Application of special inverter for road foundation on stabilized soil mixing plant

In present China inverters are widely applied in batching system in cement stabilized soil mixing station. According to the demand of the market Simphoenix has developed a special inverter for road foundation and has won a widespread use in soil mixing equipment.   

I. Introduction

With the increase of the government’s investment in constructing high grade road, the demand for the quantity and quality of construction materials, especially the materials used for road foundation which decides the quality of the whole project, is becoming higher and higher. Road foundation is made from a kind of stabilized soil, a mixture of quick lime, hydrated lime (10-15% moisture content), gravels, coal ash and cement in a certain proportion. In accordance with national requirements, qualified cement stabilized soil shall be up to the following standards: ① apply soil mixing equipment; ② use automatic batching system in its production; ③ conduct strict supervision and inspection. The key point of these standards lies in the requirement for cement stabilized soil mixing equipment, which is ingredient measuring.

Through a comprehensive survey of China’s batching systems used in cement stabilized soil mixing stations, two major modes appear: ① ingredient measuring system controlled by PLC; ② ingredient measuring system controlled by software of upper machine. Because of its convenience for operation and powerful capacity, software control system of upper machine has been widely applied by popular products in current market. Inverter adjusting and controlling has been applied in ingredient measuring system. According to the demand of the market Simphoenix has developed a special inverter for road foundation and has won a widespread use in soil mixing equipment.         
II. Composition of the equipment

1. Ingredient supply system: storage bin for aggregate (the number depends of real conditions), belt weigher with a speed controller (frequency-variable), belt conveyor:

1. Cement supply system: cement bin (cement hopper), screw conveyor with a speed controller (frequency-variable), screw electronic weigher.
2. Automatic water supply system: submersible pump, pipeline, control valve, sprayer.
3. Mixing system: motor reducing gear, continuous mixer with double horizontal shaft.  
4. Storage bin for end product: double discharging storage bin, inclined belt conveyor, air compressor.
5. Electric system: operation and control cabinet, operation room.
6. Measuring control system: manual and automatic control by microcomputer.

In this system inverter is applied to control motor speed where it is necessary. The speed of each motor is controlled by orders from microcomputer so as to control accurately.

Drawing of the equipment is as following:

III. Operating principles and requirements of the system

Principles of automatic batching system: upper machine figures out the accurate values of instantaneous flow and accumulated amount of materials conveyed out by batching scale (belt or crew) by the wight signal of the materials and the pulse signal of their speed through preset programs. Then, the upper machine will compare instantaneous flow value with set value and figure out a deviation, which will be taken as an input frequency signal for the inverter to adjust motor speed so as to equal actual flow within unit time with set flow. In this way, automatic quantitative batching in a certain proportion can be achieved. At the same time the upper machine collects water flow signal by a special water meter (or rotameter) and compares it with set value of water flow to adjust pump speed so as to match actual water flow to its set value; in addition, this system can track water proportion automatically; when production increases, more water will be added in proportion; when production decreases, water amount will be reduced in proportion; therefore, the proportion of material and water will be kept the same with delay function.       As to all raw materials, the upper machine software will collect operational data and send out signal to control inverter which will adjust speed of all motors so as to make a proper proportion of the ejection of all raw materials. The raw materials become qualified cement stabilized soil after mixture. Signal between inverter and upper machine will be transmitted by conversion device 485/232. The inverter will collect current signal from the 4-20MA electronic weigher on belt weigher. After process by software, upper machine will send out a frequency variable signal to inverter. Communication between inverter and upper machine will be connected by communication protocol MODBUS.  

According to the above principles: due to its function to connect and control, an inverter is firstly supposed to have a strong capacity to communicate data, which is matched to a special control software; secondly, during its operation, the torque shall have a big enough torque, an accurate speed, a powerful capacity, a short time to accelerate and a strong ability to correct error. In addition, because of communication and control achieved by the connection to strong and weak currents, the inverter is required to possess the following features: anti-interference, weak electromagnetic radiation, easy connection and operation.

IV. Features of Simphoenix inverter

According to the requirements the equipment, Simphoenix E380 series for general purpose are totally up to the standards. Based on it, the company has improved E380 series and developed a special machine which suits stabilized soil mixing equipment. Owing to its powerful functions and features, it has been widely used in the electronic application of stabilized soil mixing equipment:    

1. Standard 485 communication interface, connecting to upper machine conveniently;

2. MODBUS communication protocol, easy and convenient for client to program upper machine;

3. Unique low-frequency torque, big enough when started and stable while operation, which efficiently prevent blocking:

4. Accurate control of speed to meet the required speed to ensure even ejection of materials and accurate quantity to guarantee the quality;

5. Special transmitting channel for collected data–current value of sensor, which will be defined as feedback value to be transmitted to upper machine;

6. Special channel for pulse signal to be sent out, which can be used in site exclusively;

V. Electric principles and structure

As an important device which connects control system of microcomputer and power unit, the inverter in this equipment can motivate motor, collect and transmit dada, conduct orders from microcomputer and has many other important functions. In this way, the accurate proportion of raw materials can be achieved by adjustment of motor speed. The electric connection is as below(the number of inverters can be more or less according to real conditions):

Control line connection of inverters is as below:

Control line connection of inverters is as below:

Controlling orders on inverters, including data reception, data sending, order controlling and alarming processing, which are programmed according to communication protocol of them (attached no further explanation here), shall be added in microcomputer software.

As to inverters, corresponding parameters shall be set so as to communicate with upper machine normally. The following parameters shall be set to inverters of E380 series:

Correspondence address of inverters will be available by modifying F9.1 and its parameters will be set according to the address of its slave which is required by software. If self-defined protocol is used, F9.0 shall be set as 0014.

VI. Operation and maintenance  

After the setting of its parameters, the inverter will meet the requirements of production by control panel and microcomputer according to operation instructions while installing and debugging, which is easy and convenient.

It is easy to maintain Simphoenix inverters, which run stably in a long term under correct connection and operation. If it is off for a long time, it is better to charge it for 2 hours every week. Due to its exposure to air and many metal parts, lightening protection shall be made to the machine to prevent damage by lightening stroke and at the same time water shall be prevented.

VII. Conclusion     

According to field technological requirements, Simphoenix engineers, through further research, have developed a machine, which collects all data directly into an inverter in which the data is processed by a special software, to achieve quantitative batching and production by changing modes.

Simphoenix E380 series inverters have been widely applied in stabilized soil mixing equipment and have won praise from all clients by its good quality and service. Their continuous and stable operation has gained a favorable profit because of their guarantee for stable production and good quality.