Abstract: This paper introduces the methods and issues to be noted in the field installation and commissioning of medium and low voltage frequency converters, and explains the application effect of frequency converters from the user's perspective. Keywords: Medium and low voltage, installation, commissioning. Introduction: With the increasing popularity of frequency converters in industrial production, frequency converters, as a speed-regulating and energy-saving product, have received increasing attention from industrial and mining enterprises, becoming an indispensable tool in industrial production and daily life. In various industries related to national economy and people's livelihood, such as petroleum, chemical, building materials, power, mining, plastics, metallurgy, and water conservancy, frequency converters are playing a powerful role. According to statistics, in the current and next few years, the sales of frequency converters will grow at a rate of 10%-15%, that is, approximately 5-9 million KW per year. Frequency converters of various power levels, performance, and applications will be available to users. Therefore, how to enable users to effectively utilize frequency converters and truly leverage their advantages in convenient speed control, energy saving, and improved production processes is a primary concern for frequency converter manufacturers. This article takes the use of frequency converters by users as an example, and combines some problems encountered by the author during user installation and commissioning to briefly discuss the installation and commissioning issues of frequency converters for reference by a wide range of frequency converter users. I. Installation and Usage Conditions of Frequency Converters Just like other electrical equipment or devices, frequency converters, as a type of power electronic equipment, also have strict usage conditions and application scenarios. Any installation or use that violates the product usage specifications will be illegal and will inevitably lead to losses of equipment, personnel, and property. 1. Usage Environment and Precautions for Frequency Converters 1.1 The required working environment for frequency converters is briefly described as follows: 1) Working environment temperature: -10℃ to +40℃, and the change in the working environment should not exceed ±5℃/h. 2) Relative humidity: The maximum relative humidity of the air should not exceed 90%, the hourly change rate of relative humidity should not exceed 5%, and condensation should not occur. 3) The operating location should be free of conductive or explosive dust, and free of gases or vapors that corrode metals or damage insulation. 4) Permissible vibration conditions at the inverter installation location: Vibration frequency 10-150Hz, vibration acceleration not exceeding 5m/s². When the inverter may resonate due to vibration of the mounting base, vibration reduction measures should be taken to avoid the resonant frequency. 5) AC input power supply: a) Voltage fluctuation not exceeding ±20%. b) Frequency fluctuation not exceeding ±2%, frequency change not exceeding ±1% per second. c) Three-phase voltage imbalance: negative sequence component not exceeding 5% of positive sequence component. d) Power supply harmonic components: the root mean square value of the relative harmonic content of the voltage not exceeding 10%. 6) Altitude: Not exceeding 1000m. 1.2 Precautions: 1) Non-professionals should not open the cover or cabinet door for use or testing; 2) The inverter has undergone withstand voltage testing before leaving the factory, and users do not need to conduct withstand voltage testing on the inverter again; 3) Large capacitors for improving power factor should not be connected in parallel to the motor; 4) The casing must be reliably grounded; 5) Three-phase input should not be changed to two-phase input, otherwise phase loss protection will occur; 6) When operating at low frequency, the effect of the motor's built-in fan and lubrication should be considered, and when operating at high frequency, the bearing capacity should be considered. These are the most basic rules for inverter installation. These rules must be thoroughly understood and familiarized with. 2. Preparatory work before installation To install an inverter and ensure its normal operation, meeting technical and process requirements, in addition to meeting the above basic rules, the following points should be noted: 1) Before installation, it is essential to be familiar with and master the production process and technical requirements, understand its load conditions, and understand the role and position of the inverter in the system. Is the goal energy saving, improving the production process, or both? In some cases, there is no room for energy saving, but it is inappropriate to insist on energy saving by the inverter. 