What are the VFD (Variable-frequency Drive) prone to failure? and a workaround!

3.1 VFD (Variable-frequency Drive) fault classification According to the characteristics of VFD (Variable-frequency Drive) failure or damage, it can generally be divided into two categories; one is frequent automatic shutdown during operation, accompanied by certain For fault display codes, the treatment measures can be handled and resolved according to the guidance method provided in the accompanying manual. This type of fault is generally a protection action phenomenon caused by the inappropriate setting of VFD (Variable-frequency Drive) operating parameters, or external working conditions and conditions that do not meet the requirements of VFD (Variable-frequency Drive); the other type is Due to the harsh use environment, sudden failures such as short circuit caused by high temperature and conductive dust, insulation reduction or breakdown caused by humidity (in severe cases, abnormal phenomena such as ignition and explosion will occur). After this type of failure occurs, VFD (Variable-frequency Drive) will generally have no display. The treatment method is to first disassemble and check the VFD (Variable-frequency Drive), focus on finding damaged parts, and clean up and measure according to the fault area. , Replace, and then fully test, then restore the system, test run with no load, observe the waveform on the output side of the trigger circuit, when the size and phase difference of the 6 groups of waveforms are equal, load and run again to solve the problem. This paper mainly expounds the analysis and treatment methods of the second type of failure. 3.1.1 Main circuit failure According to the statistics of the actual number of VFD (Variable-frequency Drive) failures and downtime, the failure rate of the main circuit accounts for more than 60%; the failure rate caused by improper operation parameters accounts for about 20%; the control circuit board appears Faults accounted for 15; faults caused by operating errors and external abnormalities accounted for 5. From the statistics of the degree of failure and the difficulty of handling, such failures will inevitably cause damage and scrapping of components. It is the main consumption part of VFD (Variable-frequency Drive) maintenance cost. (1) Damage to the rectifier block. The damage to the VFD (Variable-frequency Drive) rectifier bridge is also one of the common faults of the VFD (Variable-frequency Drive). The VFD (Variable-frequency Drive) rectifier blocks produced in the early stage are mainly rectified by diodes. At present, part of the rectification block adopts the thyristor rectification method (VFD (Variable-frequency Drive) of voltage regulation and frequency modulation type). Medium and high-power ordinary VFD (Variable-frequency Drive) rectifier modules are generally three-phase full-wave rectifiers, which are responsible for the rectification of all output power of the VFD (Variable-frequency Drive), and are prone to overheating and breakdown. The VFD (Variable-frequency Drive) cannot transmit power, the fuse is blown, etc., and the three-phase input or output terminals show low resistance (normally, the resistance value reaches above megohm) or short circuit. When replacing the rectifier block, it is required to evenly coat a layer of silicon thermal paste with good heat transfer performance on the contact surface with the heat sink, and then tighten the screws. If there is no rectifier block of the same type, it can be replaced by another type of rectifier block with the same capacity, and the fixing screw holes must be re-drilled and tapped before installation and wiring. For example, after a VFD (Variable-frequency Drive) (7.5kVA) rectifier module (oval shape) produced by Siemens in the mid-1980s broke down, a rectifier block (rectangular shape) of the same capacity produced by Sanken was used because there were no similar rectifier block accessories. After being replaced, it has been in operation for many years and is still in normal use. (2) VFD (Variable-frequency Drive) charging resistor is damaged due to easy damage to the charging resistor. The reason is generally: if the contactor of the main circuit does not absorb well, it will burn out due to too long flow time; or the charging current will be too high and burn out. Bad resistance; or because the main circuit is powered on and the RUN signal is turned on at the same time when the heavy load is started, the charging resistor must pass the charging current and the load inverter current at the same time, so it is easy to be burned out. The characteristics of its damage are generally manifested as signs of damage such as burning, blackening of the shell, and bursting. It can also be judged by measuring its resistance with a multimeter (machines with different capacities have different resistance values, which can be determined by referring to the resistance value of the same model). (3) The inverter module is burned out. Medium and small VFD (Variable-frequency Drive) generally use three sets of IGTR (high-power transistor modules); large-capacity models use multiple sets of IGTR in parallel, so they should be measured and checked one by one. to test. IGTR damage can also cause VFD (Variable-frequency Drive) OC (pA or pd or pn) protection function action. There are many reasons for the damage of the inverter module: such as a short circuit of the output load; the load is too large, and the high current continues to run; the load fluctuates greatly, resulting in excessive inrush current; the cooling fan is ineffective; Problems such as damage, performance deterioration, and parameter changes will cause abnormal output of the inverter. For example, a FRN22G11S-4CX VFD (Variable-frequency Drive), the output voltage three-phase difference is 106V, disassembled and checked the appearance of the inverter module (6MBP100RS-120) online, no abnormalities were found, and no fault was found by measuring the 6-way drive circuit. After the module was removed and measured, it was found that one group of modules could not be turned on normally, and the parameters of this module changed greatly (compared with the other two groups). After the replacement, the power-on operation was normal. Another example is the MF-30K-380 VFD (Variable-frequency Drive) which has a DC link overvoltage tripping fault during startup. This VFD (Variable-frequency Drive) does not trip over voltage every time it is started. During the inspection, it was found that after the VFD (Variable-frequency Drive) was energized (there was no power-on display signal on the control panel), the measured DC circuit voltage reached more than 500V, because the positive pole of the DC circuit of this type of VFD (Variable-frequency Drive) was connected in series. SK-25 contactor. When there is a closing signal, it will pick up after the pre-charging process, so it is suspected that the performance of the pre-charging circuit is poor, and the pre-charging circuit is disconnected, and the situation remains the same. Check the filter capacitor with a capacitance meter and find that it has failed. After replacing the capacitor, the VFD (Variable-frequency Drive) works normally. 