Electric motor capable of reducing cogging torque

A method of reducing cogging torque in a synchronous electric motor comprising:

• providing a rotor including a plurality of magnetic pole units, each magnetic pole unit having the same shape wherein each magnetic pole unit has an outer peripheral surface and each of the magnetic pole units has a magnetic pole center P1;
• and
• providing a stator including slots facing the outer peripheral surface of the rotor, wherein,
• correcting the outer peripheral surface of each of the magnetic poles by adding a correction amount based on a sine function to a reference surface to create a corrected outer peripheral surface, wherein the corrected outer peripheral surface of each of the magnetic pole units is bulged to an outside in a radial direction so that a waveform of a magnetic flux density generated from the rotor is a sine wave shape, and
• a concave part or convex part is formed at a central part in a circumferential direction of an outer peripheral surface in each of the magnetic pole units, the concave part or convex part being small enough to prevent changing of a waveform cycle of cogging torque determined by a least common multiple of the number of slots and the number of magnetic poles of the rotor, wherein the corrected outer peripheral surface differs from the reference surface everywhere except at the magnetic pole center P1.