Electromagnetic tape stabilization
The electromagnetic strip stabilization is a system for the stabilization of rapidly passing through ferromagnetic metal strips such as iron or steel foil by means of dynamically-controlled electromagnet . The main area of application for electromagnetic strip stabilization is in hot-dip galvanizing plants for continuous strip galvanizing .
Basics
During the hot-dip galvanizing process , the strip leaves the weld pool with a zinc coating that is still liquid, depending on the strip speed. The tape is then passed through the blow-off nozzle. There, the excess zinc is blown off evenly over the entire width and on both sides with air or nitrogen under pressure in order to achieve a specified layer thickness and a homogeneous surface.
The quality of the process depends on the distance between the nozzles and the belt. The closer these are, the more accurate the process of blowing off. However, a minimum distance between the nozzle and the belt must be taken into account in order to prevent the blow-off nozzle from touching the belt passing through and thus avoiding damage.
Due to the properties of the system, such as the generation of heat and the material of the belt, the belt may be deformed in waves across its width. In addition, deflections from the ideal pass line or vibrations of the belt that superimpose the entire process are possible. These disturbances have a negative effect on the distance between the nozzle and the strip material - it must be increased. The process is no longer ideal and the zinc layer is inhomogeneously distributed. In order to be able to guarantee a minimum zinc layer thickness at every point on the strip, the strip must be "over-galvanized". The zinc layer is distributed unevenly on the material and more zinc must be applied than is ideally required.
This can be remedied by additional stabilization of the continuous belt. The conventional solution of calming the strip using guide rollers cannot be used for hot-dip galvanizing because of the liquid zinc layer. Even increasing the belt tension further has its limits and does not lead to the desired result.
This is where the electromagnetic strip stabilization process comes into play. To do this, an array of electromagnets is mounted on both sides of the belt as close as possible above the nozzle. These counteract the deformation dynamically and without contact. They dampen vibrations and keep the belt on the optimal passage line. This significantly improves the homogeneity of the zinc coating over the strip width and length. In addition, the distance between the nozzles and the belt can be reduced and the process improved.
General structure and function
The electromagnets are attached in pairs opposite one another on both sides of the belt (see schematic diagram of electromagnet pair). One pair each covers a certain width of the band. The number of magnet pairs required therefore depends on the respective bandwidth.
Pole shoes on both leg ends of each electromagnet ensure a homogeneous, large-area distribution of the magnetic field (see schematic diagram of the pair of electromagnets). This achieves an even transmission of force to the sheet metal and prevents premature, locally occurring, magnetic saturation on the strip.
A non-contact sensor for determining the distance to the belt is mounted on each electromagnet (see schematic diagram of the pair of electromagnets). This distance is measured over the entire width for each pair of electromagnets and serves as the basis for regulating the irregularities of the belt passing through. The electromagnets exert a tensile force on the belt when they are active.
If the tape is at a point between a pair of electromagnets outside of the center position, the further away magnet becomes active and "pulls" the tape back into the ideal position at this point. By arranging the pairs of electromagnets over the entire width of the belt, it is possible to dampen the vibrations, to compensate for deformations of the belt and to regulate it in the ideal central position.
Mode of action
The picture shows an example of the mode of operation of the arrangement of the electromagnets with an S-shaped deformation of the continuous belt. Here the sensors 1A, 2A and 4B and 5B are activated to compensate for the deformation. Pair 3 remains inactive because the belt is already in the target position there.