This paper presents a general multi-harmonics analytical model able to predict both torque and force behaviour of a multi-phase permanent magnet synchronous machine. This machine topology can be adopted in bearingless applications and in application where controlling the radial forces is deemed to be beneficial to reduce the system vibrations. The modelling approach is applied to a surface permanent magnet synchronous motor featuring a 6-phase symmetric winding configuration although the proposed model is general. The underlying mathematics behind the force and torque production is carefully described with emphasis on both assumptions and approximations. Finite element analysis are proposed for validation purpose of the proposed analytical model, thus highlighting benefits and limitations. Once validated, the model is used to draw general design guidelines for this kind of electrical machine highlighting the trade-off between the competitive needs of maximizing the torque and force capability while minimizing the interaction between force and torque production.