Method of magnetizing a cylindrical body

• US 6,154,352 A
• Filed: 01/22/1999
• Issued: 11/28/2000
• Est. Priority Date: 03/29/1996
• Status: Expired due to Fees

• Current Assignee: Urenco Limited
• Original Assignee: Urenco Limited
• Inventors: Atallah, Kais
• Primary Examiner(s): FLEMING, FRITZ M
• Application Number:
  US09/155,280
• Time in Patent Office: 676 Days
• Field of Search: 361/143-148
• US Class Current: 361/143
• CPC Class Codes: H01F 13/003 Methods and devices for magnetising permanent magnets permanent magnets H01F7/02
• CPC Class Codes: H02K 15/03 having permanent magnets

Abstract

In a method of magnetizing a cylindrical body comprising a substantially isotropically distributed material according to the present invention, the method comprises the steps of: applying a magnetic field to the body; and varying the magnetic field circumferentially around the body thereby to achieve at least over a part of the cylinder a generally sinusoidal magnetization circumferentially. Thus, can advantageously be achieved a sinusoidal flux density distribution for, in particular (though without limitation), rotors for electric motors in the air gap surrounding the rotor. The resultant sinusoidal back-emf waveform is ideal for high precision servo applications. In addition, when a high pole number is adopted, such bodies can be mounted on air cored rotor-hubs, which enable a substantial reduction in inertia, without substantial loss of performance.

Claims

1. A method of magnetising a cylindrical body comprising a substantially isotropically distributed material, the method comprising the steps of:

applying a magnetic field to the body; and

varying the magnetic field circumferentially around the body thereby to achieve magnetisation according to the following equation;

##EQU11## where M is the magnetisation vector, p is the required number of pole pairs and θ

m is the angle between the magnetic field direction and the radial axis of reference of the body.

2. A method of magnetising a cylindrical body according to claim 1, in which the arrangement of magnetisation is according to θ

m=(1+p)θ

for internal fields and θ

m=(1-p)θ

for external fields.

3. A method of magnetising a cylindrical body according to claim 2, in which a plurality of conductor positions has an angular distribution determined according to the solutions set out in equation (C) below of the following equations (A) and (B) for the cases in which there are an even and odd number of conductors per pole respectively:

##EQU12## in which n is the harmonic order, K is the number of conductors per half pole, J_n is the current density for the nth harmonic, α

is half the angle subtended by the conductor relative to the cylindrical axis and i is an integer.

4. A method of magnetising a cylindrical body according to claim 2 or claim 3, in which the distance d of each conductor from the body is according to the following equation (D):

##EQU13## in which R is the inner radius of the cylindrical body in the case in which the field is applied internally to the body and is the outer radius of the cylindrical body in the case in which the field is applied externally to the body, p is the number of pole-pairs and n_s the order of the lowest harmonic not eliminated by the current density distribution.

5. A method of magnetising a cylindrical body according to claim 4, in which n_s =7.

6. A method of magnetising a cylindrical body according to claim 4, in which n_s =9.

7. A method of magnetising a cylindrical body according to claim 2, in which a slotless core is provided adjacent the conductors.

8. A method of magnetising a cylindrical body according to claim 1, in which the cylindrical body has a longitudinal axis, the body comprising a reinforcing layer and a magnetic layer, the magnetic layer comprising distributed magnetic material and a binder, the magnetic layer, being provided in the form of at least one slab.

9. A method of magnetising a cylindrical body according to claim 8, in which the magnetic layer is thicker radially in a continuous radial thickness of the magnetic layer than a thickness of the reinforcing layer.

10. A method of magnetising a cylindrical body according to claim 8, in which the at least one slab is continuous axially for a substantial part of the length of the body.

11. A method of magnetising a cylindrical body according to claim 8, in which the magnetic material is distributed substantially homogeneously throughout the magnetic layer.

12. A method of magnetising a cylindrical body according to claim 8, in which the magnetic layer comprises a plurality of separate segments.

13. A method of magnetising a cylindrical body according to claim 8, in which the body comprises at least one segment defining an arc of the body, the or each segment comprising fibre-reinforced plastics material bounding magnetic filler material, the body having a longitudinal axis, the magnetic material being arranged such that in a segment at a point along the axis and at a radius from the axis, within the segment there is magnetic material at that radius throughout the arc of the segment.

14. A method of magnetising a cylindrical body according to claim 13, in which there are a plurality of segments.

15. A method of magnetising a cylindrical body according to claim 13, in which the percentage volume density of magnetic material in the segment is substantially more than 17%.

16. A method of magnetising a cylindrical body according to claim 8, in which the substrate comprises a substrate layer on which is mounted the plurality of segments.

17. A method of magnetising a cylindrical body according to claim 16, in which there is additionally provided a further layer over the plurality of segments, whereby the segments are sandwiched between the substrate and further layers.

18. A method of magnetising a cylindrical body according to claim 8, in which the substrate and/or further layers comprise a fibre-reinforced plastics material.

19. A method of magnetising a cylindrical body according to claim 8, in which the thickness of the or each segment is substantially greater than the thickness of the substrate and/or further layers.

20. A method of magnetising a cylindrical body according to claim 19, in which axially only a single segment is provided.

21. A method of magnetising a cylindrical body according to claim 8, in which the body comprises a fibre reinforced plastics material bounding magnetic filler material, an outer region of said body comprising a hoop fibre layer and a helix fibre layer.

22. A method of magnetising a cylindrical body according to claim 21, in which in an inward/outward sense from the longitudinal axis of the body, the hoop layer is inside the helix layer.

23. A method of magnetising a cylindrical body according to claim 8, in which the body comprises a substrate bearing a plurality of discrete magnetic segments each segment comprising a magnetic material and an outer fibre reinforced layer about a segment, whereby circumferentially there are a plurality of segments.

24. A method of magnetising a cylindrical body according to claim 1, in which the magnetic field generated by the body lies substantially within the outer circumference of the substrate.

25. A method of magnetising a cylindrical body according to claim 1, in which the magnetisation is according to the following equation in which the units are Amps per metre:

space="preserve" listing-type="equation">θ

.sub.H =(1+p)θ

;

M=|M| cos (pθ

)e.sub.r +|M| sin (pθ

)e.sub.θ

for an internal field and for an external field is according to;

space="preserve" listing-type="equation">θ

.sub.H =(1-p)θ

;

M=|M| cos (pθ

)e.sub.r -|M| sin (pθ

)e.sub.θ

where M is the magnetisation vector, p is the required number of pole pairs and θ

_H is the angle between the magnetic field direction and the longitudinal axis of the cylindrical body.

26. A cylindrical body magnetised according to claim 1.