Claims:1. A system for controlling cooling bed roller table motors of rolling mill comprising
a PLC processor with distributed input-output nodes;
AC drives with built-in drive to PLC processor communication means for facilitating centralized cooling bed roller table motor speed control and monitoring to ensure higher speed of roller table speed control based on rolling regime;
MCC panels operatively connected with each of the cooling bed roller table motors for protecting the motors from anyone or more from single phasing, earth fault and trip;
power distribution unit to independently control power for AC drives, PLC processor, input-output nodes and the MCC panels.

2. The system as claimed in claim 1, wherein the cooling bed roller table is divided in four groups, each group is having equal number motors and each group includes at least one AC drive having variable frequency drive (VFD) operatively connected to each motors of the group through motor cable to control speed of the motors.

3. The system as claimed in anyone of the claim 1 or 2, wherein the VFDs of each of the group are networked to constitute a VFD control circuit and programmed for centralized cooling bed roller table motor speed control and monitoring.

4. The system as claimed in anyone of the claims 1 to 3, wherein the VFD comprises sine wave filter at its output in the motor cable between the VFD and the motors to protect the motors from voltage reflection and other localised heating.

5. The system as claimed in anyone of the claims 1 to 4, wherein the MCC panel includes all motor protection circuitry including single phasing, earth fault and trip to protect the cooling bed roller table motors from damages.

6. The system as claimed in anyone of the claim 1 to 5, wherein the distributed input-output nodes includes remote I/O (RIO) nodes selectively installed in the rolling mill for facilitating the centralised roller table motor speed control and monitoring.

7. The system as claimed in anyone of the claims 1 to 6, wherein the PLC processor is configured to implement soft logics for all the operations and integrated operation of cooling bed roller table motors speed control and also provided with features of on-line diagnostics, flexibility in logic changes, modular architecture.

8. The system as claimed in anyone of the claim 1 to 7, wherein the power distribution unit is implemented with proper isolation and protection to enable the VFDs to operate independently and free from finishing mill bus bar power supply to avoid dependency of the cooling bed roller table motors from the finishing mill power supply.
, Description:FIELD OF THE INVENTION:
The present invention relates to improve controlling of motors of roller table of rolling mills of steel processing plant. More specifically, the present invention is directed to develop a control system for improved control of the group motors of the roller table of the rolling mills comprising of cooling bed roller table motors operated from an operator desk at Control Pulpit and driven by electrical systems. The present control system advantageously ensures higher speed of roller table speed control based on the rolling regime thus facilitating an increase in productivity.

BACKGROUND OF THE INVENTION:
The cooling bed roller table of the rolling mill of the steel processing plant is a continuous mill comprising of multiple stands for reduction of billets to different sizes and shapes. The mill is designed for rolling and finishing of different TMT bars.
The cooling bed roller table of the rolling mill is very critical for running of the mill operation since this is the common equipment for cooling of rolled product. Therefore, availability of this equipment is must for cooling bed operation and thus the mill operation.
The cooling bed roller table of a typical rolling mill normally consist of various mechanisms like Kick off, carry over, shuffle bar eccentric, shuffle bar lifting, and common run on roller table etc. A typical cooling bed roller table normally consist of 120 motors, which are divided into four groups each having 30 motors. The existing control system for 120 motors comprises exciter set having DC generator, AC Squirrel Cage Induction Motor, Variable Frequency Alternator (VFA) and its DC drive motor, and associated relay logic comprising 100 odd relays and speed transmitter.
A preferred layout of the typical merchant mill is given in the accompanying figure 1, whereas a preferred layout of the cooling beds is shown in the accompanying figure 2. The block diagram and interconnection of the existing system for controlling of the motors of the typical cooling bed roller table is shown in the accompanying figure 3.
The existing control system is based on DC generator, AC Squirrel Cage Induction Motor, Variable Frequency Alternator and its DC drive motor. This existing control system includes hardwired relay logic circuits and interconnections having large number of power and control cables running from machine hall to fields.
All the interconnections among the components of the control system are prone to fire hazards in the congested cable galleries and earth faults which disrupt production process and contribute to mill downtime in a major way. The hardwired relay logic circuits of the existing control system includes relays as control and logic devices, and insulated wires for the interconnections between the relays. This is time-consuming to set up and fault-finding due to involvement of unreliable large number of electro-mechanical contacts and interconnections. Also, the relay logics lacks the state of art on-line monitoring and fault diagnostic facilities which results tedious maintenance and delays in liquidation of faults, again contributing to reduction of mill efficiency.
The existing control system has drawn operative power from finishing mill bus bar. Thus, if there is any problem in the mill, the operator had to stop the operations of the cooling beds which result cobbles and reduction in productivity of the mill.
Also, the existing control system does not include motor protection at the motor control centre (MCC) panel which results delay in fault liquidation.
It is thus there has been a need for developing a new control system for efficiently controlling of the motors of the typical cooling bed roller table which will solve the above stated limitations of the existing cooling bed roller table motor’s control system.

