1. Overview With the continuous development of automation, frequency converters are increasingly used in various industries. Frequency converters are constantly being innovated, with increasingly larger functions and correspondingly improved reliability. However, improper use, incorrect operation, and untimely maintenance can still lead to malfunctions or changes in operating conditions, shortening the equipment's lifespan. Therefore, daily maintenance and repair are particularly important. 2. Precautions Operators must be familiar with the basic working principles and functional characteristics of frequency converters and possess basic electrical operation knowledge. Before inspecting and maintaining the frequency converter, the main power supply must be completely disconnected, and the frequency converter's indicator light must be completely off. 3. Daily Inspection Items Before powering on the frequency converter, the ambient temperature and humidity should be checked. Excessive temperature will cause the frequency converter to overheat and alarm, and in severe cases, it can directly damage the frequency converter's power devices and cause short circuits. Excessive humidity can cause direct short circuits inside the frequency converter. During frequency converter operation, pay attention to whether its cooling system is functioning properly, such as whether the air duct is unobstructed and whether the fan makes any abnormal noises. Generally, inverters with higher protection ratings, such as IP20 and above, can be installed in an open manner. Inverters with IP20 and below should generally be installed in a cabinet. Therefore, the heat dissipation effect of the inverter cabinet directly affects the normal operation of the inverter. The inverter's exhaust system, such as whether the fan rotates smoothly and whether there is dust or blockage at the air inlet, are areas that cannot be ignored in our daily inspections. Check whether the motor, reactor, transformer, etc., are overheating or have any unusual odors; whether the inverter and motor have any abnormal noises; whether the current display on the inverter panel is too high or the current fluctuation is too large; and whether the output three-phase voltage and current (UVW) are balanced. 4. Regular Maintenance Clean the air filter cooling duct and internal dust. Check whether screws, bolts, and connectors are loose. Check whether there is a short circuit in the input and output reactors to ground and between phases; the resistance should normally be greater than tens of megohms. Check whether the conductors and insulators are corroded; if so, wipe them clean with alcohol immediately. When conditions permit, use an oscilloscope to measure the stability of the output voltage of each circuit of the switching power supply, such as 5V, 12V, 15V, 24V, etc. Measure whether there is distortion in the waveform of each circuit of the driver circuit. Check whether the UVW phase waveform is a sine wave. Check whether there are arcing marks on the contacts of the contactor. If serious, replace with a new product of the same model or larger capacity; confirm the correctness of the control voltage and perform a sequential protection action test; confirm that there is no abnormality in the protection display circuit; confirm the balance of the output voltage of the inverter when it is running alone. It is recommended to check regularly, once a year. 5 Replacement of spare parts The inverter is composed of a variety of components. After long-term operation, the performance of some of these components will gradually decrease and age. This is also the main reason for inverter failure. In order to ensure the long-term normal operation of the equipment, the following components should be replaced regularly: (1) Cooling fan The power module of the inverter is the most heat-generating component. The heat generated by its continuous operation must be dissipated in time. The life of a fan is generally about 10kh～40kh. The fan should be replaced every 2-3 years based on the continuous operation of the frequency converter. Direct cooling fans are divided into two-wire and three-wire types. For two-wire fans, one wire is positive and the other is negative. Do not connect them incorrectly when replacing. For three-wire fans, in addition to the positive and negative wires, there is also a detection wire. Be careful when replacing them, otherwise it will cause the frequency converter to overheat and alarm. AC fans are generally divided into 220V and 380V. Do not mix up the voltage level when replacing them. (2) Filter capacitors Intermediate DC circuit filter capacitors: also known as electrolytic capacitors, their main function is to smooth the DC voltage and absorb low-frequency harmonics in the DC. The heat generated by its continuous operation, plus the heat generated by the frequency converter itself, will accelerate the drying of its electrolyte, directly affecting its capacity. Under normal circumstances, the service life of the capacitor is 5 years. It is recommended to check the capacitor capacity once a year. Generally, it should be replaced when its capacity decreases by more than 20%. 6 Tests 6.1 Static Tests (1) Test the rectifier circuit. Locate the P and N terminals of the DC power supply inside the inverter. Set the multimeter to the resistance X10 range. Connect the red probe to P and the black probe to R, S, and T respectively. There should be a resistance of about tens of ohms, and they should be basically balanced. Conversely, connect the black probe to P and the red probe to R, S, and T in sequence. There should be a resistance close to infinity. Connect the red probe to N and repeat the above steps. The same result should be obtained. If the following results are obtained, it can be determined that the circuit has an abnormality: A. The resistance is unbalanced in the three phases, which indicates that the rectifier bridge is faulty. B. When the red probe is connected to P, the resistance is infinite, which indicates that the rectifier bridge is faulty or the starting resistor is faulty. (2) Test the inverter circuit. Connect the red probe to P and the black probe to U, V, and W respectively. There should be a resistance of tens of ohms, and the resistance of each phase should be basically the same. The reverse phase should be infinite. Connect the black probe to the N terminal and repeat the above steps. The same result should be obtained. Otherwise, the inverter module is faulty. 6.2 Dynamic Test After the static test results are normal, dynamic test, i.e., power-on test, can be carried out. The following points must be noted before and after powering on: (1) Before powering on, it is necessary to confirm whether the input voltage is correct. Connecting a 380V power supply to a 220V inverter will cause the inverter to explode (capacitors, varistors, modules, etc. to explode). (2) Check whether each port of the inverter is correctly connected and whether the connection is loose. Abnormal connection may sometimes cause the inverter to malfunction, and in severe cases, it may explode. (3) After powering on, check the fault display content and preliminarily determine the fault and cause. (4) If no fault is displayed, first check whether the parameters are abnormal. After resetting the parameters, start the inverter under no-load (without motor connected) conditions and test the output voltage values ​​of the three phases U, V, and W. If there is a phase loss or three-phase imbalance, the module or drive board is faulty. (5) Under normal output voltage (no phase loss, three-phase balance), perform load testing. It is best to perform full load testing during testing. 7 Fault diagnosis (1) Rectifier module damage is generally caused by grid voltage or internal short circuit. Replace the rectifier bridge after eliminating internal short circuit. When handling faults on site, the user's grid conditions should be checked, such as grid voltage, whether there are welding machines or other equipment that pollute the grid. (2) Inverter module damage is generally caused by motor or cable damage and drive circuit failure. After repairing the drive circuit, replace the module when the drive waveform is good. After replacing the drive board during on-site service, the motor and connecting cables must also be checked. Run the inverter after confirming that there are no faults. (3) No display after power-on is generally caused by damage to the switching power supply or soft charging circuit, resulting in no DC power in the DC circuit. For example, the starting resistor may be damaged, or the panel may be damaged. (4) Overvoltage or undervoltage is generally caused by input phase loss, circuit aging, and circuit board moisture after power-on. Find the voltage detection circuit and detection point, and replace the damaged components. (5) Overcurrent or short circuit display after power-on is generally due to damage to the current detection circuit, such as Hall element, operational amplifier, etc. (6) Overcurrent display during startup is generally due to damage to the drive circuit or inverter module. (7) Normal no-load output voltage, but overload or overcurrent display after load is generally due to improper parameter settings or aging of the drive circuit or module damage.