Abstract: A motor rotor for use in a grinder or other electromotive tool in which a motor winding is prevented from being damaged by collisions with particles. The motor rotor is composed of a core having sixteen teeth extending radially relative to a rotation shaft and slots formed among the teeth, a coil of winding wound around the slots, and wedge members for covering the slots. Felt strip members are wound around the parts of the coil exposed at both ends of the core before the exposed parts are impregnated with varnish. Varnish also permeates the strip members, and the strip members are firmly adhered to the exposed parts of the coil. After the varnish dries, the strip members fix themselves and are provided with a strength equal to that of plastic. The ribs provided by the strip members act as fins blowing away any particles, thereby protecting the coil from damage from the particles.