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Application of SLANVERT Inverter in Water Supply

I  Introduction
In plant water supply and domestic water supply, the flow rate, pressure, lift and other parameters always change with the change of working conditions. In the past, due to limitation of technical level, there is no appropriate speed regulation method to solve the negative effect brought by the variables. Therefore, manual switchover of pump is low in regulation precision and will cause electric energy waste. Switchover of pump has great impact of the entire pipe network, and pumps and motors have always been running at high speed, which will reduce the normal service life of the mechanical system and electrical system.
Practice proves that in the fan and pump type loads, the application of frequency conversion control can save electric energy of 20% to 50%, and its application can bring the following benefits:

1. It has obvious energy saving effect.
2. It realizes soft start and soft stop for motor. It solves the dripping, leakage and overflow caused by impact of direct starting of motor to pumps and pipe network. There is no impact to the electric grid in starting of motor.
3. It realizes protection functions for the motor, such as motor overload, over-voltage, under-voltage, overheating, earthing, power supply phase failure, etc.
4. The whole production process realizes automation to maintain stable pressure of the pipe network.
5. It extends the service life of the pump and motor. The inverter can automatically regulate the running frequency to mitigate wear of the pump and aging of the motor.

II  Frequency Conversion Transformation Scheme for Constant-Pressure Water Supply System
The system mainly consists of three pumps, one SLANVERT SB200 Inverter, pressure transmitter, PLC and other components Steel enterprises have large water consumption, large pump capacity, and correspondingly large diameter of pipe network. Therefore, in switchover of the pump, valve should be controlled. Switch-on of the pump shall follow the principle of "switch pump before opening valve", while switch-off of pump shall follow the principle of “close the pump before stopping the pump". In this time, PLC should be used for control. The system control schematic diagram is shown in Fig. 5.

Fig. 5 Schematic Diagram of Water Supply System under Frequency Conversion and Constant Pressure

In Fig. 5, the given value of pressure is set by keyboard of the inverter, while the feedback value of the pipe network pressure is sent by the pressure transmitter. The built-in PID controller of the inverter compare the given value with the feedback value from the pressure transmitter, to control the number of pumps in operation, and the frequency-conversion also regulates the speed to achieve constant pressure of the pipe network. If small-capacity pumps are used, no PLC control valve is required, and multi-pump switchover functions can use the built-in expansion unit of SLANVERT inverter to realize control. Its application is simple and maintenance is convenient.

III  Energy Saving
To adapt to the change of actual water consumption, manual switchover of the number of station pumps was adopted in the past to regulate the water pressure and flow rate of the pipe network. This would cause pressure change (sometimes over-pressure and sometimes under-pressure) at different outlets of the pipe network. Application of frequency-conversion speed regulation technology for pump unit is to change the speed of pump by regulating the frequency of the input motor power supply. Centrifugal pump has load of square torque characteristics. The power consumption of the motor is proportional to the cube of its speed. After frequency-conversion speed regulation, a lot of electric energy can be saved to achieve the objective of energy saving and efficiency improvement.