Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Work or object holding type
Patent
1998-11-30
2000-08-15
Donovan, Lincoln
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Work or object holding type
335291, H01F 720
Patent
active
061042706
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to magnet lifters, and particularly to a lifter with electropermanent magnets provided with a safety device for controlling their working point.
As it is known, lifters are divided into three classes depending on the type of used magnets, i.e. permanent magnets, electropermanent magnets and electromagnets. Each lifter type has its own advantages and drawbacks.
The lifters with permanent magnets have the advantage of an almost negligible power consumption and of a produced magnetic force which is reliably constant and independent of outer supply sources. On the other hand, it is not possible to increase the magnetic force if necessary and the magnets are exceedingly bulky for lifting heavy loads. Furthermore, the load detachment requires the application of a considerable mechanical power amount in order to reduce the magnetic force to a value smaller than the load weight. Alternatively, the magnets are to be made movable so as to be moved away from the load, thus decreasing the magnetic attraction.
On the contrary, in the lifters with electromagnets it is possible to freely vary the magnetic force by simply adjusting the current flowing in the windings which generate the magnetic field. However, any breakdown, even if very short, of the power supply immediately cancels the magnetic force and thus causes the load detachment. It is therefore evident that safety systems ensuring the supply continuity are essential.
The lifters with electropermanent magnets substantially combine the advantages of the two aforementioned lifter types. This is due to the use of a permanent magnet of the reversible type, i.e. a magnet wherein the polarity is easily reversible through the application of an electric impulse. The reversible magnet thus generates an adjustable flux which also can direct the flux of a conventional permanent magnet combined therewith. It is thus possible to short-circuit the two magnets when the lifter is to be deactivated, or arrange them in parallel for activating the lifter. Since just an electric impulse but not a continuous supply is needed for reversing the reversible magnet, the safety problems affecting electromagnets are overcome. At the same time, even though permanent magnets are used, it is possible to vary the magnetic force within some limits, and the load detachment is easy to carry out with a minimum power consumption and without complex structures for moving the magnets.
However, the electropermanent magnets, with respect to the other two types of magnets, have a drawback in the working instability due to the particular magnetization curve of the reversible magnet. In fact the reversible magnets are usually made of an aluminum-nickel-cobalt (alnico) alloy having an hysteresis characterized in that a high induction corresponds to a reduced coercive force. This characteristic allows to direct the magnetic flux in the permanent magnet forming the electropermanent magnet.
However, the magnetization curve has a "knee" beyond which the behavior of the reversible magnet is still linear, but much more sloped than in the first region. This involves great induction variations corresponding to small coercive force variations. Practically this means that the lifters with electropermanent magnets are greatly affected by the dynamics of the lifted material. It is in fact known that the oscillations of the plates lifted by such a unit involve a variation of the air gap, and accordingly a variation of the total reluctance of the magnetic circuit, which may shift the working point of the magnetic masses of the lifter below said "knee". This dynamics is affected even by little shifts of the lifted material and thus slight bendings or hardly detectable curvings are enough to cause a considerable variation of the magnetomotive force thereby making the lifting system very unstable.
In the known lifters there is a system for measuring the magnetic force which is the same for any type of magnets. This measuring system only serves to calculate the safety operative factor by compari
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patent: 4237455 (1980-12-01), Beckley et al.
patent: 4910633 (1990-03-01), Quinn
patent: 4956625 (1990-09-01), Cardone et al.
patent: 4965695 (1990-10-01), Baumann
patent: 5642089 (1997-06-01), Lysen
Donovan Lincoln
Railfix N.V.
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