2) From an electrical perspective, the primary load of the frequency converter is the motor. Therefore, before installation, a clear understanding of the motor on site is essential, including its rated voltage, rated current, number of poles, and rated power. The installed frequency converter must be compatible with it. In some special cases, such as heavy loads, altitudes exceeding 1000m (i.e., exceeding standard altitudes), or frequency converters for coal mine hoists, the frequency converter must be one or even two power ratings higher than the load motor. Generally, it is not allowed for the frequency converter to be of a lower power rating than the load motor to avoid overloading and frequent overload protection, causing unnecessary trouble. 3) The electrical insulation of the motor must be tested before installation. Motors with poor insulation cannot be installed with frequency converters. Although frequency converters have short-circuit protection, momentary grounding can still damage some frequency converters. 4) Before installation, carefully read the frequency converter's instruction manual, considering which parameters need to be set according to the on-site process, and the methods for setting these parameters. Become proficient in these settings. 5) For certain applications, especially those requiring automatic control and necessitating auxiliary accessories, such as pressure gauges, sensors, pressure transmitters, and supporting facilities like PID controllers, temperature controllers, and timers for water supply, and sometimes remote control devices, a thorough understanding of these is essential for rapid installation and commissioning. 6) Wiring must strictly adhere to the inverter's instruction manual, including main and control lines. In some cases, specifications must exceed the manual's requirements, not fall below. Where crimping lugs is required, they must be crimped strictly according to specifications and workmanship standards. 7) In complex modern industrial control systems, electromagnetic compatibility must be considered. Influence and anti-interference measures for the inverter must be taken into account, and electromagnetic filters may be added if necessary. In some cases, where the motor is far from the inverter, output reactors and filters should be considered. 8) For potential energy loads, such as coal mine main shaft winches, hoists, and elevators, due to regenerative power generation, braking units and matching braking resistors should be installed to prevent overvoltage protection or damage to the inverter. The above issues are all things we need to understand and master before installing a frequency converter. Failure to do so may lead to unsuccessful installation or commissioning, resulting in equipment damage or malfunction. This is something we must remember. II. Installation and Commissioning of Medium-Voltage Frequency Converters The medium-voltage frequency converters we are referring to here are those with voltage levels ranging from 750V to 2300V, divided into two voltage levels: 1140V and 2300V. This type of frequency converter is generally used for submersible electric pumps in oil fields; the 1140V version is also used for pumping units. We will briefly describe the installation and commissioning methods. 1. Installation Steps a. Connect the control line and confirm the original operating status at the power frequency. First, it is necessary to understand the working process of the submersible electric pump. Submersible electric pumps (SAPs) are electric pumps placed 1000-3000 meters underground. Due to the long cables, the surface power supply equipment must compensate for cable losses. Therefore, the original power frequency unit must provide a supply voltage 100-300V higher than the motor's rated voltage. Both the pump and cable are high-temperature, high-pressure equipment, and insulation tests must be performed before installation. Using a 2500V megohmmeter, the insulation resistance should not be lower than 50MΩ. The original power frequency supply voltage and operating current must be checked for reference during commissioning. Because the submersible electric pumps are deployed at different depths in oil fields, the supply voltage varies. Therefore, for frequency converters of the same voltage level, different voltage inputs result in different control voltages. To solve this problem, a transformer is installed inside the frequency converter, ensuring the same voltage output regardless of the input voltage. For this type of frequency converter, the output voltage is limited to 220V, 110V, and 380V. 220V supplies power to the control circuit, 110V supplies power to the motor protection device, and 380V supplies power to the voltmeter. The control wiring is shown in Figure 1. [align=center] Figure 1, Transformer Tap Diagram[/align] Therefore, select the appropriate control power supply terminals based on the original power supply voltage. Connect the two wires that were left unused at the factory or the two wires connected to the highest voltage level (tested at the highest voltage at the factory) to the terminals of the original voltage level. Since there are two transformers, rewiring is required. See Figure 2. [align=center] Figure 2, Control Terminal Wiring Diagram[/align] b. Connecting the Main Line: According to the wiring specifications, connect the power lines to the three-phase input (or marked R, S, T) terminals of the frequency converter, the motor lines to the three-phase output (or marked U, V, W) terminals of the frequency converter, and the ground wire to the terminal marked "⊥" on the frequency converter. Tighten the corresponding bolts. 