Cogging-force-reduction techniques for linear permanent-magnet machines

The paper describes two techniques for reducing the cogging force associated with the finite length of the ferromagnetic armature core of a tubular permanent-magnet machine. Analytical expressions for predicting the cogging force which is associated with armature cores having skewed or stepped end faces are established, and a cogging-force-minimisation technique is employed to determine an optimal armature-core length. The validity of the analytical cogging-force predictions and the effectiveness of the cogging-force-reduction techniques are verified by three-dimensional finite-element analyses and experimental measurements.