Permanent magnet electric motor

1. A permanent magnet electric motor comprising:

a rotor having a shaft, a rotor core fixed to the shaft and having nine teeth, a commutator fixed to the shaft adjacent the rotor core and having six segments, and three rotor winding units, each winding unit having two ends directly connected to two respective segments of the commutator and three rotor windings in serial connection between the two ends and wound about the teeth of the rotor core;

a stator magnetically coupled to the rotor, the stator having four permanent magnetic poles surrounding the rotor core; and

a plurality of brushes in sliding electrical contact with the commutator for supplying electric current to the rotor windings via the commutator.

2. The motor of claim 1 , wherein

the first of the six segments of the commutator is directly connected to the first end of the first rotor winding unit;

the second of the six segments of the commutator is directly connected to the second end of the first rotor winding unit and to the first end of the second rotor winding unit;

the third of the six segments of the commentator is directly connected to the second end of the second rotor winding unit and to the first end of the third rotor winding unit;

the fourth of the six segments of the commutator is connected to the first segment by a short circuit wire and directly connected to the second end of the third rotor winding units;

the fifth of the six segments of the commentator is connected to the second segment by a short circuit wire; and

the sixth of the six segments of the commutator is connected to the third segment by a short circuit wire.

3. The motor of claim 1 , wherein two diametrically opposed segments of the commutator are electrically connected together by a respective short circuit wire and the ends of the three rotor winding units are directly connected to four adjacent segments of the commutator.

4. The motor of claim 1 , wherein the rotor windings are connected in a triangular configuration and each end of each side of the triangle is connected to two corresponding segments of the commutator.

5. The motor of claim 4 , wherein each side of the triangle comprises three windings, said three windings being wound about six of the nine teeth such that said six teeth form three pairs of adjacent teeth and each rotor winding of said three rotor windings is wound about a respective one of said three pairs of adjacent teeth.

6. The motor of claim 1 , wherein the three rotor windings of the same winding unit are wound on six different teeth belonging to three adjacent pairs of teeth of the rotor core.

7. The motor of claim 1 , wherein the diameter of the commutator is less than 10 mm.

8. The motor of claim 1 , wherein the diameter of the commutator is less than 5 mm.

9. The motor of claim 1 , wherein the diameter of the commutator is less than 3 mm.

10. A permanent magnet electric motor comprising:

a rotor having a shaft, a rotor core fixed to the shaft and having n teeth, a commutator fixed to the shaft adjacent the rotor core and having m segments, and a plurality of rotor windings wound about the teeth and coupled to the segments;

a stator magnetically coupled to the rotor, the stator having k permanent magnetic poles surrounding the rotor core; and

a plurality of brushes in sliding electrical contact with the commutator for supplying electric current to the plurality of rotor windings via the commutator;

wherein k, m and n are integers, m is greater than k and less than n, the least common multiple of n and k is greater than 24; and

wherein the plurality of rotor windings form a plurality of rotor winding units each winding unit having two ends respectively connected to two adjacent segments of the commutator and comprising more than one winding connected in series and wound on different teeth of the rotor core.

11. The motor of claim 10 , wherein in and k are even numbers and n is an odd number.

12. The motor of claim 10 , wherein the diameter of the commutator is less than 5 mm and m is equal to six.

13. The motor of claim 10 , wherein the diameter of the commutator is less than 3 mm and m is equal to six.

14. The motor of claim 10 , wherein two diametrically opposed commutator segments are electrically connected together by a respective short circuit wire and the ends of the rotor winding units are directly connected to four adjacent segments of the commutator.

15. The motor of claim 10 , wherein at least one of the segments is directly connected to one of the ends of the rotor winding units, at least one of the segments is directly connected to two different ends of two of the rotor winding units, and at least one of the segments is not directly connected to any one of the ends of the rotor winding units.