cut off type of rotor disc 1 with an insulating barrier 4 as the outermost radial portion on the q axis 7 the outermost insulating barrier 4 is considered not to have pitch points 5 rotor discs 1 are distinguished between two different designs depending on whether the outermost radial portion on a q axis 7 is a magnetically permeable segment 3 or an insulating barrier 4 a rotor disc 1 with a cut off 11 refers to a rotor type shown in fig3 wherein the outermost portion of the rotor pole sector 15 comprises an insulating barrier 4 e g air in form of a shallow recess provided in the peripheral edge of the rotor perimeter 2 the rotor disc 1 is provided with cut offs 11 in order to disable the magnetic flux at the outermost radial portions on the q axes 7 conversely a rotor disc 1 without a cut off 11 refers to a rotor type shown in fig2 wherein the outermost portion of the rotor pole sector 15 consists of a magnetically permeable segment 3 forming a substantially smooth semi circular perimeter 2 since the invention is based on modifications in the insulating barrier geometry which may not be immediately apparent without doing some measurements we here define some auxiliary reference points 8 which are useful for explaining the invention and for making the modifications readily measurable fig4 is a schematic drawing showing the definitions of the reference points 8 and reference angles 9 10 related to the reference points 8 the number of insulating barriers 4 has been chosen to be small two for the clarity of the drawing but any suitable.