise and vibration of the machine mechanical structure.
In order to highlight this problem, the paper is devoted to the investigation of the magnetic forces produced by two fractional slots PM machines, one of them belongs to the set of machines where the number of slots Ns which differs from the number of poles 2p by unity. Such investigation is carried out by both 2D and 3D finite element analysis (FEA). This done, the study is extended to a comparison of the torque production and the speed range enlargement capabilities of both machines.
2 Study background
A. The basis of magnetic forces
Magnetic interaction between rotor and stator is the source of torque production in electrical machines. This interaction leads to the generation of tangential forces, radial forces and magnetostrictive forces in the machine air gap. Tangential forces are the useful ones form the point of view of torque production. Both...