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The closed-loop PID control function is one of the important areas in inverter application technology, and it is also a key technical means for inverters to achieve their excellent performance. Those responsible for the design, operation, and maintenance of variable frequency speed control systems should be thoroughly familiar with and proficient in the basic theories of PID control, and should accumulate extensive practical experience in order to contribute to the widespread adoption of this technology. In industrial production or certain operating system devices, a stable pressure, temperature, flow rate, liquid level, or rotational speed is often required as a prerequisite for ensuring product quality, improving production efficiency, and meeting technical specifications; this is where the PID control function of frequency converters comes into play. PID control is a method in a closed-loop control system that enables the controlled physical quantity to rapidly and accurately approach the control target as closely as possible. To achieve closed-loop PID control, the PID function must first be set to active. There are two specific methods: the first is to preset the function parameter codes of the inverter; for example, in the Converge CVF-G2 series of inverters, setting parameter H-48 to 0 disables the PID function ； When set to 1, it is normal PID control ； Setting it to 2 results in a PID for constant-pressure water supply. Secondly, it is determined by the status of the external multi-function terminals of the inverter. For example, in the Yaskawa CIMR-G7A series of inverters, as shown in Figure 3-1, by selecting any one of the multi-function input terminals S1~S10 and setting the function codes H1-01～H1-10 (corresponding to terminals S1~S10) to 19, that terminal will have the function of determining whether PID control is active. This terminal is ineffective when the common terminal SC is set to “ON” and effective when it is set to “OFF”. It should be noted that most frequency converters have both of the aforementioned preset methods, but a few brands of frequency converters only have one of them. In some systems where the control requirements are not very strict, it is sometimes sufficient to use only the PI control function without activating the D function, which simplifies the system debugging process. The feedback logic of PID: The terms used to describe the feedback logic in various frequency converters vary, and there are even cases where similar terms are used but have opposite meanings. The system design should be based on the instructions provided in the manual of the selected frequency converter. The so-called feedback logic refers to the control polarity of the inverter’s output frequency, which is determined by the feedback signal generated by sensors detecting the controlled physical quantity. For example, in central air conditioning systems, the return water temperature is used to control the output frequency of the inverter and the speed of the pump motor. During heating in winter, if the return water temperature is low, the feedback signal decreases, indicating that the room temperature is low; this prompts an increase in the output frequency of the inverter and the speed of the motor, thereby increasing the flow rate of hot water. During cooling in summer, if the return water temperature is low, the feedback signal decreases, indicating that the room temperature is too low. From an energy-saving perspective, it is possible to reduce the output frequency of the inverter and the speed of the motor, thereby decreasing the flow rate of cold water. As can be seen from the above, when the temperature is low, the feedback signal decreases; however, as indicated in Table 1 for the functional selection of the feedback logic, the direction of frequency change in the inverter is opposite. This is why feedback logic is introduced. Target signal and feedback signal: To keep a certain physical quantity in a frequency conversion system at the desired target value, the PID control circuit of the inverter continuously compares the feedback signal with the target signal, and adjusts the output frequency and the speed of the motor in real time based on the results of this comparison. Therefore, the PID control of an inverter requires at least two control signals: a target signal and a feedback signal. The target signal mentioned here refers to the electrical signal corresponding to the expected stable value of a certain physical quantity, also known as the target value or set value ； The electrical signal corresponding to the actual value of this physical quantity as measured by a sensor is called the feedback signal, also known as the feedback value or current value. In the functional diagram of PID control, there is a PID switch; the PID function can be enabled or disabled through the functional parameter settings of the inverter. When the PID function is active, the switch is in the downward position, and it is the PID circuit that determines the operating frequency ； When the PID function is disabled, the switch is closed upward, and the operating frequency is determined by the frequency setting signal. The PID switch, action selection switch, and feedback signal switching switch all have their operating states determined by the settings of the functional parameters. Given the target value, as for how to transmit the command information of the target value (target signal) to the inverter, different inverters use various methods; generally speaking, there are mainly the following two approaches. The first is the automatic conversion method: when the PID function is preset in the inverter, its frequency setting function during open-loop operation automatically switches to target value setting, as seen in the Yaskawa CIMR-G7A and Fuji P11S inverters in Table 2. The second is the channel selection method; examples include the Convo CVF-G2, Senlan SB12, and PuChuan PI7000 series of frequency converters listed in Table 2. The input channels for the target signal have been described above; next, it is necessary to determine the value of the target. Since the target signal and the feedback signal are usually not the same physical quantity, making a direct comparison is difficult; therefore, the target signal of most frequency converters is expressed as a percentage of the sensor’s range. For example, if the air pressure in a gas storage tank is required to remain at 1.2 MPa and the range of the pressure sensor is 2 MPa, then the percentage corresponding to 1.2 MPa is 60%, and thus the target value is 60%. In the parameter list of some frequency converters, there are parameters corresponding to the upper and lower limits of the sensor’s range. For example, in the Fuji P11S frequency converter, setting parameter E40 (display coefficient A) to 2 corresponds to the upper limit of the pressure sensor’s range, which is 2 Mpa ； Set parameter E41 (display coefficient B) to 0, meaning the lower limit of the range is 0 ； Then the target value is 1.2, that is, the stable pressure value is 1.2 Mpa. The target value is the absolute value of the expected stable value. Connection of the feedback signal: All types of frequency converters have several frequency setting input terminals. Among these input terminals, once one is designated as the input channel for the target signal, the other input terminals can