DC motors are used to drive the Rolls in cold rolling mills. One of the main reasons of their breakdown is the breaking of armature risers. Solutions are sought to mitigate these failures
We produce Tinplated sheets by cold rolling followed by electrolytic tin plating. In the Cold Rolling Mill (CRM) the thickness of the coil is reduced to the desired specifications. CRM is a reversing mill in which the thickness of the coil is reduced through multiple passes. In this mill there are:
The function of these rolls is:
Typically coil with an input thickness ranging from 1.6mm to 3.2mm is reduced to a thickness in the range of 0.18mm-0.32mm in this mill. A coil is passed 6 to 7 times through these rolls to get the desired reduction in the thickness. Basic rolling takes place by putting the sheet in tension by pulling it from both sides (reversing reel 1 & 2/ pay-off reel) and then applying roll force with the help of the back-up rolls.
The work rolls are driven by two motors, while the other roles i.e. intermediate rolls and backup rolls are friction driven as shown in fig 1.
These 2 motors called Mill Stand Motor # 1 and # 2 (termed as MSM-1 and MSM-2) are coupled and they together drive the Work Rolls through gear box (see fig 2). Both the motors are of same capacity, 1925 KW, for more details refer table 1.
There are 4 motors in total of which 2 are in continuous operation and 2 are kept as backup. When one gets damaged, it is replaced by a spare one. Breakdown of the motors happen because of many reasons like:
We are able to identify and fix most of these problems except for the Riser failure faults, which occur due to breaking of the risers. When this problem occurs, heavy electrical sparking under the carbon brushes starts and we are unable to run the motor. Hence, we have to stop the mill completely. This adversely affects our productivity.
We have tried many methods to identify the onset and prevention of the breakage of Risers like:>
but none of these yielded good results. We are therefore, inviting solutions that will enable us to identify the onset and take corrective actions to prevent breakage of risers. We are also seeking solutions that will facilitate us to fix the broken risers in our motors permanently.
The solution must meet the following criteria:
Further information about the electrical configuration of motors, circuit breakers etc. is as below.
Both the motors (MSM-1 & MSM-2) are controlled by individual AVTRON DC Digital drive and converters. However, they work in Master-Slave configuration. MSM-1 drive acts as Master and MSM-2 as Slave. The picture of motors along with Tacho and other equipment is shown in the fig 2. Two high speed Circuit Breakers that are interlocked together, one for each drive, are used to trip the armature current, in case of over current. It means breakers of MSM-1 and MSM-2 drive pick up together (motorized) and if one trips or opens because of any reason, the other one also trips or opens.
To feed the armature of the motor, two converters are operated in parallel in order to cater to high current requirements of the motor. The armature converter firing is directly controlled and generated by AVTRON drive. Since it has to fire two parallel converters, a Pulse Distribution Amplifier is used which has one input and two outputs for each of the twelve pulses. It provides the amplification of the firing pulse. Isolation is provided by Pulse Transformer card which is provided before Gate-Cathode connection of each thyristor.
The Armature converters are fed from 6000 kVA Converter transformer with two Secondary windings – one connected to MSM-1 and another to MSM-2. Two secondary windings belong to DY11 vector group i.e. one of the secondary is Delta connected and another Star connected. They have a phase difference of 30 degrees. The AC incoming to Armature converters are tapped and sent to Drive for sensing the incoming voltage, frequency and phase. Because of high voltage, the voltage is stepped down using synchronizing transformer (3 nos. – 1 no. in each phase) and then dropped further to around 2 Volts using feedback processing board which comprises of potential divider network.
To get information about the main components of a DC motor and their function you may refer to the following link: http://www.electricaleasy.com/2012/12/basic-construction-and-working-of-dc.html.