Electricity: electrical systems and devices – Control circuits for electromagnetic devices – Systems for magnetizing – demagnetizing – or controlling the...
Reexamination Certificate
1999-08-04
2001-03-20
Fleming, Fritz (Department: 2836)
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
Systems for magnetizing, demagnetizing, or controlling the...
C361S139000, C361S156000
Reexamination Certificate
active
06205012
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for altering the magnetic state of a permanent magnet and particularly, though not exclusively, to a magnetiser or demagnetiser for use with highly permanently magnetic Rare-Earth Transition Metal magnets such as Nd—Fe—B or Sn—Co based magnets
FIELD OF THE INVENTION
Demagnetisation of ferromagnetic components is often necessary in industry to facilitate handling or coating of the components. In addition, demagnetisation also prevents unwanted pick up of magnetic debris.
One way of achieving demagnetisation is to heat the components to a temperature above their ferromagnetic Curie temperature; on cooling back down to below the Curie temperature, the permanent magnetism is lost. This is a costly, time-consuming process which is not suitable for many materials due to corrosion problems or for an assembly containing plastics material, for example.
Demagnetisation of a ferromagnetic component may also be achieved magnetically by applying successively smaller opposing magnetic fields to the magnetised component so as to drive the component around successively smaller magnetic hysteresis loops, until the component is demagnetised. For materials which are only slightly permanently magnetic, such as mild steel, the magnetic demagnetisation may readily be achieved by slowly withdrawing the component from the centre of a magnetic field generated by a mains driven A.C coil inductor.
For materials which are more permanently magnetic, such as harder steels, and ferrite and Alnico permanent magnets, a single shot “ringing” capacitor discharge demagnetiser is used. capacitor discharge magnetisers work by discharging a charged bank of capacitors through a coil inductor thereby producing a magnetic field which magnetises the component. Conventional capacitor discharge demagnetisers work on a similar principle, but the demagnetising circuit is designed such that on discharge, a decaying resonance or ringing occurs, with electromagnetic energy transferred successively between the coil inductor and the capacitor bank. This ringing phenomenon, combined with the natural loss of energy associated with coil inductors, ensures the generation of a reversing magnetic field of decaying amplitude which demagnetises the component.
There are many difficulties with magnetically demagnetising the most permanently magnetic materials such as Rare-Earth Transitional Metal magnets based on Nd—Fe—B or Sm—Co. Use of a single-shot ringing demagnetisation circuit is not possible for these magnetic materials because any such circuit would not ring with sufficient efficiency, that is with a high enough Q-factor, at the high power levels required for these materials. At present, the only way of magnetically demagnetising Rare-Earth Transition Metal permanent magnets is to apply about 20 or more magnetic pulses of reversing sign and decreasing amplitude with a conventional capacitor discharge demagnetiser. After the discharge of each pulse, the operator has to wait for the capacitors to recharge up to the new level and has to reverse the connections to the demagnetising coil. This is a very time-consuming procedure and is not practicable in an industrial environment.
SUMMARY OF THE INVENTION
It is desired to overcome the above-mentioned problems and to provide an apparatus which is capable of altering the magnetic state of a permanent magnet in an efficient, controllable and relatively quick manner.
According to one aspect of the present invention there is provided an apparatus for altering the magnetic state of a permanent magnet, said apparatus comprising: a magnetic field inducing device for generating and applying an induced magnetic field to said permanent magnet, said device being provided in circuit between two charge storage elements; and means for transferring charge alternately in opposed directions between said storage elements through said magnetic field inducing device to generate a series of alternating polarity magnetic field pulses of decreasing magnitude in said device.
Preferably the apparatus is arranged to demagnetise a column of Nd—Fe—B permanent magnets, for example 200 or more magnets, in a single operation. This can be achieved by the magnetic field inducing device being a coil inductor which is long enough to accommodate the column of magnets. The uniform demagnetisation or a column of permanent magnets is considerably more difficult than the demagnetisation of a single magnet. This difficulty is due in part to the differences of the degree of permeability at the ends of the column as compared with the middle of the column. However, the present invention advantageously overcomes these problems and permits the demagnetisation of relatively large numbers of permanent magnets in a single operation.
Preferably the transferring means is also arranged to discharge charge stored in the storage elements into the magnetic field inducing device to generate a single magnetic field pulse of sufficient amplitude to magnetise the magnet. The apparatus may also be arranged to connect together both of the storage elements to provide a single charge storage means which has a greater charge storage capacity than either of the individual charge storage elements. In this way, the apparatus can advantageously be arranged to carry out both magnetisation and demagnetisation in a fast and efficient manner.
Preferably, the apparatus further comprises adjusting means for comparing the amount of charge present in the storage elements with a predetermined set level and for adjusting the amount to be equivalent to the set level between each charge transfer. The provision of adjusting means advantageously allows the charge received by a storage means to be topped up to a predetermined set level before the next charge transfer. Accordingly, the size of the decreasing envelope of magnetic pulses can be accurately controlled and, in particular, the amplitude of step size between successive magnetic pulses can be set by the operator. The step size is important because if it is too large, the magnet, will be left with an undesirable residual magnetism after the demagnetisation procedure, and if the step size is too small, the demagnetisation procedure will take too long and not provide an industrially practical solution.
The apparatus may be arranged to commence each operation for altering the magnetic state of the magnet from a different storage element to that used in the previous operation. By alternating the starting storage element, the working life of the storage elements is advantageously maximised.
The magnetic field inducing device may comprise a plurality of individual magnetic field inducing devices, such as coil inductors, which are arranged to be selectively connected into circuit after each operation for altering the magnetic state of the magnet. The provision of several magnetic field inducing devices advantageously reduces the time period between successive demagnetisation or magnetisation operations which would otherwise be required for the magnetic field inducing device to cool down between operations.
According to another aspect of the present invention there is provided an apparatus for changing the magnetic state of a permanent magnet to a desired magnetic state, said apparatus comprising: means for charging a first charge storage element to a predetermined level; means for generating a magnetic field pulse by discharging said first storage element into a second storage element via a magnetic field inducing device; means for generating another magnetic field pulse of a different polarity and a different magnitude than that of said previous pulse by discharging said second storage element into said first storage element via said magnetic field inducing device; said generating means being arranged to be operated alternately to provide a series of alternating polarity magnetic field pulses of decreasing magnitude in said device.
According to another aspect of the present invention, there is provided an apparatus for demagnetizing a permanent magne
Fleming Fritz
Pollock, Vande Sande & Amernick, R.L.L.P.
Redcliffe Magtronics Limited
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