be used as input terminals for the feedback signal. One of them can be selected for use by using the corresponding function parameter code. Several typical options for selecting feedback signal channels in frequency converters are shown in Table 3. Pre-setting and adjustment of P, I, D parameters: The PID function of an inverter utilizes the difference between the target signal and the feedback signal to adjust the output frequency. On one hand, we want the target signal and the feedback signal to be as close as possible to each other, that is, for the difference to be small, in order to achieve the desired level of precision in regulation ； On the other hand, we also want the control signal to have a certain amplitude to ensure the sensitivity of the control. The way to resolve this contradiction is to amplify the difference signal in advance. The proportional gain P is used to set the amplification factor of the difference signal. For any frequency converter, parameter P has a settable value range. During initial debugging, P can be preset at a value slightly above the middle range, or the factory default value can be used temporarily; fine adjustments can then be made based on actual operating conditions once the equipment is in use. As mentioned above, the larger the proportional gain P, the higher the adjustment sensitivity. However, due to the inertia in both the transmission system and the control circuit, it is not possible to stop immediately once the adjustment reaches its optimal value; this results in \"overshoot,\" followed by counter-regulation that leads to further overshoot, thus creating oscillations. To this end, an integration element I is introduced; its effect is to cause the difference signal amplified by the proportional gain P to increase (or decrease) gradually over the integration time, thereby slowing down its rate of change and preventing oscillations. However, if the integration time I is too long, it will be difficult for the controlled physical quantity to recover quickly when the feedback signal changes abruptly. Therefore, the value of I is related to the time constant of the dragging system: when the time constant of the dragging system is small, the integration time should be shorter ； When the time constant of the drag system is large, the integration time should be longer. The differential time D determines a corresponding adjustment action in advance, based on the rate of change of the difference signal, thereby reducing the adjustment time and overcoming the drawback of recovery delay caused by an excessively long integration time. The value of D is also related to the time constant of the dragging system: when the time constant of the dragging system is small, the differential time should be shorter ； Conversely, when the time constant of the drag system is large, the differentiation time should be longer. The preset values of the P, I, and D parameters complement each other; on-site adjustments should be made according to actual conditions as follows: if the controlled physical quantity oscillates around the target value, first increase the integration time I. If oscillations persist, the proportional gain P can be reduced slightly. Once the measured physical quantity changes, it is difficult to restore it to its original state. First, increase the proportional gain P; if the recovery is still slow, the integration time I can be reduced appropriately, or the derivative time D can also be increased. In this application example, the Chuangshi CSBG type frequency converter is used, and its PID function is employed to automatically control the chilled water circulation system of the central air conditioning system in the office building of a local procuratorate. For the control method of the chilled water circulation system, several options are available: the first is constant temperature difference control, which uses the difference between the return water temperature and the outlet water temperature as the basis for control. The PID function of the temperature difference controller is utilized to generate a frequency setpoint signal for the inverter; this approach does not require activating the PID function of the inverter. The second is constant pressure difference control, which involves regulating based on the difference between the outlet pressure and the inlet pressure of the chilled water pump. Third is constant temperature control. If the outlet temperature of the chiller is relatively stable, by measuring only the return water temperature and utilizing the PID function of the inverter, it is possible to achieve the same control effect as that of constant temperature difference control. This example adopts exactly this solution for cooling in summer. Application circuit The application circuit is shown in Figure 3. The frequency converter shown in the figure is of the Chuangshi CSGB type, with a capacity of 30 kW; its parameter settings are listed in Table 3-4. During setup, all parameters are first restored to their factory defaults using P126. The advantage of doing this is that, although there are over a hundred parameters for this inverter, a considerable number of them are not actually relevant in this example ； Moreover, some of the useful parameters can use their default factory values, which makes parameter setting relatively simple. The setting of parameter P98 activates the PID function, with the feedback logic operating in a positive manner. The Creation inverter has dedicated feedback signal input channels, namely the PID/FB1 and GND terminals; the feedback signal is set to a voltage input of 0~5V via parameter P99. The target signal is set by parameter P03. Since parameters P103 and P104 have already defined the upper and lower limits for the range of the temperature sensor, it is possible here to set the absolute value of the desired temperature of the return water; this value can be 5–10°C higher than the temperature of the outlet water, depending on the cooling requirements of the air-conditioned room. The chilled water circulation pump is not allowed to stop operating, so parameters P16 and P105 have been set. Other effective application parameters of the inverter are not listed in Table 4; the factory default values are used by default. The temperature meter is the CHANGHUI YR-GFC display controller; its temperature measurement range is set at 0~100°C, with a corresponding output signal of 0~5V. In other words, a voltage signal of 5V is generated when the temperature is 100°C, while a voltage signal of 0V is generated at 0°C, which corresponds to the feedback signal selection set by parameter P99. This temperature measurement output signal is the feedback signal to the frequency converter. Application results: After the frequency converter and air conditioning system were installed, they were powered on for a trial run. By pressing the RUN button on the frequency converter’s panel (parameter P00 in Table 4 sets the operation command to be controlled by the panel’s RUN/STOP button), the motor started running. Subsequently, parameters such as the proportional gain P in P100, the integration time I in P101, the acceleration time in P21, and the deceleration time in P22 were adjusted appropriately, after which the system was put into full operation. This resulted in a 25% reduction in energy consumption and stable room temperatures. Source: Inverter http://yunrun.com.cn/tech/list_63.html