Design of permanent magnet flat linear motors for standstill applications

The machines to be considered have a planar airgap and can be thought to be the result of cutting a conventional cylindrical machine along a radial plane and unrolling it. The machines use skewed permanent magnet poles to eliminate the magnetic cogging effects due to the teeth on the primary. An inverter is used to supply the machines. The phase of its current outputs are locked to the excitation pole position in the same way as in rotary brushless d.c. motor. It is assumed that the supply currents are quasi square in form. They are constant over 60 degree time periods. During this time the force produced by the machine will change for two reasons, first the fields due to the excitation and primary change in space position and secondly the magnetic end effect force [1] is altered. This variation is shown in fig. 1 for a four pole experimental machine. Both experimental and calculated values are given. The calculated results were obtained from a two dimensional finite element analysis. Account was taken of skew in chis analysis by adding together 3 space phase shined versions of the curve shown at fig. 1(a). This results in the curve shown at fig, 1(b) which compares well with the practical values and confirms that the finite element approach given above is suitable for design purposes.