Rotor windings for DC motor

1. 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, and wherein:

k is 2 and n is 2; and

each tooth has four coils wound around it, which belong to four winding units.

2. The DC motor of claim 1 , wherein the rotor winding is formed by winding a single continuous wire without cutting the wire until the rotor winding is formed.

3. The DC motor of claim 1 , wherein each winding unit is connected to a first segment and a second segment, and the second segment is under the same magnetic polarity as another segment adjacent to the first segment.

4. The DC motor of claim 1 , wherein of the four winding units, two winding units are connected in parallel between a pair of segments, and the other two winding units connected in parallel between another pair of segments.

5. The DC motor of claim 4 , wherein the rotor comprises a plurality of equalizers, and each equalizer links a group of segments under the same magnetic polarity.

6. The DC motor of claim 1 , wherein each coil is wound around a single tooth, and the P coils of each winding unit are wound on P teeth under the same magnetic polarity.

7. The DC motor of claim l, wherein:

the stator comprises at least one pair of brushes slidably contacting the segments of the commutator, each pair of brushes comprises a positive brush and a negative brush; and

each layer of windings forms two parallel branches electrically connected between the at least one pair of brushes.

8. The DC motor of claim l, wherein

the rotor winding is formed by two wires, and each wire has a diameter in the range of 0.6 millimeter (mm) to 0.8 mm.

9. The DC motor of claim 1 , wherein P is 3 and m is 3.

10. The DC motor of claim 1 , wherein the rotor winding is formed by winding a wire having a diameter in the range of 0.9 millimeter (mm) to 1.1 mm.

11. A DC motor comprising:

a stator having 2P magnetic poles, wherein P is an integer greater than one; 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 therebetween, wherein m is an odd integer greater than 1;

a commutator fixed to the rotor shaft 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 k×m winding units, each winding unit connected to only two segments and comprising P coils directly connected in series between the two segments,

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, and wherein n is 2, and the rotor winding is formed by two wires, and each wire has a diameter in the range of 0.6 millimeter (mm) to 0.8 mm.

12. The DC motor of claim 11 , 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 two winding units, each segment in the second set is directly connected with only one winding unit, and each segment in the third set is directly connected with none of the winding units.

13. The DC motor of claim 11 , wherein each winding unit is connected to a first segment and a second segment, and the second segment is under the same magnetic polarity as a third segment adjacent to the first segment.

14. The DC motor of claim 13 , wherein the rotor comprises a plurality of equalizers, and each equalizer links a group of segments under the same magnetic polarity.

15. The DC motor of claim 11 , wherein each coil is wound around a single tooth, and the P coils of each winding unit are wound on P teeth under the same magnetic polarity.

16. The DC motor of claim 11 , wherein:

the stator comprises at least one pair of brushes slidably contacting the segments of the commutator, each pair of brushes comprises a positive brush and a negative brush; and

the rotor winding forms two parallel branches electrically connected between the at least one pair of brushes.

17. The DC motor of claim 11 , wherein P is 3 and m is 3.