Due to the global economic situation in general and the Chinese boom in particular, the steel sector is facing high demand for sheets and semi-finished products made of steel.

Therefore, the cranes at Dillingen, Germany-based rolled steel producer Dillinger Hütte have to run around the clock at full utilisation.

The sheet handling cranes were recently subjected to a retrofit within which the complete electrical equipment was replaced by new industrial controls.

As these cranes are applied in the hot area, contactor technology was preferred for the motor controls over converter technology as this concept allowed for the abolishment of the electronics room’s thermal insulation and air-conditioning, while also reducing conversion-induced downtime and costs.

Köhler Elektrotechnik of Völklingen-Geislautern, Germany took on the planning and realisation of the retrofitting process in accordance with specifications of the new construction department at Dillinger Hüttenwerke.

“The key requirement of the project was to implement a rugged and durable concept, which, at the same time, facilitated an extremely short commissioning period,” says Stefan Jammas, of the technical department at Köhler.

Two so-called magnetic travelling cranes, which convey the heavy sheets in the plate rolling mill, were equipped with new industrial controls.

Subsequently, the production cranes, which are also operated around the clock, were retrofitted. The portal cranes’ drives for horizontal and vertical movements are driven in any direction at four different speeds. The switch-over is realised by 3RT12 vacuum contactors from the Sirius product range by Siemens.

These devices were especially developed for applications which are subjected to disproportionately high switching frequencies paired with extreme ambient conditions.

Stefan Jammas, of Köhler, says: “The manufacturer offers a mechanical service life of 10 million switching cycles for these devices, which was the major argument in favour of their selection for this project.”

The Sirius vacuum contactors are available up to 250kW, a power rating which was almost completely utilised by the cranes. The magnetic travelling cranes, which originally date back to 1973, transport sheets which weigh up to 50t and feature lengths of up to 40m.

In contrast to the conventional 3RT10 contactors, the main contacts of the vacuum contactors do not switch in the air under atmospheric conditions, but inside hermetically enclosed vacuum switching tubes.

This prevents the formation of open arcs and switching gases during the switching process, thereby automatically prolonging the maintenance intervals. As no minimum distances to grounded parts are required, the complete circuit can be assembled compactly.

A further advantage of the employed vacuum contactors is that they no longer have to be controlled with 230 V AC as with the previous solution, but they can also switch with 24 V DC.

“We clearly prefer the new solution,” says Jammas. This concept not only does away with the complete coupling line between the PLC and the contactor. If 230 V AC were applied for switching, either PLC relay cards would have to be used or an additional coupling relay, which would not only be more expensive, but would also result in additional space requirements on the control panels and additional wiring expenditures.

The 3RT vacuum contactors are generally designed for two control options. Via a DIP switch, the user can select between AC control in the range between 40 to 60 Hz or DC control.

Prior to deciding in favour of this new control philosophy, both the electric fitter and the end user wanted the concept to be directly tested on the crane. Jammas says: “The test went perfectly and cleared the way for a unanimous and immediate decision.”

Parallel installation

Not only is the aspect of control interesting with this project, so is the retrofitting method. As the cranes are in continuous use and, moreover, the electrical equipment was not to be accommodated in separate control cabinets, time had to be saved in a different way.

The complete control technology was installed on wall-mounted control panels inside the E-girders, which serve as the portal cranes’ cross beams. The new control was also completely assembled on control panels on the opposite wall during ongoing operation.

This was possible as the up to 35m long closed girder profiles feature a height of approximately 2.5m and a width of roughly 1.5m, thanks to which they are accessible. However, the installation on the driving cranes demanded a high resistance to so-called “rough sea” conditions from the technicians.

The installation conditions of this site were further aggravated by the fact that vast, still glowing sheets are frequently conveyed in this area, whose ascending heat produces temperatures between 30 to 50 degrees C inside the cross beams in which the control had to be assembled.

The temperature-resistant design of the 3RT contactors could withstand these conditions. The Simatic S7 control with the CPU 315 DP was “stowed away” in an air-conditioned control cabinet. However, to be on the safe side, special withdrawable cards were also selected for the component to allow for an increased temperature range.

During the actual conversion phase, within the scope of which only the existing slip-ring induction motors were left in place, the Köhler electricians completely “de-cored” and re-installed the cranes within only 14 days. They also removed all electrical cables.

To reduce the wiring expenditures, all cables were pre-wired on the bottom of the new cable drag chains. Using truck-mounted cranes, these “complete packages” could then be lifted and affixed, after which the wiring was connected to the new control inside the E-girder.

Operator panel

To be able to document the actual load of the complete system in practical application for the first time, the control was supplemented by an additional OP27 operator panel by Siemens. This panel allowed for the visualisation of specific processes and conditions. The Köhler experts, for example, programmed a PLC counter, whose values were displayed on the OP27. This way, all plant technicians could immediately detect the number of the vacuum contactors switching cycles to assess their loading.

For this purpose, one auxiliary contact of the vacuum contactors was connected to one counter input. The 3RT devices can be laterally equipped with up to eight such auxiliary contacts in blocks of size S0 to S12. Up to four contacts may be used as NC contacts.

Either a conventional or electronic drive can be selected for the control concept. The advantage of the electronic variant lies in the magnet coil’s targeted supply with the required power for safe switching and stopping via upstream control electronics.

For maintenance, a fast coil replacement is, of course, advantageous. With the 3RT vacuum contactors, the magnetic coil can be simply drawn out upwards without tools by operating the unlocking mechanism to be replaced by any other coil of the same size.

The practical aspect underlines the significance of technology for the faultless operation of rolling mills. So-called maintenance days are only scheduled every two to three months at the Dillingen plant, when the complete rolling mill is shut down and all maintenance is carried out on all systems in just one shift.

Following this, sheet production is continued immediately.