Repair of hoisting magnets
Whether for scrap loading and sorting, for handling coils, slabs, billets and other raw pieces, hoisting magnets are predominantly used in steel processing and steelmaking. They are used for different purposes. The Vogelsang Elektromotoren GmbH in Bochum has been repairing electromagnetic machines, such as hoisting magnets or magnetic separators, for many years. The production range of the company, which has its own coiling department and welding shop, offers special advantages, which can be demonstrated by the example of the repair of a winding damage.
The magnets are particularly stressed during use by the harsh conditions of operation and due to the thermal load of the magnet coil supplied with current. Frequent switching operations or breaking away of cables leading to undesired voltage peaks are also load factors. In addition to the influences due to operation, in many cases there are also thermal loads caused by the transport material itself. The hot magnets are particularly designed so that the radiant heat of the material handled is screened off from the electrical part of the magnet as far as possible. Depending on the application, round or cornered magnets are used. For coil handling there are magnets with mobile poles to improve the supporting surface of the magnetic pole. Since hoisting magnets and magnetic separators are electromagnetic machines, the repair has been part of the service portfolio in the Bochum-based works of the specialist for industrial repair for many years.
As regards the design, hoisting magnets are a combination of magnetic and non-magnetic components. It is important for the quality to select a material combination so that the magnetic flow is optimally guided to achieve an optimum load-carrying capacity. Furthermore, the magnet must be a robust device with a long service life, but also with a low own weight to make optimum use of the load carrying capacity.
Magnets exist as welded or cast units. It is not often possible to find a magnet coil with copper windings, but frequently with aluminium profile wire windings, and more and more with aluminium strip windings. They offer the best space-weight-ratio, are particularly insensitive to heat and, meanwhile, they represent the majority of magnet coils. During repair these boundary conditions have to be restored or, if possible, even optimized. The mechanical possibilities of the lathe and welding shop of Vogelsang Elektromotoren GmbH as well as the special expertise of the coiling department in the electrical division offer special advantages to the customers regarding the repair of hoisting magnets (Figs. 1 and 2).
The main cases of damage to hoisting machines that are repaired in Bochum are:
1. Mechanical damage to the suspension devices, to other movable parts and to the terminal boxes;
2. Electrical failure due to overvoltage or thermal overload as well as
3. Used antiwear linings.
In the case of damaged windings, the repair steps are generally similar depending on the individual damage. It is demonstrated by the following example. After measuring the electrical input, the magnet is ripped to get to the coil inside. In the case of round magnets with a diameter of up to 2000 mm, a vertical turning and boring mill is used. Rectangular magnets are ripped as on the boring mill.
With this work, special attention should be paid to remove as little material of the magnet as possible and only to rip the weld. After dismantling the antimagnetic cover plate (frequently of manganese), the sealing material consisting of silicone and sand is dismantled in the case of cast coils.
When the coil has been laid open, it can be dismantled from the casing. If the coil is still operational, attention must be paid that it is not damaged. However, in most cases this is not possible so that the coil has to be replaced. In particularly intractable cases it may be necessary to treat the magnet in the pyrolysis furnace to be able to dismantle the coil from the magnet casing.
In cases with profile wire winding, the winding is converted into an aluminium strip winding so as to achieve an improvement of the magnet simultaneously with the repair. Now the newly wound coil is inserted in the magnet and fixed in the casing by means of a corresponding sealing compound. Then the cover plates are welded again and the magnet is lined again for wear protection.
At the same time, damaged lifting rings and fastening points are replaced in the welding shop. Terminal boxes are repaired. After the repair, the magnet is tested in the test department during a heating test and the lifting force is checked by means of a crane balance. After finishing this work, the freshly painted magnet is supplied to the operator to be re-mounted. As a rule, it can be operated reliably for many years after this type of repair (Fig. 3). For further information please contact:
