Asymmetric axial permanent magnet machines having axial rotors with irregular magnets
A rotor for an axial flux permanent magnet machine is described. The machine has a stator comprising a stator housing enclosing a set of coils wound on stator bars or teeth and disposed circumferentially at intervals about an axis on the machine, and a rotor bearing a set of permanent magnets and mounted for rotation about the said axis. The rotor and stator are spaced apart along said axis to define a gap therebetween in which magnet flux in the machine is generally in an axial direction. The magnets are disposed circumferentially around said rotor and define a plurality, n, of matching sets of magnets. Each set of magnets includes a plurality of magnets, wherein said n sets of magnets on said rotor have n-fold rotational symmetry. Within a said set, the magnets have different shapes and/or relative circumferential spacings of adjacent magnets within the set of magnets are irregular.
1. A rotor for a 3-phase axial flux permanent magnet machine, the machine having a stator comprising a stator housing enclosing a set of coils wound on stator bars or teeth and disposed circumferentially at intervals about a rotational axis of the machine, and a rotor bearing a set of permanent magnets and mounted for rotation about the said axis, and wherein said rotor and stator are spaced apart along said axis to define a gap therebetween in which magnet flux in the machine is generally in an axial direction, wherein said magnets are disposed circumferentially around said rotor, wherein each magnet disposed on the rotor is a magnetic rotor pole and there are a different number of magnetic rotor poles to magnetic stator poles, wherein said magnetic rotor poles are grouped into a plurality, n (a positive integer), of matching sets of magnetic rotor poles, wherein each set of magnetic rotor poles includes two or more magnetic rotor poles and each set of magnetic rotor poles has the same number of magnetic rotor poles as the other sets, wherein all of the magnetic rotor poles in a particular set of magnetic rotor poles are equally spaced from the axis in a radial direction, wherein said n sets of magnetic rotor poles on said rotor have n-fold rotational symmetry, wherein regularly spaced circumferential positions of the magnetic rotor poles are 360/N degrees, N being a positive integer of the number of magnetic rotor poles that are equally radially spaced from the axis, and within each of said sets, at least one of the magnetic rotor poles is displaced circumferentially away from the magnetic rotor pole's regularly spaced circumferential position.
2. The rotor for a 3-phase axial flux permanent magnet machine, as claimed in claim 1 , wherein within each said set, relative angular positions of pairs of magnetic rotor poles about an axis of said rotor are displaced in opposite directions at regularly spaced positions.
3. A 3-phase axial flux permanent magnet machine as claimed in claim 1 wherein within one or more of said sets, all the magnetic rotor poles have substantially the same shape.
4. The 3-phase axial flux permanent magnet machine as claimed in claim 1 wherein within one or more of said sets, all the magnetic rotor poles have different shapes.
5. The rotor for a 3-phase axial flux permanent magnet machine as claimed in claim 1 wherein n>2.
6. The rotor for a 3-phase axial flux permanent magnet machine as claimed in claim 5 wherein n=4.
7. A 3-phase axial flux permanent magnet machine including the rotor of claim 1 .
8. A 3-phase axial flux permanent magnet machine as claimed in claim 7 wherein a ratio of a number of poles of said machine to a number of said magnetic rotor poles of said machine is 3:2, wherein said number of poles is defined by a number of said stator coils.
9. A 3-phase axial flux permanent magnet machine as claimed in claim 7 comprising two of said rotor, each with corresponding matching sets of magnetic rotor poles and each driven by said stator, wherein one of said rotors is twisted with respect to the other.
10. The 3-phase axial flux permanent magnet machine as claimed in claim 9 wherein a ratio of a number of poles of said machine to a number of said magnetic rotor poles of said machine is 3:2, wherein said number of poles is defined by a number of said stator coils.
11. A method of reducing cogging in a 3-phase axial flux permanent magnet machine, the method comprising employing a rotor with N magnetic rotor poles that are equally radially spaced from a rotational axis of the rotor, wherein the N magnetic rotor poles have a n-fold rotational symmetry (where N and n are positive integers), wherein regularly spaced circumferential positions of the magnetic rotor poles are 360/N degrees, each of n symmetric segments of the rotor comprise a plurality of the magnetic rotor poles and each segment has the same number of magnetic rotor poles as the other segments, the magnetic rotor poles in each segment being equally radially spaced from the rotational axis of the rotor, and within each of n symmetric segments of the rotor, at least one of the magnetic rotor poles is displaced circumferentially away from the magnetic rotor pole's regularly spaced circumferential position, wherein there are a different number of magnetic rotor poles to magnetic stator poles of the machine.
12. The method as claimed in claim 11 wherein n>2.
13. The method as claimed in claim 12 wherein n=4 and a ratio of stator poles to magnetic rotor poles of said machine is 3:2.
14. The method as claimed in claim 12 , employing two said rotors driven by a common stator, the method further comprising twisting one of said rotors with respect to the other by a twist in the range 1° to 20°.
15. The method as claimed in claim 14 , wherein the method further comprising twisting one of said rotors with respect to the other by a twist in the range 1° to 10°.
16. A method of shaping back emf in a 3-phase axial flux permanent magnet machine, the method comprising employing a rotor with N magnetic rotor poles that are equally spaced from a rotational axis of the rotor in a radial direction, wherein the N magnets have a n-fold rotational symmetry (where N and n are positive integers) and, N being a positive integer of the number of magnetic rotor poles that are equally radially spaced from the axis, the machine having a stator comprising a stator housing enclosing a set of coils wound on stator bars or teeth and disposed circumferentially at intervals about an axis of the machine, wherein regularly spaced circumferential positions of the magnetic rotor poles are 360/N degrees, each of n symmetric segments of the rotor comprise a plurality of the magnetic rotor poles and each segment of magnetic rotor poles has the same number of magnetic rotor poles as the other segments, all of the magnetic rotor poles in each segment being equally spaced from the rotational axis of the rotor in a radial direction, and within each of n symmetric segments of the rotor, at least one of the magnetic rotor poles is displaced circumferentially away from the magnetic rotor pole's regularly spaced circumferential position, wherein there are a different number of magnetic rotor poles to magnetic stator poles of the machine.