The inverter consists of main circuit, power circuit, IGBT drive and protection circuit, cooling fan and other parts. Its structure is mostly unitized or modular. Due to the incorrect use method or unreasonable setting environment, it will easily cause the inverter to malfunction and malfunction, or fail to meet the expected operating effect. In order to prevent problems before they occur, it is particularly important to carefully analyze the cause of the failure in advance.
1. Analysis of common faults in the main circuit
The main circuit is mainly composed of three-phase or single-phase rectifier bridge, smoothing capacitor, filter capacitor, IGBT inverter bridge, current limiting resistor, contactor and other components. Many of these common failures are caused by electrolytic capacitors. The life of an electrolytic capacitor is mainly determined by the DC voltage applied to both ends and the internal temperature. The type of capacitor has been selected in the circuit design, so the internal temperature plays a decisive role in the life of the electrolytic capacitor. Therefore, on the one hand, the appropriate ambient temperature should be considered during installation, and on the other hand, measures can be taken to reduce the pulsating current. The use of AC or DC reactors with improved power factor can reduce the pulsating current and prolong the life of electrolytic capacitors.
During capacitor maintenance, the degradation of electrolytic capacitors is usually judged by the relatively easy-to-measure electrostatic capacity. When the electrostatic capacity is lower than 80% of the rated value and the insulation resistance is below 5MΩ, replacement of the electrolytic capacitor should be considered.
2. Typical fault analysis of the main circuit
Fault phenomenon: The inverter trips due to overcurrent during acceleration, deceleration or normal operation.
First of all, it should be distinguished whether it is due to the load or the inverter. If it is the fault of the inverter, the current at the time of tripping can be checked through the history record, which exceeds the rated current of the inverter or the setting value of the electronic thermal relay, and the three-phase voltage and current are balanced, it should be considered whether there is overload. Or sudden changes, such as motor stall, etc. When the load inertia is large, the acceleration time can be appropriately extended, and the inverter itself will not be damaged during this process. If the current at the time of tripping is within the rated current of the inverter or the setting range of the electronic thermal relay, it can be judged that the IGBT module or related parts are faulty. First, you can judge whether the IGBT module is damaged by measuring the forward and reverse resistance between the main circuit output terminals U, V, W of the inverter and the P and N terminals of the DC side respectively. If the module is not damaged, the drive circuit is faulty. If the IGBT module overcurrent or the inverter trips due to short circuit to ground during deceleration, the module of the upper half bridge of the inverter or its drive circuit is generally faulty; while the IGBT module overcurrent during acceleration, it is the module of the lower half bridge or its driver. Part of the circuit is faulty, and the reasons for these faults are mostly caused by the entry of external dust into the inverter or the humidity of the environment.
3. Control loop failure analysis
The control loop affects the life of the inverter is the power supply part, which is the smoothing capacitor and the buffer capacitor in the IGBT circuit board. , so its life is mainly determined by temperature and power-on time. Since the capacitors are all welded on the circuit board, it is difficult to judge the deterioration by measuring the electrostatic capacity. Generally, it is estimated whether the service life of the capacitor is approaching according to the ambient temperature and the use time of the capacitor.
The power supply circuit board provides power for the control circuit, IGBT drive circuit, surface operation display panel and fan, etc. These power supplies are generally obtained from the DC voltage output from the main circuit, and then rectified separately by the switching power supply. Therefore, if a certain power supply is short-circuited, in addition to the damage to the rectifier circuit of this circuit, it may also affect the power supply of other parts. A short circuit in the power supply causes other power sources to lose power, etc. It is generally easier to find out by observing the power supply circuit board.
The logic control circuit board is the core of the inverter. It integrates large-scale integrated circuits such as CPU, MPU, RAM, EEPROM, etc., and has high reliability. The terminals are closed at the same time, causing the inverter to have an EEPROM fault, which only needs to be reset to the EEPROM.
The IGBT circuit board contains drive and buffer circuits, as well as overvoltage and phase-missing protection circuits. The PWM signal from the logic control board inputs the voltage drive signal into the IGBT module through optical coupling. Therefore, while detecting the mode speed, the optocoupler on the IGBT module should also be measured.
4. Cooling system
The cooling system mainly includes heat sinks and cooling fans. Among them, the cooling fan has a short service life. When the service life is approaching, the fan will vibrate, the noise will increase and finally stop, and the inverter will trip due to overheating of the IGBT. The life of the cooling fan is limited by the bearing, which is about 10,000 to 35,000h. When the inverter runs continuously, the fan or bearing needs to be replaced every 2 to 3 years. To prolong the life of the fans, some products have fans that only run when the inverter is running and not when the power is on.
5. External electromagnetic induction interference
If there are interference sources around the inverter, they will penetrate into the inverter through radiation or power lines, causing the control loop to malfunction, causing abnormal operation or shutdown, and even damage to the inverter in severe cases. The specific methods to reduce noise interference are as follows: on the control coils of all relays and contactors around the inverter, install an absorbing device to prevent surge voltage, such as RC surge absorber, the wiring should not exceed 20cm; try to shorten the control loop more than 5mm , keep a distance of more than 10cm from the main circuit; when the inverter is far away from the motor (more than 100m), on the one hand, the cross-sectional area of the wire can be increased to ensure that the line voltage drop is within 2%, and the output reactance of the inverter should be installed at the same time. The device is used to compensate the charging current of the distributed capacitance caused by the long distance wire. The grounding terminal of the inverter should be grounded in accordance with the regulations, and must be reliably grounded at a dedicated grounding point, and cannot be used together with electric welding and power grounding; a radio noise filter is installed at the input end of the inverter to reduce the input high-order harmonics, thereby reducing the amount of noise from the power line to the ground. Noise effects of electronic equipment; at the same time, a radio noise filter is also installed at the output end of the inverter to reduce the line noise at its output end.
Detection method:
1. Determine the polarity
First, set the multimeter to the R&TImes; 1KΩ block. When the multimeter measures, if the resistance value between one pole and the other two poles is infinite, and the resistance value between this pole and the other two poles is still infinite after changing the test lead, then judge this pole grid (G ) Use a multimeter to measure the remaining two poles. If the measured resistance value is infinite, the measured resistance value is smaller after replacing the test leads. In the measurement with a smaller resistance value, it is judged that the red test lead is connected to the collector (C); the black test lead is connected to the emitter (E).
2. To judge whether it is good or bad
Set the multimeter to the R&TImes; 10KΩ block, connect the black test lead to the collector (C) of the IGBT, and the red test lead to the IGBT emitter (E). At this time, the pointer of the multimeter is at the zero position. Touch the gate (G) and collector (C) at the same time with your finger, at this time the IGBT is triggered and turned on, the pointer of the multimeter swings to the direction of the smaller resistance value, and can stand to indicate a certain position. Then touch the gate (G) and emitter (E) at the same time with your finger, at this time the IGBT is blocked and the pointer of the multimeter returns to zero. At this point, it can be judged that the IGBT is good.
3. Precautions for testing
Any analog multimeter can be used to detect IGBTs. Note that when judging the quality of the IGBT, be sure to set the multimeter to the R&TImes; 10KΩ block, because the internal battery voltage of the multimeter below the R&TImes; 1KΩ block is too low, the IGBT cannot be turned on when the test is good, and the IGBT cannot be judged. . This method can also be used to detect the quality of power field effect transistors (P-MOSFETs).
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