Application of Altivar38 VFD (Variable-frequency Drive) in water treatment

introduction

The principle of frequency conversion energy-saving technology: Generally, the design of electric drive equipment considers the situation of short-term overload operation, and the power configuration of the motor is often greater than about 10% of the maximum power of the load, or even larger.

1 Raising the question

The water treatment center in the steel rolling area is part of the second-phase technical transformation of Xinyu Iron and Steel Co., Ltd. It is mainly responsible for processing and supplying the turbid ring water and clean ring water required by the plate factory and wire rod. The turbid ring water system of the medium and thick plate factory is a 24-hour continuous water supply. Since the operation, due to frequent roll changes and different production process requirements, the water consumption has been unstable, resulting in high and low water levels in the cold water pool, unstable operation, and instable operation. It is safe and often overflows, causing waste of resources and environmental pollution. For this reason, the water level of the pool must be adjusted by frequently operating the return valve and turning on and off the pump. According to statistics, from June to August 2006, on average, the backflow was adjusted at least 18 times a day, and the pump was started and stopped at least 3 times. The frequent power frequency start of the pump shortens the service life of the motor. The current impact during power frequency start, the pressure impact of the pipe network, and the water hammer effect during instantaneous power failure lead to a high failure rate of equipment, frequent maintenance, and increased maintenance costs. A large part of the electric energy consumed by the motor frequency operation is used for the backflow of the system, doing useless work and increasing the power consumption. The original system is an open-loop system, which only plays the role of starting and stopping the pump, and cannot be adjusted automatically. It is difficult for the operator to monitor, and it needs to be monitored from time to time. A little carelessness will affect the adjustment of the water level. In addition, the water level fluctuates greatly, the liquid level gauge is not properly selected, and the computer display cannot truthfully reflect the on-site situation. The operator must often go to the site to check, and the adjustment data cannot be provided in time and accurately, which makes the adjustment lag and increases the workload.

2 Design and determination of system transformation plan

2.1 Selection

The current mode of operation is that pumps No. 1 to No. 4 in the degreasing pump room supply water to the turbid ring cold water pool of medium and thick plates. According to production needs, it is decided to add a frequency conversion device to carry out frequency conversion transformation, and adopt the "one drive two" form, that is, one set of frequency conversion device It can interchangeably drive two water pump motors, which are used for pump motors No. 3 and No. 4 in the degreasing pump room. In this way, one unit operates with frequency conversion and the other is for standby. Unit maintenance does not affect the work of the frequency conversion system. The selected motor models are all Y2-3l5M-4 (380 V 132 kW 240 A), and the frequency conversion device is Altivar38 energy-saving VFD (Variable-frequency Drive) of Schneider Telemecanique Company, with a power of 132 kW. It complies with IEC, UL and CSA standards, and has wide speed range, good acceleration and deceleration braking performance, adaptive adjustment and energy saving, and protection functions such as power overvoltage, motor phase loss, motor overheating, VFD (Variable-frequency Drive) overheating, etc. .

There are many ways to set the variable frequency speed regulator (VVVF for short). Here, the VFD (Variable-frequency Drive) panel PI setting and the current feedback detected by the liquid level gauge are used to form a closed-loop automatic PID adjustment through the setting of internal parameters. Control System.

2.2 Control loop

The original control circuits of the two water pump motors are reserved as the power frequency bypass of the new control system, which is determined by the power frequency/variable frequency selection switch on the frequency conversion cabinet. There are electrical interlocks between the frequency conversion of the two water pump motors and between the frequency conversion to avoid misoperation.

2.3 Main circuit

Since the original power frequency main circuit uses Sunwoele’s model HSSR-CX AC motor soft starter, in order to ensure that the power frequency will not be affected after the transformation, the output wiring terminal of the VFD (Variable-frequency Drive) motor main circuit must be connected to the The output terminal of the soft starter.

2.4 Operation mode

The operation of this system can realize three-site control of the frequency conversion panel/main control room/on-site, and all three can control the start and stop of the water pump motor. The frequency conversion panel control can also realize speed adjustment, parameter setting and LED display of system operation, which is mainly convenient for debugging. The control in the main control room is PLC remote control, and the operation mode under normal conditions is convenient for operators to control. On-site control is an operation box next to the motor, which is mainly convenient for the staff to observe the restart of the water pump and the motor after maintenance, and stop in case of an accident.

2.5 Monitoring system

The original power frequency has been controlled by PLC, and Siemens' S7-300 PLC and STEP7 special software of ladder diagram visual programming language are selected to process the data from the electric control cabinet and field operation box.