Electricity: electrical systems and devices – Control circuits for electromagnetic devices – Systems for magnetizing – demagnetizing – or controlling the...
Patent
1999-01-22
2000-11-28
Fleming, Fritz
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
Systems for magnetizing, demagnetizing, or controlling the...
H01F 1300
Patent
active
061543527
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to methods of magnetising cylindrical bodies and especially, though not exclusively, to such bodies for use in rotors in electrical devices.
BACKGROUND OF THE INVENTION
It is known for instance from international patent application no. WO 94/06193 to provide a magnetic and fibre material composite cylindrical rotor. WO 94/06193 discloses a composite cylindrical rotor with helically wound tows in a plurality of layers and magnetic material in an epoxy resin in gaps between the tows.
The use of composites can provide a material of relatively high Youngs Modulus (about 120 to 350 GPa compared with about 200 GPa for steel) but with a far lower density (about 1600 kg/m.sup.3 compared with 7800-8000 kg/m.sup.3 for steel). Therefore, for fixed rotor dimensions the stiffness and strength to weight ratios of the composite are 3 to 9 times those of steel. This permits high speeds and faster acceleration.
It is desirable to use such a rotor in eg motors, but so far it has been difficult to achieve the necessary magnetisation distribution, in particular for applications for which multiple poles are desirable.
It is an aim of preferred embodiments of the present invention to overcome or obviate disadvantages encountered with versions of the prior art, whether referred to herein or otherwise.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention, there is provided a method of magnetising a cylindrical body comprising a substantially isotropically distributed material, the method comprising the steps of: achieve at least over a part of the cylinder a generally sinusoidal magnetisation 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.
Suitably, the magnetic field varies substantially continuously circumferentially around the body.
Suitably, the step of applying a magnetic field to the body comprises the steps of: and magnetising magnetic field.
The plurality of conductor positions enable an arrangement of magnetisation according to .theta.m=(1+p).theta. for internal fields and .theta.m=(1-p).theta. for external fields.
Suitably, the plurality of conductor positions are discrete.
Suitably, the current density omits at least one low order harmonic, desirably the 3rd order harmonic, preferably the 3rd and 5th order harmonics and more preferably the 3rd, 5th and 7th order harmonics.
Suitably, the body is magnetised such that it exhibits a generally sinusoidally varying field in the internal or external gap around the body.
It is almost impossible to generate an exact sinusoidal magnetisation distribution according to, for instance for an internal field .theta.m=(1+p).theta. and the present invention provides an advantageous approximation thereto.
The conductor positions can be located at specified angular positions to achieve the desired magnetising field distribution throughout the cylindrical body. Suitably, the angular distribution of the conductor positions is 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: ##EQU1## in which n is the harmonic order, K is the number of conductors per half pole, J.sub.n is the current density for the nth harmonic, .alpha. is half the angle subtended by the conductor relative to the cylindrical axis and i is an integer.
Suitably, the distance d of each conductor from the body is according to the following equation (D): ##EQU2## in which R is the inner radius of the cylindrical body in the case in which the field is applied inter
REFERENCES:
patent: 3366811 (1968-01-01), Wullkopf
patent: 5204569 (1993-04-01), Hino et al.
patent: 5280209 (1994-01-01), Leupold et al.
patent: 5424902 (1995-06-01), Shida et al.
patent: 5557248 (1996-09-01), Prochazka
Fleming Fritz
Urenco (Capenhurst) Limited
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