1. Debugging: a. Pull down the isolating switch on the cabinet door and turn the "Variable Frequency Start" button on the cabinet door to disconnect it. After power is restored, use an MF-47 or 500 multimeter in the "2500V" high-voltage range to measure the three-phase input voltage (be extremely careful). It should be within the specified range and the three phases should be balanced. If incorrect, disconnect the power supply and check the power source. b. After the power supply is normal, close the isolating switch and use a multimeter to check the 220V voltage of the control power supply (measure the junction box inside). Check if it is within the specified range, allowing for ±20% fluctuation. It should be within 220V ± 20V. If it exceeds this range, pull down the high-voltage isolating switch and adjust the control transformer connections until the requirements are met. c. Connect one wire from a single-phase plug to terminal "C" of the control terminal block, and connect the other two phases to the junction box (for older models, connect one wire to terminal "C" and the other to terminal "D"). Close the isolating switch. The inverter should receive control power, and the panel should display "43.21". After a few seconds, the "PRO" light should illuminate (for newer models), or only the "PRO" light should illuminate (for older models). If no display is shown, reverse the plug and reconnect it. d. After the panel display is normal, check if the inverter's control function is normal. Use a multimeter to check the voltage across the time-delay thyristor (i.e., the voltage across the time-delay resistor). It should be around 1.00V. Turn the inverter's "On/Off" switch to the "On" position. The inverter should start. Adjust the "Frequency Adjustment" knob; the inverter frequency should increase from "2.00" to "50.00". Use a multimeter to measure the three-phase output terminals of the frequency converter (measurement should be taken from the converter side, as the cabinet is not connected to main power and the converter's main contactor is not engaged). The three-phase voltages should be balanced, and the voltages should also be balanced relative to the neutral line. The approximate voltages are: approximately 15V for 2300V and approximately 10V relative to the neutral line; approximately 9V for 1140V and approximately 5V relative to the neutral line. If unbalanced, check for any disconnections or other problems during transport until the voltages are completely balanced. e. Setting operating parameters. This includes setting the frequency converter parameters and the motor protection device parameters. a) Setting frequency converter parameters. Since frequency converters are generally designed for 2300V/125KW or 1140V/75KW at the factory, they should be reset according to the requirements of the on-site load, including rated current and overload protection current. Other parameters generally do not need to be modified. b) Setting motor protection device parameters. The parameters of the motor protection device include underload current, overload current, undervoltage setting, and overvoltage setting. Here's an explanation: Because submersible pumps are deeply embedded in the oil layer of the well, they rely on oil circulation for cooling during operation. If the well fluid supply is insufficient, the motor will detach from the oil layer and run dry, easily burning out the pump. Therefore, underload protection should be set. When the operating current is lower than 80% of the rated current, the motor will stop running, protecting it. The parameter setting method can be found in the corresponding instruction manual. There are two main types: BK-3 and BK-J1. For the BK-3 type, the method is as follows: When the display shows "P" (just after power-on or after pressing the "Up" key four times), the parameter value can be modified. Press the corresponding number key to the required value, then press the "Up" or "Down" key, and finally press the corresponding function key to complete the parameter value modification. The BK-JI method is as follows: In the ready screen (just powered on or after pressing the "Up" key four times), press the "Up" key once, then press the "Setting Value" key once. The screen will display the setting value operation screen. A cursor will flash on the screen; you can enter the corresponding data at the flashing cursor position. Press the "Move" key to move the cursor up and down. The arrow ("↑" or "↓") in the upper right corner of the screen indicates the current movement direction. Press the "Up" and "Direction" keys to change the direction of the arrow. After inputting the