US Patent 8680738 Universal motor with auxiliary magnetic poles

A universal motor includes a stator and a rotor rotatably installed in the stator. The stator includes a stator core having a yoke, n primary poles with windings wound thereon and n auxiliary poles. The primary poles and auxiliary poles are alternately arranged on the radially inner side of the yoke in a circumferential direction thereof. When the windings are electrified, n primary magnetic poles and n auxiliary magnetic poles are formed at the primary poles and auxiliary poles respectively, n being an integer greater than 1. All the primary magnetic poles have the same polarity and the polarity of each auxiliary magnetic poles is opposite the polarity of the primary magnetic poles.