OBJECT OF THE INVENTION:
It is thus the basic object of the present invention is to develop a control system for controlling cooling bed roller table motors of rolling mill to ensure higher speed of roller table speed control based on the rolling regime and increase in productivity.

Another object of the present invention is to develop a control system which would be adapted to control speed of roller table group motors operating at a high speed.

Another object of the present invention is to develop a PLC based control system for controlling cooling bed roller table motors of rolling mill which would be adapted to implement soft logics for all the operations and integrated operation of cooling bed roller table motors speed control, on-line diagnostics, flexibility in logic changes, modular architecture.

Yet another object of the present invention is to develop a control system for controlling cooling bed roller table motors of rolling mill which would be adapted to independently drive the cooling bed roller table motors without drawing operative power from finishing mill bus bar.

SUMMARY OF THE INVENTION:
Thus according to the basic aspect of the present invention there is provided a system for controlling cooling bed roller table motors of rolling mill comprising
a PLC processor with distributed input-output nodes;
AC drives with built-in drive to PLC processor communication means for facilitating centralized cooling bed roller table motor speed control and monitoring to ensure higher speed of roller table speed control based on rolling regime;
MCC panels operatively connected with each of the cooling bed roller table motors for protecting the motors from anyone or more from single phasing, earth fault and trip;
power distribution unit to independently control power for AC drives, PLC processor, input-output nodes and the MCC panels.

According to another aspect in the present system for controlling cooling bed roller table motors of rolling mill, the cooling bed roller table is divided in four groups, each group is having equal number motors and each group includes at least one AC drive having variable frequency drive (VFD) operatively connected to each motors of the group through motor cable to control speed of the motors.

According to another aspect in the present system for controlling cooling bed roller table motors of rolling mill, the VFDs of each of the group are networked to constitute a VFD control circuit and programmed for centralized cooling bed roller table motor speed control and monitoring.

According to another aspect in the present system for controlling cooling bed roller table motors of rolling mill, the VFD comprises sine wave filter at its output in the motor cable between the VFD and the motors to protect the motors from voltage reflection and other localised heating.

According to another aspect in the present system for controlling cooling bed roller table motors of rolling mill, the MCC panel includes all motor protection circuitry including single phasing, earth fault and trip to protect the cooling bed roller table motors from damages.

According to another aspect in the present system for controlling cooling bed roller table motors of rolling mill, the distributed input-output nodes includes remote I/O (RIO) nodes selectively installed in the rolling mill for facilitating the centralised roller table motor speed control and monitoring.

According to another aspect in the present system for controlling cooling bed roller table motors of rolling mill, the PLC processor is configured to implement soft logics for all the operations and integrated operation of cooling bed roller table motors speed control and also provided with features of on-line diagnostics, flexibility in logic changes, modular architecture.

According to another aspect in the present system for controlling cooling bed roller table motors of rolling mill, the power distribution unit is implemented with proper isolation and protection to enable the VFDs to operate independently and free from finishing mill bus bar power supply to avoid dependency of the cooling bed roller table motors from the finishing mill power supply.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 shows preferred layout of the typical rolling mill of the steel processing plant.

Figure 2 shows a preferred layout of the cooling bed of the rolling mill of the steel processing plant.

Figure 3 shows block representation of interconnection of existing system for controlling of the motors of a typical cooling bed roller table.

Figure 4 shows block representation of the present control system for centralised roller table motor speed control and monitoring.

Figure 5 shows a preferred embodiment of the redundant power distribution unit of the present control system.

Figure 6 shows VFD control circuit associated with a preferred embodiment of the present control system.

Figure 7 shows circuit design of MCC for the cooling bed roller table motors.