data, press the "Up" key four times to save the changes. As shown in the figure. Some wells may have insufficient fluid supply and require reduced frequency operation. Therefore, depending on the specific situation, the parameters can be reset according to the actual operating current after the pump is running. f. Load operation. After the parameters are set, the plug on the junction box can be unplugged, and the wires on the external "C" (or "C" and "D") points can be disconnected to allow load operation. Press the "Variable Frequency Start" button on the cabinet door. After the set delay time, the internal contactor will engage, and the inverter will display "43.21" with "PRO" lit (new models) or only "PRO" lit (older models). Turn the "Start/Stop" switch on the inverter to the "Start" position. The inverter will then gradually increase from the lowest frequency. Adjust the "Frequency Adjustment" knob to raise the frequency to 30Hz. Use a current meter to measure the three-phase output current; it should be basically balanced, with an imbalance not exceeding 20%. Then raise the frequency to the user's required frequency. Use a current meter again to measure the input and output currents; all three phases should be basically balanced. If the frequency does not increase during adjustment—that is, the frequency does not increase when adjusting the frequency adjustment knob, and the output current continues to increase, triggering speed limit protection and eventually overcurrent protection—this may be due to insufficient low-frequency compensation. This can be resolved by adjusting the compensation potentiometer on the main control board (older models) or by setting the inverter's "Low-Frequency Compensation" parameter (newer models). During adjustment, ensure the output current decreases while the frequency increases; avoid excessive adjustment to prevent overcompensation, which could lead to overcurrent in the motor and inverter, causing malfunctions. After normal operation, readjust the motor protection parameters based on the operating current to achieve reliable protection and normal operation. Confirm the motor's forward and reverse rotation. Generally, forward rotation draws more current and indicates higher wellhead pressure. Reverse rotation typically doesn't raise the pressure gauge. In some wells with slow oil production, waiting a while is necessary to confirm this. Since this inverter cabinet also has a backup power frequency to temporarily switch in case of inverter problems, ensuring production isn't interrupted, test power frequency operation after the inverter is working correctly, ensuring forward and reverse rotation is also normal. When adjusting forward and reverse rotation, adjust the inverter output and the power frequency input to ensure both operate normally. After adjustment, run the frequency converter normally. Once normal operation is achieved, start the motor protection device. After the protection device displays normal operation, turn off the "frequency converter start" knob (this activates the motor protection device; it will not work if not turned off). All instrument readings on the cabinet should be normal, indicating the debugging is complete. III. Installation and Debugging of Low-Voltage Frequency Converters Low-voltage frequency converters handle a wide variety of loads, resulting in significant differences in installation and debugging. From the perspective of using the frequency converter, low-voltage loads can be broadly categorized into those with inertia and those without. The most significant characteristic of loads with inertia is that they generate electricity during rapid shutdown. How to handle this energy is crucial for the normal operation of the frequency converter. (I) Installation: Before initial installation or long-term storage, a comprehensive inspection of the frequency converter is necessary. The methods are as follows: 1) Visual inspection: Check for any damage, rust, or frost/condensation on metal parts. If frost/condensation is present, dry the parts for 4 hours (60℃) or allow them to ventilate at room temperature for 24 hours. 2) For low-power frequency converters, gently turn the casing over and check for any abnormal noises inside. If any abnormal noises are heard, open the casing and locate and remove any foreign objects. 3) For higher-power frequency converters, open the casing and check for any loose wires or bolts that may have come loose during transportation or storage. If any are found, resolder or tighten them. Several points should be noted here: a) The installation of the frequency converter should meet the aforementioned working environment requirements. b) Frequency converters are available in wall-mounted and cabinet-type configurations; they must be installed securely to ensure safety during operation. For ventilation and heat dissipation, the frequency converter must be installed vertically, as shown in Figures 5 and 6. c) For cabinet-type structures, sufficient space should be left around the converter for easy operation and heat dissipation: at least 1.5 meters in front and at least 1 meter behind and to the sides. For wall-mounted converters, sufficient ventilation space should also be provided around the converter; the top of the converter should be at least 1 meter from the ceiling, and the bottom should be at least 1 meter from the ground to ensure smooth ventilation and reliable operation. [align=center] Figure 5. Installation of a single frequency converter a) b) Figure 6. Cabinet installation of a single frequency converter[/align] a) External cooling method b) Internal cooling method Some rooms are tightly sealed, requiring the installation of exhaust fans or air conditioners in cooperation with the user. In some environments that are dirty and humid, it is important to take isolation measures to prevent dust and moisture. When two or more frequency converters are installed in a control cabinet, they should be installed side by side (horizontal arrangement) as much as possible. If a vertical arrangement is necessary, a shelf should be added between the two frequency converters to prevent hot air from the lower frequency converter from entering the upper frequency converter. As shown in Figure 7. [align=center] Figure 8. Cabinet installation of two frequency converters[/align] a) Horizontal arrangement b) Vertical arrangement 1.1 Wiring of the main circuit 1. Basic wiring The installation of the main line is relatively simple. Connect the power cord to the input (or terminals marked R, S, T) of the frequency converter, connect the motor wire to the output (or terminals marked U, V, W) of the frequency converter, and reliably ground the frequency converter's grounding terminal through the ground wire. See Figure 8. [align=center]Figure 8, Basic Wiring of the Main Circuit[/align] Note: a. The input and output terminals of the frequency converter must never be connected incorrectly. If the power input is incorrectly connected to the output terminal, regardless of which inverter tube is conducting, it will cause a short circuit between the two phases and quickly burn out the inverter tube. See Figure 9. [align=center]Figure 9, Consequences of Incorrect Power Connection[/align] b. A circuit breaker is generally required, mainly for disconnecting the circuit. When the frequency converter malfunctions, especially when the rectifier circuit or main circuit is damaged, the large current can promptly trip the circuit breaker, disconnecting it from other circuits in the grid and preventing impact on other circuits. c. Wire crimping must be reliable. Generally, use wire lugs of equivalent capacity for crimping. 1. Ensure tightness to prevent overheating and burnout of wiring or terminals during long-term operation under high current. 2. Wire diameter selection: Generally, select the wire diameter according to the wiring requirements of the motor. In special cases, select a larger specification, especially if the motor is far from the inverter. Choose a larger size rather than a smaller one. 3. Grounding: Each inverter has a dedicated grounding terminal "E" or "⊥". The user should reliably connect this terminal to the earth. When the inverter is grounded with other equipment, or multiple inverters are grounded together, each equipment must be connected to the ground wire separately. It is not allowed to connect the grounding terminal of one equipment to the grounding terminal of another equipment before grounding. As shown in Figure 10. [align=center] Figure 10. Grounding of inverter and other equipment[/align] a) Correct connection method b) Incorrect connection method 1.2. Wiring of control circuit: After the main wiring of the inverter is completed, the inverter can run. However, in general, for the convenience of control and monitoring, it is necessary to bring the operation and display parts of the inverter to a convenient location. In some cases, the on-site environment is poor, making it unsuitable to install the frequency converter. Instead, it is installed in a better-equipped power distribution room, with the control section brought to the site. In other cases, such as hoist frequency converters, the control section needs to interface with the original system, requiring control lines to be brought out. Control lines are divided into analog and digital signals. 1. Analog signals. These mainly include: input side setpoint signal lines and feedback signal lines; output side frequency signal lines and current signal lines. Analog signal lines have poor anti-interference capabilities and must use shielded cables. The end of the shield closest to the frequency converter should be connected to the common terminal of the control circuit, but not to the frequency converter's ground (E) or earth. The other end of the shield should be left floating, as shown in Figure 11. [align=center] Figure 11, Shielded cable connection method[/align] The wiring principles should be followed: A. Keep at least 100mm away from the main circuit. B. Avoid crossing with the main circuit as much as possible. If crossing is unavoidable, a perpendicular crossing should be used. 2. Switching signals. Control lines for start/stop, jogging, and multi-speed control are switching control lines. Generally speaking, the wiring principles for analog control lines also apply to digital control lines. However, digital signals have strong anti-interference capabilities, so unshielded wires are allowed when the distance is not very far, but the two wires of the same signal must be twisted together. (II) Debugging the frequency converter There is no fixed mode for debugging the frequency converter. It can be roughly divided into several steps: "first no-load, then light load, then heavy load". 