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

When Mitsubishi VFD (Variable-frequency Drive) is selected in the manufacture of water supply/drainage equipment, the PID function is often used. The technical manual has a detailed description of it. However, everyone has different understandings of this function, which is mainly reflected in: model selection, function selection, direction determination, set value definition, upper/lower limit effect and PID parameter adjustment. Here we make some simple summaries from previous practical applications, which can be regarded as a supplement to the product information, so that users can understand and easily complete the debugging of this function in a short time.

1. Q: Which Mitsubishi VFD (Variable-frequency Drive) has PID control function, and what are the differences between them?

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

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

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

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

A: The specific operation method in this process is different for each series of products: except for E500, the above other series of VFD (Variable-frequency Drive) can redefine a PID enable terminal in the unused input terminal, that is, the terminal is connected for PID control mode, and disconnected for ordinary V/F control mode. For example: when 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); in E500, the operation mode is determined by setting parameter P128, P128=0 for ordinary V/F control mode, P128=20 or 21 for PID control mode. Therefore, for occasions where two modes need to be switched frequently, it is recommended to use products other than E500.

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

A: When the process of deviation value = set value - measured value is completed by VFD (Variable-frequency Drive), the measuring sensor should be connected to terminal 4 (A500, F500, F500J, F700, E500 and S500); if the operation process (dashed box) is completed by special equipment other than VFD (Variable-frequency Drive) (setting of set value and input of measuring sensor and deviation value operation result), then only the deviation value output terminal needs to be connected to terminal 1 (A500, F500 and F700). In addition, in V500, both the measurement signal and the deviation signal are connected to terminal 1, and only the parameter content is used to distinguish them. Generally, if there is no special need, in order to simplify the system structure, most people use VFD (Variable-frequency Drive) to complete the deviation value operation.

5. Q: How to determine the PID action direction, in other words, in which occasions should positive action or reverse action be used?

A: This is the first step of the entire debugging process, which is very critical and must be accurately selected according to the system requirements of the industry. Generally speaking, it should be a reverse action during water supply, flow control, and heating. In layman's terms, when the measured value (water pressure, liquid flow, temperature) increases, the execution amount should be reduced, otherwise it should be increased. In drainage and cooling, it is a positive action. When the measured value (water pressure, temperature) increases, the execution amount should be increased, otherwise it should be reduced.

6. Q: How should the set value be defined? Is its role related to adjusting the motor speed?

A: In this project, some users tend to confuse it with the speed setting in V/F mode. In PID mode, it refers to determining a value that meets the field control requirements in the full range of measured values, and using this value as the target value, so that the system will eventually stabilize 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 needs to be maintained at 0.7MPa, so 0.7Mpa is the set value. It can be given by analog quantity, that is, the corresponding voltage (5V*70%=3.5V) is applied between terminals 2 and 5 in external operation mode; it can also be given in the parameters, let P133=70% (only valid in PU and PU/EXT mode). When the system does not reach the set pressure, the motor runs at the upper 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. Q: How should the PID parameters be determined and what values are appropriate?

A: Because the structural characteristics of each system are different, and it is difficult to calculate the accurate value of PID, the default PID parameters in VFD (Variable-frequency Drive) need to be readjusted. For the sake of simplicity in debugging, generally only P and I control are needed in water supply and drainage and flow control. The D parameter is difficult to determine and is easily confused with interference factors. It is also unnecessary in such occasions and is 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 system reacts. However, if it is too small, it will cause oscillation and affect the stability of the system. It plays a role in stabilizing the measured value. I is to eliminate static error. Even if the measured value is close to the set value, it should not be too large in principle. During the trial operation, the P and I parameters can be repeatedly adjusted while observing the changes in the measured value under online conditions until the measured value is stable and close to the set value.

8. Q: What is the role of the upper/lower limit setting? Does it affect the operation of the system?

A: This setting is not necessary. It only serves as a reminder and has no effect on the operation of the system. It can be decided whether to use it or not according to the actual situation. In some cases, when the measured value is lower than the lower limit or higher than the upper limit, an alarm is required and other related equipment needs to be driven to cooperate. Two points can be selected within the measurement range as the upper/lower limit values. When the condition is activated