Electrical machines with reduced cogging
An electrical machine has at least two separate groups of at least two circumferentially adjacent magnet poles. One of the circumferentially outer magnet poles in one of the groups of magnet poles is defined as being in its reference position. The reference position of each other magnet pole is defined as the position each other magnet pole would occupy if all the magnet poles were equally circumferentially spaced around the first or second body and the one circumferentially outer pole was in its reference position. At least one of the circumferentially outer magnet poles in each group is sited in its reference position. At least one magnet pole in each group is a displaced magnet pole and is sited in a position that is displaced from its reference position by an amount that is not equal to an integral multiple of the reference angular pitch of the winding slots. The displacement of the magnet poles provides a pronounced reduction in cogging.
1. An electrical machine including a first body having a plurality of circumferentially spaced magnet poles formed at its radially inner surface, and a second body located within the first body and having a plurality of circumferentially spaced winding slots formed at its radially outer surface, one of the first and the second bodies being a rotatably mounted rotor, and the other of the first and the second bodies being a fixedly mounted stator, the winding slots having a reference angular pitch that is equal to 360° divided by a number of the winding slots in the electrical machine;
the magnet poles comprising at least a first group including at least four circumferentially adjacent magnet poles, and a second group including at least four circumferentially adjacent magnet poles, the first group and the second group including a same number of magnet poles and having a same displacement pattern;
each magnet pole having a reference position, the reference positions being equally spaced around a circumference of the first body;
the displacement pattern of the magnet poles in the first and second groups being determined according to a formula:
D
=
2
(
p
-
1
)
N
where:
D=an angular displacement in winding slot pitches of magnet pole p from its reference position,
P=a magnet pole number within the first or second group, and counted from the closest adjacent magnet pole group, and
N=the number of magnet poles in the first or second group;
at least one magnet pole in each of the first and second groups being a circumferentially outermost magnet pole and being located in its reference position as a result of said formula; and
at least one magnet pole in each of the first and second groups being a displaced magnet pole and being located in a position that is displaced from its reference position as a result of said formula by an amount that is not equal to an integral multiple of the reference angular pitch of the winding slots.
2. The electrical machine according to claim 1 , wherein the first body is the stator, and wherein the second body is the rotor.
3. The electrical machine according to claim 1 , wherein the first body is the rotor, and wherein the second body is the stator.
4. The electrical machine according to claim 1 , wherein no magnet pole is displaced from its reference position by more than the reference angular pitch of the winding slots.
5. The electrical machine according to claim 1 , wherein no two adjacent magnet poles are in contact with, or impinge upon, each other.
6. The electrical machine according to claim 1 , wherein the displacement pattern of the magnet poles around the circumference of the first body has at least two-fold rotational symmetry about a rotational axis of the rotor.
7. The electrical machine according to claim 1 , wherein both circumferentially outermost magnet poles in each of the first and second groups are located in their reference positions.
8. The electrical machine according to claim 1 , wherein the first group includes an even number of magnet poles, and wherein the second group includes an even number of magnet poles.
9. The electrical machine according to claim 1 , wherein the first group includes eight magnet poles, and wherein the second group includes eight magnet poles.
10. The electrical machine according to claim 1 , wherein the winding slots are uniformly circumferentially spaced.
11. The electrical machine according to claim 1 , wherein each magnet pole is substantially parallel to a rotational axis of the rotor.
12. The electrical machine according to claim 1 , wherein each magnet pole is skewed relative to a rotational axis of the rotor.
13. The electrical machine according to claim 1 , further comprising an air gap between the first body and the second body, and wherein an outer surface of each magnet pole is shaped such that the air gap is not uniform.