1. No-load check and parameter preset The wind and light frequency converter can be checked without connecting the main power supply, but only the control power supply. Other types of frequency converters do not have this convenience. For low-power frequency converters, the shorting piece on the main terminal block can be removed to supply 380V power to the three-phase input. High-power frequency converters generally have control voltage input terminals. A one- or two-core cable can be used to connect 380V single-phase power to check the frequency converter (Note: before connecting, the two wires connected to the three-phase input on both ends should be removed). Refer to the instruction manual to familiarize yourself with the use of each key and the parameter setting method. After familiarizing yourself with the system, turn on the inverter and raise the frequency to 50Hz. Use a multimeter (preferably a pointer type) to measure the three-phase output. The voltage should be completely balanced. After checking, turn off the power. For low-power inverters, put the removed shorting piece back in its original position (note that the two terminals of the shorting piece on the main circuit should be discharged before putting it back). For high-power inverters, remove the two external wires of the control terminal and restore the original wiring. Do not connect the three-phase output of the inverter to the motor wire. Connect the three-phase input of the inverter to a 380V power supply and observe the inverter's no-load operation. Other types of inverters can also follow this procedure. (1) Familiarize yourself with the keyboard, that is, understand the function of each key on the keyboard, perform trial operation, and observe the changes in the display. (2) Perform basic operations such as "start" and "stop" according to the instructions, observe whether the inverter is working normally, and further familiarize yourself with the keyboard operation. (3) Preset the parameters. Preset the main parameters according to the method described in the instructions. Check whether the inverter's performance matches the preset values ​​for several easily observable items such as acceleration and deceleration time, jogging frequency, and frequency of each gear when using multiple speeds. (4) Connect the external input control lines and check the performance of each external control function item by item. (5) Use a multimeter (preferably an analog type) to check whether the three-phase output voltage is completely balanced. 2. Load operation After the above tests, it is proven that the inverter is normal and can be operated with a load. The load operation of the inverter includes light load test operation and heavy load operation, i.e., normal operation. If conditions permit, you can also test run the motor with no load first, but in general, it is directly operated with a load. Before the test operation, the insulation of the motor should generally be checked. For low-voltage 380-660V motors, use a 1500V megohmmeter to test, and the insulation resistance should generally not be lower than 50 megohms. The insulation of water pump loads may be lower, but it should not be lower than 2 megohms. You should also understand the operation of the load to have a clear understanding. Low-voltage loads are diverse, including fans, pumps, mixers, wire drawing machines, plastic machinery, hoists, air compressors, belt conveyors, chemical fiber machines, machine tools, and many more. Different loads have significantly different operating conditions, therefore requiring differentiated treatment. Some special machinery necessitates the use of dedicated frequency converters. For example, hoists, being potential energy loads, require regenerative energy processing and necessitate dedicated hoist frequency converters. Similarly, some chemical fiber and machine tool loads also require specialized frequency converters. These converters incorporate special hardware and software modifications tailored to these specific loads, ensuring reliable operation. Centrifugal loads, such as centrifugal fans and centrifuges, have significant inertia, resulting in longer acceleration and deceleration times. Shorter settings can lead to overcurrent during acceleration and overvoltage during deceleration, potentially causing damage. Therefore, unless rapid shutdown is required, the time can be appropriately extended, generally