Analysis of PLC program for variable frequency constant pressure water supply with one to two drive

A. Main circuit and control circuit diagram of variable frequency constant pressure water supply system:

The main circuit structure is a frequency conversion one to two form. The control principle is briefly described as follows:
The system consists of a frequency converter, a PLC, and two water pumps. Utilized the PID and other related functions of the frequency converter control circuit, and cooperated with PLC to implement frequency conversion one to two automatic constant pressure water supply. Equipped with automatic/manual switching function.
When there is a frequency conversion fault, the water pump can be switched to manual control for operation.
Control process: When the pressure of the water pipeline network is low, the frequency converter starts the 1 # pump. After running at full speed for a period of time, if the pressure signal from the remote pressure gauge still does not reach the set value, the PLC controls the 1 # pump to switch from frequency conversion to power frequency operation, and then the frequency converter starts the 2 # pump to run. The speed of the 2 # pump is randomly adjusted according to the pressure of the pipeline network to achieve the goal of constant pressure water supply. When the water feature decreases and the pipeline pressure increases, and the 2 # pump drops to zero speed, but the pipeline pressure remains high, the PLC controls to stop the 1 # power frequency pump, and the 2 # pump implements constant pressure water supply. When the pressure in the pipeline network is low again, switch the 2 # pump from frequency conversion to power frequency operation, use the frequency converter/5 to activate the 1 # pump, adjust the speed of the 1 # pump, and maintain a constant pressure water supply. This cycle is endless.
It should be noted that the frequency converter must set the relevant parameters for PID operation and output the relevant working status contacts in conjunction with PLC control. For detailed adjustments, please refer to the manual of Dongyuan M7200. In this example, the following parameters need to be roughly adjusted. 1. Set the start/stop control of the frequency converter to operate on external terminals; 2. Set to free parking mode to avoid impact on the output end of the frequency converter during frequency conversion/power frequency switching; 3. Set the PID operation mode, and the pressure setting value is entered from the AUX terminal. The feedback signal enters from the VIN terminal; 4. Set up the control terminals – output terminals for the frequency converter. When setting RA and RC as frequency conversion faults, the contact action outputs; When setting R2A and R2C as variable frequency zero speed, the contact action output; When DOl and DOG are set to the full speed of the frequency converter (frequency reaches), the contact action outputs.

The above diagram shows the PLC control wiring diagram. The fault signals of the water pump and frequency converter were not processed by the PLC, but were summarized to relay KA2. Its manual/automatic switching control relay KA1 switches. The operation of frequency conversion/power frequency is interlocked by contactor contacts to improve operational safety. It can be seen that R2A and D01 are the two key input signals of PLC. In the control action output of PLC, the switching from frequency conversion to power frequency is carried out through D01 (zero speed signal of frequency converter); The switching from power frequency to frequency conversion is carried out through R2A (frequency converter frequency arrival signal).
2、 Step by step program diagram of PLC:
Due to the one to two format, control is relatively simple. In fact, after four steps from S20 to S23, a cycle has been completed. The time for switching between frequency conversion and power frequency conversion is adjustable, which is adjusted by two potentiometers D8030 and D8031 attached to the FX1S type PLC. The values of the two potentiometers are directly placed in the two registers mentioned above. This facilitates the adjustment of switching time. In addition, for the start/stop control of the frequency converter, the AC contactor connected to the output end is first connected, and then the frequency converter operation command is given; When the frequency converter needs to be switched to power frequency, a stop command is given first. After the frequency converter stops, the contactor is disconnected. There is a time gap of 0.5 seconds, which effectively avoids the impact on the frequency converter.
The program is executed using step instructions in conjunction with set and reset instructions. Step control actually only has two instructions. STL, step control begins. After all the step control is completed, use a return command RET to return to the starting step S0, and then loop down. From one STL to the next, there is a “step”; SET is a set command, which sets the coil to 1 state – “energized and engaged”, and RST is a reset command; Reset the coil to 0 state – power loss release; ZRST is a batch reset command, such as resetting all five output coils such as Y0-Y5 at once; M8002 is a special relay that instantly closes when its contacts are powered on (equivalent to a rising edge pulse), and then becomes normally open. Used here for initialization processing when the program is powered on. When the program reaches step S23, it returns to step S20 and continues in this loop.
Due to the simplicity of the program itself and its smooth writing, coupled with wiring diagrams and annotations, the specific process is easy to understand at a glance, so there is no need to elaborate further here.

Reminder: With the advancement of technology, the functions of frequency converters are becoming increasingly powerful. Many frequency converters themselves have the function of one to three or even one to six, and such programs are soon becoming “cultural relics”; In terms of configuration, using an automated instrument to perform the PID function, while the frequency converter only works passively, is also a good solution; The frequency converter only drives one water pump in a fixed manner and does not perform frequency conversion/power frequency switching. When water needs to be replenished, the second pump can be directly switched from the power frequency. Due to the timely pressure regulation (speed regulation) of the frequency converter, the pipeline pressure will be more stable during operation. In fact, there are multiple solutions for constant pressure water supply, which are not limited to the structure in this article.
As the program diagram is synthesized using a “screenshot” method and is not clear enough, it can be zoomed in and viewed again.