MAGNET LIFTER

Invention relates to a magnet lifter comprising a lifting tool, which can be magnetized and demagnetized with the help of a coil and electrical current directed through it, an electrical generator forming magnetization current, a hydraulic motor for rotating a generator, a rectification arrangement forming a rectified magnetization current, and a control unit for adjusting electrical functions.

Previously a magnet lifter used with the help of a hydraulic pressure is known from a Japanese patent publication JP9100083 in which publication a hydraulic motor and a generator are located on top of a magnetic tool. Magnetization is performed by starting up the hydraulic motor and demagnetization is performed by shutting down the hydraulic motor. Residual magnetism is not removed from the magnetic tool so that the ablation of the smaller items from the tool cannot be ensured.

Further the removal of the residual magnetism with the help of an alternating current is described in the Japanese patent publication JP 9086855. The alternating current is rectified during the magnetization. Demagnetization is done by passing the rectifier and by using alternating current with decreasing amplitude. Decreasing is controlled with the help of a computer.

The disadvantage of the solutions described above is the slow removal of the residual magnetism or that it does not even become removed, like in the JP9100083 -solution. Also during the demagnetization caused by alternating current electrical power has to fed all the time. With this arrangement the removal of the residual magnetism cannot be reached as fast as possible.

In order to remove the disadvantages of the above mentioned solutions and in order to achieve an efficient and fast removal of the residual magnetism, a new magnet lifter has been developed to which it is characteristic that a capacitor belonging to the rectification arrangement is coupled in such a way that the power it stores can be discharged in the demagnetization situation through an inverter coupling coupled between the rectifier and the coil in order to form demagnetization current in turn directed to both directions.

The advantage of the invention is the fact that residual magnetism can quickly be totally removed also when the generator is being shut down. Capacitor that has stored energy can be discharged at the demagnetization stage and its charging can be formed to be demagnetizing alternating voltage. The alternating voltage can be formed at the desired frequency and the excess power can be directed to the discharging resistance.

Common cooling can be arranged for the generator and the rectifier and a hydraulic motor, a generator, a rectification arrangement and an electronical unit and discharging resistance can all be located to the magnetic tool. Further the electronical unit located between the generator and the coil can easily be formed to be a modular section, such as for example a circuit board unit that can be coupled parallel in order to multiply the magnetization current that can be directed to the coil, if larger generators are used.

In the following the invention is described more detailed by referring to the accompanying drawing in which

Figure 1 shows the coupling of the magnet lifter as a diagrammatic view. Figure 2 shows the general coupling as a diagrammatic view.

A coupling according to the invention is shown in the figure 1 in which a three-phase voltage Ll, L2, L3 comes from the generator to the rectification arrangement 5. The rectified voltage ends up in + and - tracks L4 and L5. The capacitor 6 that is related to the rectification is also connected between these tracks. Magnetization current is not directed directly to the coil 2 of the magnetic tool 1 from the tracks L4 and L5, but it is directed through a H-bridge connection 7 to the coil 2. Through the H-bridge connection magnetization current can be directed both as direct current to the coil 2 and as frequency adjusted alternating current. The control unit 9 comprises adjusting devices and the measurements of the voltage and the current. As an adjusting device it includes the ^* controlling to the H-bridge connection 7 in such a way that rectified current is directed to the coil 2 at the magnetization stage. At the demagnetization stage it directs the H-bridge connection 7 aκ(d the limiting coupling of the voltage in su^h, a yrøy that the H-bridge connection 7 conyeys alternating current to the coil 2 at the desired frequency and also with the desired voltage.

A line 11 is directed from the measuring, direction and the adjustment section 9 which line is a magnetization line for the H-bridge connection 1, the line 12 is a demagnetization line and the line 13 is a limiting line of the voltage and the control of the power for the resistance 8. In the diagram the measuring of the voltage is shown with the sign U and the measuring of the current is shown with the sign I.

A voltage delimitation arrangement comprises the measuring of the voltage between the tracks L4, L5, a switch 10 through which the charging of the capacitor 6 or the inductive residual energy left in the magnet can be directed to the discharge resistance 8 and a voltage delimitation arrangement further comprises a control system for this function in the control unit 9. This arrangement functions mainly when the magnetization is being switched off and when the residual magnetization is being removed when the H- bridge connection 7 is at the frequency adjustment stage.

A hydraulic motor 4 and a generator 3 are also shown in the figure 2. Cooling current 15 from the section 14 is directed to the generator 14. The placement of the additional sections 14.1 - 14.3 (circuit boards) is shown in the figure in a diagrammatic way. One such modular section 14 can be for example a section that produces approximately 10 A magnetization current.