equivalent to the equipment's free stop time (experiencedly around 300 seconds). If rapid shutdown is required, a braking circuit should be added. Once the above issues are clarified, operation under load is possible. Connect the inverter output to the motor wiring. Power on. 1) Jog or test run at low frequency. Observe the forward and reverse directions of the motor. If it reverses, adjust using the inverter's forward and reverse terminals, or adjust the inverter's output wiring after power is off. Some machinery may not allow reverse rotation; in this case, first disconnect the motor and machinery coupling, run the motor idle, adjust the direction of rotation, and then reconnect the coupling. For loads like submersible pumps, where the direction of rotation is not visible above the well, experience can be used: forward rotation results in lower current, increased pressure gauge reading, and higher water flow; reverse rotation results in lower pressure gauge reading, lower water flow, and higher current. 2) Start-up test. Gradually increase the operating frequency from 0Hz and observe whether the drive system can start. At what frequency should it start? If starting is difficult, try to increase the starting torque. Specific methods include: increasing the starting frequency, increasing the U/f ratio, and using vector control. For some loads, such as submersible pumps with long motor leads, there may be a long-line effect, meaning that due to the large harmonic components in the inverter output, the voltage at the motor terminals may increase. Therefore, during adjustment, pay attention to the changes in motor current. If a continuous increase in motor current is observed, stop the machine immediately, and consider installing an output reactor. 3) Starting test. Apply the given signal to its maximum and observe: a) the change in starting current; b) whether the entire drive system runs smoothly during the speed-up process. If the circuit breaker trips due to excessive starting current, the speed-up time should be appropriately extended. If the starting current is too high in a certain speed range, try to solve the problem by changing the starting method (S-shaped, half-S-shaped, etc.). 4) Shutdown test. Adjust the operating frequency to the highest operating frequency, press the stop button, and observe the shutdown process of the drive system. a) Whether the circuit breaker trips due to overvoltage or overcurrent during the shutdown process. If so, the deceleration time should be appropriately extended. b. When the output frequency is 0Hz, check if the drive system exhibits creeping behavior. If so, apply appropriate DC braking. 5) Load Test of the Drive System The main contents of the load test are: a. If fmax > fN, a load capacity test should be performed at the highest frequency. That is, whether it can be driven under normal load. b. At the lowest operating frequency of the load, examine the motor's heating. Make the drive system operate at the minimum speed required by the load, apply the maximum load at that speed, and conduct a low-speed operation test for the continuous running time required by the load, observing the motor's heating. c. Overload test can be conducted according to the possible overload conditions and duration of the load, observing whether the drive system can continue to work. After adjustment, the inverter should be put into formal load operation, generally observed for more than two hours to ensure reliable operation. These are the most basic steps in inverter commissioning. During inverter commissioning, various situations may be encountered, such as inverter interference and anti-interference, power factor compensation, closed-loop operation, etc., which must be gradually mastered through specific practice. Here, we will discuss closed-loop operation. This is one of the most important functions of a frequency converter. In many situations, the advantages of automatic control can only be realized through closed-loop operation. 1. Principle of a Closed-Loop System Closed-loop operation involves selecting a physical quantity (such as temperature, pressure, tension, liquid level, etc.) of the driven system. At a certain point (this point should play a crucial role in the operation of the entire system or have universality), a corresponding sensor or transmitter (such as thermocouples, remote pressure gauges, temperature transmitters, pressure transmitters, tension sensors, etc.) detects this quantity and sends it to a PID controller. This PID controller performs proportional, integral, and derivative calculations with the system's desired value (which can be set on the PID controller), and then sends the result to the frequency input terminal of the frequency converter to adjust the frequency of the frequency converter, thereby adjusting the motor speed, so that the entire driven system is in a state of automatic regulation and stable operation. The detected value of the system is called the feedback signal, and the desired value is called the setpoint signal or target signal. The