Overview:

The water supply system is an indispensable part of the national production and life. The traditional water supply system has some disadvantages that it covers a large area, water quality is easy to pollute, and the infrastructure investment is large. Furthermore, the water pressure cannot be kept constant, resulting in waste of energy. Variable frequency drives speed regulation technology is a new type of mature AC motor step-less speed regulation technology. It is widely used in the field of speed control with its unique and excellent control performance, especially in the water supply field. Safe production and strict water supply quality cry out for stable constant pressure water supply system. So the VFD has been applied more widely. The constant pressure water supply method features constant water pressure, convenient operation, reliable running, energy saving, and higher automation degree. At present, most developed countries have widely adopted AC drives control technology. And in China, frequency inverter project is promoted nationally as a key technology for energy saving.

i The working principle:

1.1 Working conditions of the original water supply system:

The site is a sugar factory. The original water supply system consists of one route pipe network system, pump motor 155KW, one starter. Pumps switching is controlled according to the upper and lower pressure limits, and the water pressure varies greatly due to the production demand and the pumps switching. The pumps switching has large impact to the power grid. In production, return valve is used to get suitable pressure adjusting.

1.2 Frequency conversion energy-saving retrofit plan:

Tell from the above working conditions that VFD energy saving plan can be used in this application site. Retrofit plan is as follows:

Part of the system adopts frequency conversion control. Pressure sensor is installed on the pipe network to constantly detect the water pressure of the pipe network. The pump motor speed is adjusted according to the production water volume demand and thus ensure the constant water pressure of the pipe network. A starter is added to make make the pumping system can be switched to grid power supply at any time in case of any VFD problem. After the transformation, the power saving effect is manifested, and the constant pressure water supply greatly improves the production process. The control system control diagram is as follows:

II The system configuration:

2.1 Constant pressure water supply realization:

AD300 frequency conversion system built-in PID function is used. Pipe network remote pressure sensor detects the instant pressure in pipe network and converts it into 0~10V voltage signal which is being sent directly to AD300 inverter analog voltage input terminal. Set pressure reference value and PID parameters. The inverter built-in PID will calculate and output required frequency to the pump motor. System parameters can be adjusted during actual operation to make the system control response fast. The pipe network pressure reference can be given freely according to the production process, and the frequency inverter automatically follows the reference change to keep the pressure constant and same as set, which not not only save energy but also much better satisfy the water pressure required by production process.

2.2 VFD control and original grid power supply switching:

The sugar industry water supply system requires very high system reliability. Once the water supply has malfunction, it has great impact to the production process. So a switching function has been added to the system. In case of any VFD problem, the system can immediately switch to grid power supply and start the pumping system with original standby auto-transformer starting. This ensures the continuity and reliability of the system. The two sets of starter systems are electrically interlocked to prevent mistake operation and ensure safe running of the system.

iii Parameter setting:

F0.02=1 Operation control mode is external terminal

F0.03=7 frequency reference is set to internal PID

F6.01=1 X1 terminal is running forward

F6.03=8 RST reset

F7.03=16 Inverter fault output

F8.00=0 Reference use PID digital setting (default)

F8.01=0 AI1 feedback value selection

F8.02=XX Analog PID digital reference (Set this value according to the required pressure)

iv Retrofit benefits:

4.1 Extend the service life of equipment and pipe network by reducing system maintenance work.

VFD’s soft starting removes the torque shock on the motor and also eliminate the pipe burst which might happen in original system at starting. Equipment service life is extended and maintenance fee greatly reduced.  

4.2 Improve water supply system’s quality and reliability

After adopting the variable frequency constant pressure control system, the water supply pressure can be adjusted very smoothly and freely as the production process changes. Due to the adoption of the new system, the failure rate is greatly reduced, and the two sets of starting systems can be standby to each other, which ensures the continuity of the system and provides a guarantee for system economical optimization.

4.3 Improve the electricity consumption in the factory.

The new system removes the electricity impact to the grid at starting. New system power factor is up to 0.95 and efficiency is up to 0.98. The system working current is lower and cable wearing is reduced, and thus improve the factory’s overall electricity consumption condition.

4.4 Remarkable energy-saving efficiency and economic benefits

To hardly keep the constant pressure, the original system requires frequent pumps switching, which not only consumes lots of electricity but also generate great impact to the electricity grid. And the extra return-vale does not help reduce the pump power consuming. In contrast to this, the VFD system automatically keeps the constant pressure and easily adjust the required pressure. Generally, the system’s energy saving efficiency is around 30%, giving remarkable economic benefits.