The perfect number of roll setsReversing stand drives
In the reversing rougher section, the slabs with a standard thickness of 220 mm arriving from the continuous caster are rolled to form strips with thickness ranging between 30 and 38 mm, depending on the desired final thickness.
Originally, ArcelorMittal Gent possessed two stands: a reversing stand for several passes and a continuous stand for one pass. "Pass" describes each passage of a slab through the stand.
In order to increase the capacity of the roughing section, we faced the decision between adding further stands or employing a single, powerful reversing stand. The latter option enabled us to decommission the existing reversing stand.
The analysis and hot strip mill departments together selected a powerful four-high reversing stand, which has two work rolls and two back-up rolls. The working rolls are driven without reduction by two variable-speed synchronous motors, each with a rating of 12 MW and a base speed of 58.5 rpm. The back-up rolls are necessary for the transmission of the rolling force.
With a stand of this type, rolling speeds of 5 to 7 m/s and rapid accelerations of up to 5 m/s˛ can be attained. As a result, the temperature drop of the rolled material is limited. The temperature of the strip at the furnaces' exit is 1,235 °C, and seldom lower than 1,100 °C at the entrance of the finishing mill. During the successive rolling passes, the temperature drops at an increasing rate, because the surface area of the rolling product and its contact with the water-cooled rolls increases with reduction.
The reduction in the number of stands substantially reduces the number of work and back-up roll sets in use, which in turn considerably lowers the costs. The rolls must be replaced at regular intervals.
One of the most important reasons to select a synchronous motor was its inertia, which is lower by a factor of four than that of equivalent DC motors, and also the lower maintenance costs.
Reversing stand drive
What torque must be delivered by the drives, and how is rolling performed?
The maximum nominal power of DC motors is limited. When power values in the order of 12 MW are required, several DC motors must be cascaded, which is disadvantageous for the dimensions and robustness of the installation.
It is also possible to place motors of larger diameter. However, this results in a much higher inertia.
The development of a vector control has enabled the torque of a synchronous motor to be controlled. Few industrial reference installations were available at that time.
When the rolling product passes between the cylinders, a compressive force is generated which is determined by the thickness reduction and by the width and hardness of the material. The hardness in turn strongly depends on the temperature, and to a lesser degree on the rolling speed.
Figure 1 (below) shows the force characteristic.
Figure 1: distribution of the rolling force in the contact zone between slab and roll. He indicates the thickness prior to passage through the roll, hs the thickness following the passage. The resultant of the forces is represented by vector F. The peak of the force distribution characteristic represents the point at which the speeds of the rolling product and the rolls are synchronized. At this point, the friction forces between the rolling product and the rolls also change sign.
Thermal cycle of the motor
Rolling is almost always performed at the maximum capacity of the motors, i.e. 25 MW per motor. An optimum pass plan is calculated for each slab which best exploits the motor capacity. The rolling rate is thus optimised.
To prevent the motors from being overloaded, i.e. to prevent the temperature of the motor windings from exceeding a defined value, it was necessary to integrate idle cycles. The idle cycle is described as the thermal cycle of the motor.
Block diagram of the principle drive concept
The control and damper winding principle
Figure 2: block diagram of the drive concept
Power factor (cos phi): 1Number of pole pairs: 6Rotor type: with flat polesFrequency (Fn): 5,85 Hz/12 HzSpeed (Nn): 58,5 tr/min - 120 tr/minVoltage between phases (Un): 5900 VCurrent per phase (In): 1212 A/2873ATorque(Cn): 1960 kNm/4457 kNm
Power supply to the synchronous motors
Rated / maximum characteristics: (Pn):
Figure 3 shows a typical power supply system for a synchronous motor.
Figure 3: block diagram of the drive concept
Details of the power supply
Power supply to the reversing stand ( cycloconverter)
This power supply type (cycloconverter) is robust and reliable. It does, however, have certain disadvantages. The table below provides a summary. Voltage source inverters (VSIs) appeared some years ago. They are used to supply very high power to the synchronous motors of reversing mills. VSIs are multilevel inverters equipped with IGBTs or IGCTs. They have been included in the table for comparison.