14. An electrical machine including a first body having a plurality of circumferentially spaced magnet poles formed at its radially inner surface, and a second body located within the first body and having a plurality of circumferentially spaced winding slots formed at its radially outer surface, one of the first and the second bodies being a rotatably mounted rotor, and the other of the first and the second bodies being a fixedly mounted stator, the winding slots having a reference angular pitch that is equal to 360° divided by a number of the winding slots in the electrical machine;
the magnet poles comprising at least a first group including at least two circumferentially adjacent magnet poles, and a second group including at least two circumferentially adjacent magnet poles;
each magnet pole having a reference position, the reference positions being equally spaced around a circumference of the first body;
at least one magnet pole in each of the first and second groups being a circumferentially outermost magnet pole and being located in its reference position;
at least one magnet pole in each of the first and second groups being a displaced magnet pole and being located in a position that is displaced from its reference position by an amount that is not equal to an integral multiple of the reference angular pitch of the winding slots;
the magnet poles having a reference angular pitch that is equal to 360° divided by a number of the magnet poles in the electrical machine; and
winding coils having an angular pitch equal to the reference angular pitch of the magnet poles.
15. An electrical machine including a first body having a plurality of circumferentially spaced magnet poles formed at its radially inner surface, and a second body located within the first body and having a plurality of circumferentially spaced winding slots formed at its radially outer surface, one of the first and the second bodies being a rotatably mounted rotor, and the other of the first and the second bodies being a fixedly mounted stator, the winding slots having a reference angular pitch that is equal to 360° divided by a number of the winding slots in the electrical machine;
the magnet poles comprising at least a first group including at least two circumferentially adjacent magnet poles, and a second group including at least two circumferentially adjacent magnet poles;
each magnet pole having a reference position, the reference positions being equally spaced around a circumference of the first body;
at least one magnet pole in each of the first and second groups being a circumferentially outermost magnet pole and being located in its reference position;
at least one magnet pole in each of the first and second groups being a displaced magnet pole and being located in a position that is displaced from its reference position by an amount that is not equal to an integral multiple of the reference angular pitch of the winding slots;
the magnet poles having a reference angular pitch that is equal to 360° divided by a number of the magnet poles in the electrical machine; and
winding coils having an angular pitch equal to two-thirds of the reference angular pitch of the magnet poles.
16. An electrical machine including a first body having a plurality of circumferentially spaced magnet poles formed at its radially inner surface, and a second body located within the first body and having a plurality of circumferentially spaced winding slots formed at its radially outer surface, one of the first and the second bodies being a rotatably mounted rotor, and the other of the first and the second bodies being a fixedly mounted stator, the winding slots having a reference angular pitch that is equal to 360° divided by a number of the winding slots in the electrical machine;
the magnet poles comprising at least a first group including at least two circumferentially adjacent magnet poles, and a second group including at least two circumferentially adjacent magnet poles;
each magnet pole having a reference position, the reference positions being equally spaced around a circumference of the first body;
at least one magnet pole in each of the first and second groups being a circumferentially outermost magnet pole and being located in its reference position;
at least one magnet pole in each of the first and second groups being a displaced magnet pole and being located in a position that is displaced from its reference position by an amount that is not equal to an integral multiple of the reference angular pitch of the winding slots; and
each magnet pole being formed by permanent magnets having an angular width that is substantially 2¼ times the reference angular pitch of the winding slots.
17. An electrical machine including a first body having a plurality of circumferentially spaced magnet poles formed at its radially inner surface, and a second body located within the first body and having a plurality of circumferentially spaced winding slots formed at its radially outer surface, one of the first and the second bodies being a rotatably mounted rotor, and the other of the first and the second bodies being a fixedly mounted stator, the winding slots having a reference angular pitch that is equal to 360° divided by a number of the winding slots in the electrical machine;
the magnet poles comprising at least a first group including at least two circumferentially adjacent magnet poles, and a second group including at least two circumferentially adjacent magnet poles;
each magnet pole having a reference position, the reference positions being equally spaced around a circumference of the first body;
at least one magnet pole in each of the first and second groups being a circumferentially outermost magnet pole and being located in its reference position;
at least one magnet pole in each of the first and second groups being a displaced magnet pole and being located in a position that is displaced from its reference position by an amount that is not equal to an integral multiple of the reference angular pitch of the winding slots;
an air gap between the first body and the second body; and
each winding slot at the air gap having a width substantially equal to one half of the reference angular pitch of the winding slots.