Effects of loading on radial magnetic forces in low-speed permanent magnet machine with concentrated windings

This paper investigates the effects of loading on radial magnetic forces in a low-speed PM machine with non-overlapping concentrated windings. Vibration level of the PM machines with concentrated windings can be potentially higher compared to the traditional machines with distributed windings. It is mainly due to the presence of low order harmonics in the spatial distribution of the radial forces. In this paper, magnetic flux density distribution in the airgap of a 120-slot/116-pole machine is computed using finite element method (FEM). Maxwell's stress tensor method is then employed to calculate radial and tangential forces in the airgap. Flux density, radial and tangential force distributions are studied in different loading conditions and the effect of d-axis and q-axis currents are investigated. It is found that d-axis current can significantly change the amplitude of the lowest spatial harmonic of radial force distribution and consequently the vibration behavior. Experimental tests are carried out to compare the vibration level of the prototype PM generator supplying resistive, inductive and capacitive loads.