Consideration of radial magnetic forces in brushless DC motors

The permanent magnet (PM) synchronous machine (PMSM) is widely used in hybrid and electrical automotive applications because of its high power density. Machines having fractional number of slots per number of poles machine and using concentrated windings have several advantages in comparison with the machines having distributed windings, mainly in terms of copper volume and simplicity of manufacturing. Nevertheless, the literature has also shown that this type of machine has some disadvantages linked to the harmonic content of the stator MMF. This could lead to high PM losses and strong variations of the radial magnetic forces (RMF) applied to the stator teeth. This last effect could be at the origin of vibrations and acoustic noise. In this context, three models of PMSM, 28 poles-30 teeth (28P 30S), 28 poles-42 teeth (28P 42S) and 28 poles-84 teeth (28P 84S) have been investigated in this paper. The RMF of theses machines has been carefully studied, in order to determine the vibration modes of each machine. Those modes have been compared with the resonance modes of the machines in order to compare the risks of vibration and acoustic noise of the different machines.