Supplementary explanation of Mitsubishi VFD (Variable-frequency Drive) in the application of PID function

The PID function is often used after Mitsubishi VFD (Variable-frequency Drive) is selected in the manufacture of water supply/drainage equipment, and it has been described in detail in the technical manual. Different people have different understandings of this function, which are mainly reflected in: model selection, function selection, direction determination, setting value definition, upper/lower limit function and PID parameter adjustment, etc. Here is a simple summary from the past practical applications, even if it is a supplement to the product information, so that users can understand and easily complete the debugging of this function in a relatively short period of time.

1. Question: Which Mitsubishi VFD (Variable-frequency Drive) has PID control function, and what is the difference between them?

Answer: At present, all Mitsubishi VFD (Variable-frequency Drive) have PID (single pump) control function, and the product series include: A500, F500, F500J, F700, V500, E500, S500. Among them, F500J, F500 and F700 are dedicated products for fans and pumps, while F500 and F700 (0.75-55kW) also have advanced PID control function, that is, multi-pump switching function (up to 4 units). For the electrical schematic diagram and specific operation methods, please refer to the corresponding user manual of each product.

2. Question: How to choose the sensor for measurement?

Answer: In A500, F500L (above 55kw), F500J, E500, and S500, only current type (4-20mA) sensors can be used; F700 can use current (4-20mA) and voltage type (0-5V, 0- 10V) sensor; in F500 (0.75-55kW), if the advanced PID control (multi-pump switching) function is used, both types of sensors can be used, the difference is that the output of the voltage sensor is connected to the VFD (Variable-frequency Drive ) of the No. 1 terminal; and in the V500, only voltage sensors can be used.

3. Question: How to make VFD (Variable-frequency Drive) enter PID control mode?

Answer: The specific operation method in this process is different for each series of products: except E500, in the above series of VFD (Variable-frequency Drive), you can redefine a PID enable terminal in the unused input terminal, that is, This terminal is connected to PID control mode, and disconnected is common V/F control mode. For example, if the RL input terminal is not used for other purposes, it can be set to X14, that is, P180=14 (for A500, F500, F700 and V500) or P60=14 (for F500J and S500); while in E500, the operation mode is determined by setting parameter P128, P128=0 is the ordinary V/F control mode, P128=20 or 21 is the PID control mode . Therefore, it is recommended to choose products other than E500 for occasions where frequent switching between the two modes is required.

4. Question: In the PID control mode, some VFDs (Variable-frequency Drive) have both detection signal input and deviation value input. How to distinguish and use them?

Answer: When VFD (Variable-frequency Drive) completes the process of deviation value = set value - measured value, the measuring sensor should be connected to terminal 4 (A500, F500, F500J, F700, E500 and S500); if the calculation process ( Dotted line frame) is completed by special equipment other than VFD (Variable-frequency Drive) (set value setting, measurement sensor input and deviation value calculation results), then only need to connect the deviation value output terminal to No. 1 terminal (A500 , F500 and F700). In addition, in the V500, both the measurement signal and the deviation signal are connected to the No. 1 terminal, and are only distinguished by the parameter content. Generally, if there is no special need, in order to simplify the system structure, VFD (Variable-frequency Drive) is used to complete the calculation of deviation value.

5. Question: How to determine the direction of PID action, in other words, in which occasions to use positive action or reverse action?

Answer: This is the first step in the whole debugging process, which is very critical and must be accurately selected according to the system requirements of the industry. Generally speaking, when the water supply, flow control, and heating should be reversed, generally speaking, when the measured value (water pressure, liquid flow, temperature) rises, the execution amount should be reduced, otherwise, the execution amount should be increased. However, it is a positive effect during drainage and cooling. When the measured value (water pressure, temperature) rises, the execution amount should be increased, otherwise, the execution amount should be reduced.

6. Question: How should the setting value be defined? Is its function related to adjusting the motor speed?

Answer: In this project, individual users tend to confuse it with the speed setting in the V/F mode. In the PID mode, it refers to determining a value that meets the field control requirements in the entire range of the measured value, and Take this value as the target value, and finally stabilize the system at the level or range of this value, and the closer the better. For example, the measurement range of the sensor selected in the water supply system is 0-1Mpa, and the pressure of 0.7MPa needs to be maintained, so 0.7Mpa is the set value, which can be given by analog, that is, No. 2 and No. 5 in the external operation mode Apply the corresponding voltage between the terminals (5V*70%=3.5V); it can also be given in the parameters, so that P133=70% (valid only in PU and PU/EXT modes). When the system does not reach the set pressure, the motor runs at the upper limit speed (P1), and when it reaches or exceeds the set pressure, the motor slows down or stops running, so it has nothing to do with the setting of the motor running speed.

7. Question: How should the PID parameters be determined, and what is the appropriate value?

Answer: Because the structural characteristics of each system are different, and it is difficult to calculate the exact value of PID, it is necessary to readjust the default PID parameters in VFD (Variable-frequency Drive). For the sake of easy debugging, generally only need to use P, I control in water supply and drainage, flow control, D parameter is difficult to determine, it is easy to be confused with interference factors, it is not necessary in such occasions, usually used in temperature control occasions . Among the PI parameters, P is the most important. Qualitatively speaking, since P=1/K, the smaller the P, the faster the response of the system, but if it is too small, it will cause oscillation and affect the stability of the system. It stabilizes the measured value role. And I is to eliminate the static error, even if the measured value is close to the set value, it should not be too large in principle. During the test run, the P and I parameters can be adjusted repeatedly while observing the change of the measured value under the online condition until the measured value is stable and close to the set value.

8. Question: What is the function of the upper/lower limit setting, and does it affect the operation of the system?

Answer: This setting is not necessary. It only serves as a reminder and has no effect on the operation of the system. You can decide whether to use it or not according to the actual situation. In some occasions, when the measured value is lower than the lower limit or higher than the upper limit, it is necessary to alarm and drive other related equipment to cooperate with the operation. Two points can be selected in the measurement range as the upper/lower limit.