Application of Nancal low voltage industry AC drives in stainless steel cold rolling mill

1 -Abstract
In general AC variable frequency drive (VFD) area, domestic VFD has taken up large portions of market share after many years of development. However, due to the sophisticated process, the production line of metallurgical main rolling mill has strict requirements for AC drive system. Hence, Siemens, ABB and other international brands monopolize this market section.
Through efforts over years, Shanghai Nancal Electric Co.,Ltd has launched a high performance industry drives which can satisfy the strict requirements for AC drive system in metallurgical rolling mill, the features of product contain multi-drives with common DC bus, modular design, high power density, high performance vector control, high control accuracy of speed and torque, four-quadrant operation and can drive IM motors and SM motors, and so on.
This article mainly introduces the application of NC EVFD series low voltage industry drives in a stainless steel plant production line of cold-rolling mill in Ningbo China. This production line requires the thinnest stainless steel sheet less than 0.1mm, the power of the entire system is up to megawatt level, which requires high reliably and high accuracy control of speed and tension.
2-NC EVFD series low voltage industry AC drives introduction
The output voltage of NC EVFD series industry drive is 400V/480V/690V. It adopts common DC bus multi-drive scheme. The DC bus capacitor adopts long life metal film capacitor. The product is modular design, with maximum power up to 6000kW by power module parallel. The control algorithm adopts high-performance field orientated vector control, which can drive asynchronous motor and synchronous motor, supports four-quadrant operation, and is suitable for applications requiring high performance and high reliability. The specific features are shown below:
1)Modular design, multi-drives with common DC bus.
2)Diode rectifier, IGBT 6-pulse rectifier or IGBT PWM rectifier.
3)DC capacitors use metal film capacitors, maintenance free in whole life cycle.
4)Isolated air duct design, no electric devices in airflow path, suitable for harsh environment.
5)DSP+FPGA+ARM three core control platform, high performance, high reliability and strong operational capability.
6)The main components of control hardware are with latest generation, risk-free for suspend production or technology outdated.
7)A great variety of communication interfaces, no need extensions in general application. Standard support Ethernet、Modbus、CAN、Profibus and other communication protocols, and can communicate with almost all  industry network.
8)Variety of expansion interfaces to meet most applications.
9)Customized fiber optic protocol, only need a pair of fiber optic between main control panel and power module to implement the real time transmission of data and synchronous pulse. A main control panel can connect 16 power modules at same time (no need additional reactor for module paralleling).
10)Rapid dynamic response and excellent torque performance at low speed based on advanced motor flux model and control algorithm.
11)Two vector control modes: open loop vector control (OLVC) & close loop vector control (CLVC). Control system keeps estimating motor speed in real time through the motor model, so that it can seamlessly switch to OLVC mode in the event of a speed sensor failure.
12)Suitable for asynchronous motor and permanent magnet synchronous motor.

Figure 1: structure of Nancal EVFD drive 

3 Application of NC EVFD series low voltage industry AC drives in Main Rolling Mill of a Stainless Steel Plant in Ningbo
Shanghai Nancal Electric Co.,Ltd provides overall solution and project implementation of low voltage electric drive and automatic control system for 500-combination-forming single-stand reversible cold rolling mill.
The project includes 8 machines, including 2 sets 400 kW main rolling machine, 2 sets 200 kW main winding/unwinding machine, 2 sets 55 kW small winding/unwinding machine and 2 sets 15 kW pressuring motor, and all motors are driven by NC EVFD series industry drive. The input voltage of VFD is 690V.The rated frequency of main machine and winding/unwinding machine is 30 Hz, the maximum operating frequency is 90Hz, accuracy of tension control requires less than 1% and the maximum rolling line speed is 200 meters/min. 2 sets 15kW pressuring motor use single-drive with chopping braking module and braking resistor, the other drives use multi-drive with common DC bus and IGBT 6-pulse rectifier unit with regeneration function.

Figure 2: system diagram

The overall control scheme of the reversible cold rolling mills is based on high-performance industrial microcomputers, programmable logic controllers (PLCs) and AC drive control systems. The latest all-digital system is used to form a fully distributed networked modern control system. An industrial fieldbus is used for data communication between the drive control system and the automation control system. The operation console is designed according to the principle of advanced, centralized and convenient.

Figure 3: Nancal EVFD industry AC drives

The PLC is mainly responsible for converting the line speed and tension into the speed and torque limit acceptable by the VFD, and then transmitting it to the EVFD control system through the DP bus communication. The EVFD drive the motor according to the system setting mode, basing on the received speed reference and torque limit. The industrial microcomputers is responsible for the speed control and the load balance calculation of the upper and lower rollers.
During the production, NC EVFD series industry drive runs stably with high accuracy control of tension and speed, and the entire system control performance are comparable to import brands, which can fully meet the strict process requirements. 

Figure 4: Waveform in tension establish process

Figure 5: The current output waveform of winding/unwinding machine (speed =0)

NC EVFD series low voltage industry AC drives completely satisfy the requirements of stainless steel cold rolling mill, with a series of technical features such as modular design, multi-drives with common DC bus, high accuracy control of speed and tension, high power density and high reliability.
The whole production line of the stainless steel rolling mill project in Ningbo uses NC EVFD series drive, which realizes the successful application of all domestic AC drive in the metallurgical main rolling mill production line the first time. It is of great significance for the localization of AC drive in the field of metallurgical main rolling mill.