3.1.2 Auxiliary control circuit failure VFD (Variable-frequency Drive) drive circuit, protection signal detection and processing circuit, pulse generation and signal processing circuit and other control circuits are called auxiliary circuits. After the auxiliary circuit fails, the cause of the failure is more complicated. Except for the loss of the curing program or the damage of the integrated block (this kind of fault treatment method can only be replaced by the whole control board or the integrated block), other faults are easier to judge and deal with. (1) Faulty driving circuit The driving circuit is used to drive the inverter IGTR, and it is also prone to failure. Generally, there are obvious signs of damage, such as bursting, discoloration, and disconnection of devices (capacitors, resistors, transistors, and printed boards, etc.), but there will be no damage to the drive circuit. The processing method is generally according to the schematic diagram, each group of driving circuits is reversely checked, measured, replaced, and compared step by step; or checked against another genuine (new) driver board to find the fault point step by step. Troubleshooting steps: First, clean the entire circuit board of dust and dirt. If the printed circuit is found to be disconnected, it will be repaired; if the damaged device is found, it will be replaced; according to the analysis of the author's practical experience, the suspected components should be judged by means of measurement, comparison, and substitution, and some devices need to be measured offline. After the drive circuit is repaired, use an oscilloscope to observe the output waveforms of each group of drive circuit signals. If the three-phase pulse size and phase are not equal, the drive circuit still has abnormalities (the parameters of the replaced components do not match, which will also cause this type of failure. Phenomenon) should be checked and dealt with repeatedly. The damage of the drive circuit where the high-power transistor works is also one of the reasons for the action of the over-current protection function. The most common phenomenon of drive circuit damage is phase loss, or three-phase output voltage unequal, three-phase current imbalance and other characteristics. (2) Damage to the switching power supply A more obvious feature of the damage to the switching power supply is that there is no display after the VFD (Variable-frequency Drive) is powered on. For example: Fuji G5S VFD (Variable-frequency Drive) adopts a two-stage switching power supply. The principle is that the DC voltage of the main DC circuit is reduced from 500V to about 300V, and then after a first-stage switch step-down, the power output is 5V, 24V, etc. Multiple power supplies. Common damages to switching power supplies include breakdown of switching tubes, burnout of pulse transformers, damage to secondary output rectifier diodes, and excessive use of filter capacitors, resulting in changes in capacitance characteristics (capacity reduction or large leakage current), and voltage stabilization capabilities. , It is also easy to cause damage to the switching power supply. Fuji G9S uses a waveform generator chip dedicated to switching power supply. The chip is often damaged due to the high voltage of the main circuit. Since this chip is rarely available in the market, the damage caused is difficult to repair. In addition, VFD (Variable-frequency Drive) has no display after being powered on, which is also one of the more common fault phenomena. Most of the causes of such faults are caused by damage to the switching power supply. For example, the switching power supply of the MF series VFD (Variable-frequency Drive) adopts the more common flyback switching power supply control method. A short circuit in the output stage circuit of the switching power supply will also cause damage to the switching power supply, resulting in VFD (Variable-frequency Drive) ) is not displayed. (3) Faults in feedback and detection circuits In the process of using VFD (Variable-frequency Drive), it is often encountered that VFD (Variable-frequency Drive) has no output. Damage to the drive circuit and damage to the inverter module may cause VFD (Variable-frequency Drive) to have no output. In addition, failure of the output feedback circuit may also cause such failures. Sometimes in practice, if VFD (Variable-frequency Drive) has output frequency but no output voltage (the actual output voltage is very small, it can be considered as no output), then you should consider whether it is caused by a fault in the feedback circuit. The feedback resistor used for step-down in the feedback circuit is one of the components that are more prone to failure; the damage of the detection circuit is also the cause of the action of the VFD (Variable-frequency Drive) display OC (pA or pd or pn) protection function, the detection Due to the influence of environmental factors such as temperature and humidity, the operating point of the current Hall sensor is prone to drift, resulting in an OC alarm. In short, the common faults of VFD (Variable-frequency Drive) include overcurrent, overvoltage, undervoltage and overheating protection, and corresponding fault codes. Different models have different codes. For the meaning of the codes, please refer to the accompanying instruction manual. Handling measures to solve. Overcurrent is often caused by damage to the GTR (or IGBT) power module