Figure 8 shows means for displaying status of complete integrated control system to monitor various instantaneous statuses of equipments.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS:
As stated hereinbefore, the present invention discloses a control system for efficiently controlling of the motors of the cooling bed roller table of the rolling mill of the steel processing plant. The present control system is designed for controlling speed of the motors at its prefixed locations.
In the present control system, AC drive technology is used instead of technically outdated combination of exciter set having DC generator, AC Squirrel Cage Induction Motor, Variable Frequency Alternator and its DC drive motor and associated relay logic comprising 100 odd relays and speed transmitter.
The present control system ensures a higher speed of the motor of the roller table speed control based on the rolling regime thus facilitating an increase in productivity. The present control system includes centralised control system based on PLC & Remote I/O based distributed architecture to improve the overall performance by reducing the mean time to repair as well as mean time between failures and also improving the coordination in the rolling mill operation.
The present control system comprises PLC processor with distributed input-output nodes, advanced AC Drive with built-in Drive-PLC communication for control, redundant configuration and sine wave filters. The present control system also includes a redundant power distribution unit and a new MCC panels for the motors. The accompanying figure 4 shows a block representation of the present control system architecture.
In the present control system, the distributed input-output nodes includes remote I/O (RIO) nodes which are selectively installed in the DCO MCC Room 1, 2, 3, 4 and in CP#10 for centralised roller table motor speed control and monitoring as shown in the accompanying figure 4. PLC processor (PLC panel at Motor Room) is connected with all the RIO located in the DCO MCC Room 1,2,3,4 and CP#10 over a communication network. The PLC processor receives all the feedback/ status signals like MPCB trip, earth trip, roller table speed etc. from RIOs and calculates all related computation. Output is being generated for motor status LEDs, rpm meter, speed display meters etc. located at DCO MCC Room 1,2,3,4, CP#10 and also at the Drive panels.
The present controller system is built over the PLC processor and the remote input/output platform on profibus and modbus TCP/IP. The implementation of a reliable network based PLC system drastically reduces the frequent mal-operation of the mill equipment due to hardwired connections.
The redundant power distribution unit of the present controller system is developed and implemented for AC drives or the VFD feeder circuit as well as control power for PLC, Remote I/O, MCC panels and others equipments. The power distribution unit is depicted in the accompanying figure 5.
As shown in the accompanying figure 5, the power distribution unit includes transformer 1 and 2 which are preferably existing Dry Type Transformer 250KVA, 3 phase and it is used to feed the power supply to the VFDs and other auxiliaries. power distribution unit also includes Lightning arrestor to protect the insulation and conductors of the system from the damaging effects of lightning, Change over Switch for manual selection of incomer power from transformer with mechanical interlock. When system is running only one transformer is selected for feeding the power supply.
As stated hereinbefore, the cooling bed is divided in four groups and each group is having 30 motors. In the present control system, the AC drive includes one VVVF drive (VFD) panel is installed for each group and all the four groups’ VFD are networked and programmed in such a way that to take care the proper operation of cooling bed.
The motor cable length between the VFD and the motor is much above than the normal motor cable length for VFDs and in the present control system, sine wave filter is provided in the output of VFD circuit to take care of voltage reflection and other localised heating which could damage the motors. The VFD control circuit is depicted in the accompanying figure.
In the present control system, a new MCC panels is designed and implemented for the all the motors of the cooling bed roller table with all motor protection circuitry including single phasing, earth fault and trip. The circuit design of MCC for one motor is depicted in the accompanying figure 7. The MCC circuit comprises Motor protection circuit breakers (MPCB) to protect motors from any faults in the MCC panel. The present circuit includes shunt trip coil to trip the MPCB in case of any abnormality. A current transducer is also used to measure the quantity of the current flowing in the circuit based on which value the trip setting is done. A Single phase and earth leakage relay is also provided in the circuit to sense the single phasing of the motor (is any one out of three phase is gone) and earth leakage is to sense whether motor is earthed or not (in this case leakage current will vary from the normal current)
Features of the present system include node and equipment diagnostics through PLC. Status of complete integrated control system is available to monitor various instantaneous statuses of equipments etc. (figure 8). These measures have helped in providing an efficient maintenance to reduce downtime in this critical section of mill.
The main advantages of the present control system vis-a-vis the earlier control system can be summarised as follows:
(i) The relay logic based control system has been replaced with reliable centralised Programmable Logic Controller based system. The present PLC processor of the control system is configured to implement soft logics for all the operations and integrated operation of cooling bed roller table motors speed control and also provided features of on-line diagnostics, flexibility in logic changes, modular architecture for enhancement of system features in future and so on.
(ii) For speed control of roller table motors, VFD based control system implemented. For each group of 30 motors individual VFD is installed. All the four groups VFD, PLC and remote I/O is connected through communication. This has helped in optimization of roller table speed based on rolling regime through Variable Frequency Drive (VFD) as well as starting of cooling bed roller table motors from the last speed.
(iii) Two new transformers (which were already installed earlier) were used to feed the VFD panel for running the cooling bed roller table motors. Redundant power distribution scheme was implemented with proper isolation and protection. This has enabled the VFD system to operate independently and there is no linkage with the finishing mill bus bar power supply which was earlier is the case. It is avoiding the dependency of cooling bed roller table from finishing mill power supply.
(iv) 120 new MCC panels is designed and implemented for the all the motors of cooling bed roller table with all the protection of motor including single phasing, earth fault and trip.
Although the embodiments of the invention have been illustrated as a control system for controlling cooling bed roller table motors of rolling mill which advantageously ensures higher speed of roller table speed control based on the rolling regime thus facilitating an increase in productivity, the present control system in fact, can be used for similar applications of roller table group motors speed control of steel or other stocks operating at a high speed.
While the present invention may have been described through reference to specific embodiment, the invention is not limited to these specific embodiment. Other changes and modifications known to those of ordinary skill are intended to be included within the scope of the present invention.