system regulation process is the repeated comparison and calculation of these two signals to make them as close as possible. Here, we will use constant pressure water supply as an example. (See Figure 12 below.) [align=center] Figure 12, Schematic diagram of constant pressure water supply system[/align] The water pump motor in the figure is powered by the frequency converter VVVF. SP is a pressure sensor that detects the pressure on the pipeline. It can also be a pressure transmitter, remote pressure gauge, etc., which are powered by the frequency converter with +24V or +5V. After detecting the pipeline pressure, it converts it into a 4-20mA current or a 0-5V voltage signal and sends it back to the frequency converter. After the frequency converter is set to PID effective, the frequency converter has two analog signal input terminals: (1) Target signal input terminal. That is, the setpoint terminal VRF. It is a value corresponding to the pressure control target. It is set by the potentiometer on the frequency converter. It can also be given directly by the keyboard. When a dedicated PID controller is used, it is set by the dedicated SV setting window. In addition to being related to the required pressure control target, it is also related to the range of the pressure transmitter SP. When setting it, it should be equivalent to the relevant range. (2) Feedback signal input terminal. That is, the auxiliary setpoint terminal VPF. It receives the signal fed back from the pressure sensor SP. 2. Control Process: Let XT be the target signal, the magnitude of which corresponds to the required pipeline pressure. XF is the feedback signal from the pressure transmitter. The output frequency fx of the frequency converter is determined by the synthesized signal (XT—XF). If the pipeline pressure p exceeds the target value, then XF>XT→(XT—XF)<0→the output frequency fx of the frequency converter decreases→the motor speed nx decreases→the pipeline pressure p decreases→until it matches the required target pressure (XT≈XF). Conversely, if the pipeline pressure p is lower than the target value, then XF… 0→Inverter output frequency fx↑→Motor speed nx↑→Pipe pressure p↑→Until it matches the required target pressure (XT≈XF). There is a contradiction in the above process: On the one hand, we require the actual pressure of the pipeline (its magnitude is proportional to XF) to be infinitely close to the target pressure (its magnitude is proportional to XT), that is, (XT—XF)→0; on the other hand, the inverter output frequency fx is determined by the result of subtracting XT and XF. It can be imagined that if (XT—XF) is directly used as the given signal XG, the system will not work. How to solve the above problem leads to the use of PID. (1) The method of solving the above problem by the proportional (P) link is to amplify (XT—XF) and then use it as the frequency given signal, that is: XG =KP(XT-XF) Where KP——proportional gain (i.e., amplification factor). [align=center] Figure 13, Relationship between quantities before and after proportional amplification[/align] The above relationship is shown in Figure 14. Since XG is the result of proportional amplification of (XT-XF), this stage is called the proportional stage. Obviously, the larger KP is, the smaller (XT-XF) = XG/KP is, and the closer XF is to XT. Here, XF can only be infinitely close to XT, but cannot be equal to XT. That is to say, there will always be a difference between XF and XT, usually called steady-state error, denoted by ε. The steady-state error should be as small as possible. The introduction of the proportional gain stage brings a new contradiction: in order to reduce steady-state error, the proportional gain should be increased as much as possible, but because the system has inertia, if KP is too large, it is easy to cause the controlled variable (pressure) to fluctuate, forming oscillation. As shown in Figure 14. [align=center] Figure 14, Schematic diagram of the combined effect of P, I, and D[/align] a, P regulation b, Oscillation phenomenon c, PI regulation d, PID regulation (2) Integral (I) stage The purpose of introducing the integral stage is a, to make the change of the given signal XG proportional to the integral of the product KP (XT-XF) over time.就是说，尽管KP（XT-XF）一下子增大（或减小）了很多，但XG只能在“积分时间”内逐渐的增大（或减小），从而减缓了XG的变化速度，防止了振荡。积分时间越长，XG的变化越慢。 b、只要偏差不消除，（XT——XF≠0），积分就不停止，从而能有效的消除静差。如图所示。 但积分时间太长，又会发生在被控量（压力）急剧变化时，被控量（压力）难以迅速恢复的情况。 （3）微分（D）环节微分环节的作用是：可根据偏差的变化趋势，提前给出较大的调节动作，从而缩短调节时间，克服了因积分时间太长而使恢复滞后的缺点。如图14。 变频器一般在内部都设定了简单的PI调节器。这对于较简单的闭环控制，如供水、简单的风压控制，可以满足要求。但对于较复杂的控制场合，如空压机、空调、离心风机闭环，温度，液位等，一般都要加专用的PID调节仪才能得到较好的控制。PID调试时，要参照对应的说明书，仔细的调整各个参数，以期达到最佳的运行效果，使系统能自动的、稳定的运行。四、结语 本文在自己多年从事变频器安装调试的基础上，粗落的谈了下变频器最基本的安装调试方法，以及在安装调试过程中一些简单问题的处理方法。目的是总结自己的经验，并与业界同仁相互交流，共同探讨变频器的使用方法，以期更好的发挥变频器的强大作用，更好的服务于广大的工矿企业用户。同时也为变频器的使用者提供一份不太成熟的手册似的工具。当然，随着控制系统的复杂，越来越多的先进的控制方法正不断应用于工控系统，如PLC、DCS控制、工控机等，因篇幅有限，这里不能一一陈述，还望读者海涵。因本人能力有限，文章中的缺点和错误也在所难免，望广大的同行不吝指教。现在随着我国国民经济的发展，节能降耗已成为殛待解决的问题，相信我们所从事的行业必将成为国家节能降耗的最直接的手段，为国民经济的发展付出自己的努力。 参考文献： 1、风光变频器中、低压使用说明书2、《SPWM变频调速应用技术》 张燕宾编著机械工业出版社2005年7月第三版