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VFD control loop circuit faults analysis

The affection on variable frequency drive life in the control loop circuit is the power part, the buffer capacitor in smoothing capacitor and IPM board. The ripple current pass the capacitor is a fixed value which won't be affected by the main circuit, so its life is mainly determined by the temperature and power-on time. Since the capacitors are soldered to the circuit board, it difficult to determine the capacitor deterioration by measuring the electrostatic capacity. Generally, we calculate its life base on the ambient temperature and service time.

Power supply circuit provides power to the control circuit board, IPM drive circuit, operation display panel and cooling fan, the power is obtained from the main circuit DC voltage rectified by the switching power supply. Therefore, if one power short circuit, besides itself damaged, also affect other parts power supply, such as misoperation causes power source and the public ground short circuit, result in switching power supply circuit board damaged, the fans power supply short circuit etc. Generally it's easy to find out by observing the power supply circuit board.

Logic control circuit board is the core of a variable frequency drive, it includes CPU, MPU, RAM, EEPROM etc large scale integrated circuits, the failure rate is very rare due to high reliability. But sometimes all control terminals closed simultaneously during startup which will cause the VFD drive appear EEPROM fault, in such case, just reset the EEPROM.

IPM circuit board contains drivers and buffer circuit, and over-voltage, phase loss protection circuits. Logic control panel PWM signal input to IPM module by voltage drive signal optical coupling, so, we should measure the IPM module optical coupling during module detection.

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