Analysis of the causes of 8 major faults of inverters and preventive measures

The frequency converter is composed of main circuit, power circuit, IPM drive and protection circuit, cooling fan and so on. Its structure is mostly in a modular or modular form. Due to incorrect use or unreasonable setting environment, the inverter may malfunction or malfunction, or it may not meet the expected operation results. In order to prevent problems before they occur, it is especially 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, IPM inverter bridge, current limiting resistor, contactor and other components. Many of these common faults are caused by electrolytic capacitors. The life of an electrolytic capacitor is mainly determined by the DC voltage and internal temperature applied to both ends. The type of capacitor has been selected in the loop design, so the internal temperature determines the life of the electrolytic capacitor. Electrolytic capacitors will directly affect the service life of the inverter. For every 10°C increase in temperature, the life will be halved. 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. An AC or DC reactor with improved power factor can reduce the ripple current and thus extend the life of the electrolytic capacitor.

In the maintenance of the capacitor, the deterioration of the electrolytic capacitor is usually judged by the electrostatic capacity which is relatively easy to measure. When the electrostatic capacity is less than 80% of the rated value and the insulation resistance is 5 MΩ or less, the electrolytic capacitor should be replaced.

2. Typical fault analysis of the main circuit

Symptom: The inverter trips overcurrent during acceleration, deceleration or normal operation.

The first thing to distinguish is due to the load or the cause of the inverter. If it is the fault of the inverter, the history can be used to check the current at the time of trip, which exceeds the rated current of the inverter or the set value of the electronic thermal relay. If the three-phase voltage and current are balanced, then it should be considered whether there is overload. Or sudden changes, such as motor stalls. When the load inertia is large, the acceleration time can be extended appropriately, and the process is not damaged by the inverter itself. If the current at the time of the trip is within the rated current of the inverter or within the set range of the electronic thermal relay, it can be judged that the IPM module or related part has failed. First, it is possible to determine whether the IPM module is damaged by measuring the positive and negative resistances between the main circuit output terminals U, V, and W of the inverter and the P and N terminals on the DC side. If the module is not damaged, the drive circuit has failed. If the IPM module is over-current or the inverter trips to ground during deceleration, it is generally the module of the upper half of the inverter or its drive circuit is faulty; and when the IPM module is over-current during acceleration, it is the module of the lower half of the bridge or its drive Part of the circuit is faulty. The cause of these faults is mostly caused by external dust entering the inverter or the environment is humid.

3. Control loop failure analysis

The control loop affects the life of the inverter. It is the power supply part. It is the smoothing capacitor and the snubber capacitor in the IPM board. The principle is the same as the above, but the pulsating current passing through the capacitor here is basically the value that is not affected by the main loop load. Therefore, its life is mainly determined by temperature and power-on time. Since the capacitors are soldered to the Circuit Board, it is difficult to judge the deterioration by measuring the electrostatic capacity. Generally, it is estimated whether the battery life is close to its service life based on the ambient temperature of the capacitor and the time of use.

The power circuit board supplies power to the control loop, the IPM drive circuit, the surface operation display panel, and the fan. These power supplies are generally obtained by rectifying the DC voltage output from the main circuit through the switching power supply. Therefore, if a certain power supply is short-circuited, in addition to the damage of the rectifier circuit of this circuit, it may affect other parts of the power supply. For example, the control power supply is short-circuited with the common ground due to misoperation, resulting in partial damage of the switching power supply on the power supply circuit board. A short circuit in the power supply causes other power supplies to be powered down, etc. It is usually easier to find by observing the power board.

The logic control circuit board is the core of the inverter. It concentrates on large-scale integrated circuits such as CPU, MPU, RAM, EEPROM, etc. It has high reliability, and the probability of failure itself is very small, but sometimes it is completely controlled by booting. The terminals are closed at the same time, causing an EEPROM fault in the inverter, as long as the EEPROM is reset again.

The IPM board contains drive and buffer circuits, as well as overvoltage and missing protection circuits. The PWM signal from the logic board is used to input the voltage drive signal into the IPM module through optical coupling. Therefore, the optocoupler on the IPM module should be measured while detecting the mode fast.

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