Rotor windings for dc motor

A DC motor comprising:

• a stator having 2P magnetic poles, wherein P is an integer greater than 1;
• and
• a rotor rotatably mounted to the stator and comprising:
• a rotor shaft;
• a rotor core fixed to the rotor shaft and having a plurality of teeth defining m×P slots, wherein m is an odd integer greater than 1;
• a commutator fixed to the rotor shaft, adjacent to the rotor core, and having k×m×P segments, wherein k is an integer selected from 1 and 2;
• a rotor winding received in the slots of the rotor core and connected to the segments of the commutator, the rotor winding comprising n layers of windings, wherein n is an integer smaller than P;
• each layer of windings comprising k×m winding units and each winding unit comprising P coils directly connected in series,
• wherein the segments comprise a first set of segments, a second set of segments and a third set of segments, each segment in the first set is directly connected with n winding units, each segment in the second set is directly connected with 2n winding unit, and each segment in the third set is directly connected with none of the winding units.