The rotor assembly contains a number of polar anisotropic ring magnets sequentially stacked, a number of supporting rings within the stacked ring magnets, and an axle threading through the supporting rings. The ring magnets are of a same diameter and length. Each supporting ring contains three co-centric rings: an inner steel ring, a middle rubber ring, and an outer steel ring. The supporting rings are positioned with substantially equal spacing and with a gap away from each aperture of the stacked ring magnets, so that additional weight elements could be positioned and adhered within the gap. The periodic positioning of a number of supporting rings could significantly reduce the weight of the motor assembly with much improved steadiness. Additionally, the middle rubber rings of the supporting rings provide a buffer mechanism for absorbing vibration and noise reduction.