Back EMF improvement and force ripple reduction in PM linear motor drives

Permanent magnet linear synchronous motor (PMLSM) drives may exhibit cogging force and high harmonic content in the EMF waveform. Both of them cause a force ripple, under low and high load, introducing trouble in the positioning precision. To reduce the force ripple, there are two techniques: an appropriate PMLSM design, and a suitable control of the current waveforms. This paper presents an innovative and simple technique based on the PM shifting for reducing both the cogging force and the harmonic content in the back EMF of the PMLSM drive, in order to achieve a smooth force, without a current harmonic compensation. The method to calculate the appropriate PM shift is presented as well as its effect on the EMF harmonics and force ripple. Experimental results are reported confirming the results obtained by means of this technique.