Inverter safety matters and maintenance - Solutions - Huaqiang Electronic Network

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The basic principle of AC-DC-AC frequency conversion involves converting three-phase AC power into DC through a bridge rectifier circuit, and then using an inverter to convert the DC back into three-phase AC with variable frequency. This is widely used in motor drives and power supplies. To determine the fault range of the inverter, start by checking the DC bus voltage after powering on the system. If the measured voltage equals approximately 1.35 times the input AC voltage, it suggests that the inverter itself might be faulty. Otherwise, components like the rectifier, pre-charging circuit, or filter capacitors could be damaged. In some inverters, a contactor is used for pre-charging the DC bus. If the contactor fails to activate, simulate the "no fault" signal and external start command to manually engage it. Then measure the DC bus voltage again to identify the issue. Be aware that the trigger signal may be a pulse rather than a continuous level. For static testing of the rectifier unit, use a multimeter in diode mode. Connect the red lead to the DC bus positive terminal and the black lead to the three-phase input. All readings should be similar. Reverse the leads and check again. A normal diode will show around 0.4–0.6 V in forward bias and infinity in reverse. Any deviation indicates a faulty component. Similarly, test the inverter’s IGBT modules by checking the resistance between the DC bus and the output terminals. Each phase should have consistent values. If one phase shows low resistance in both directions, it likely means a damaged IGBT or freewheeling diode. When diagnosing the control circuit, look at the status lights on the main board. A green LED usually indicates normal operation, while a red light may signal a protection trip. Check for trigger signals on the gate drivers using an oscilloscope. If no signal is present, there could be a problem with the control board or the drive circuit. Common issues in the control circuit include power supply failures, overcurrent alarms, and temperature-related faults. For example, if the power supply outputs abnormal voltages like +24V or ±15V, check the load and the board itself. Overcurrent alarms often point to a faulty IGBT or current sensor. Temperature alarms may indicate a failed fan or a damaged NTC thermistor. Safety is crucial during maintenance. Always ensure the system is powered off before touching any components. Wait at least 5 minutes after disconnecting power to allow the capacitors to discharge fully. Avoid touching IC pins on the control board to prevent static damage. Also, remember that the output terminals may still carry up to 200V DC even when the inverter is in standby or running at zero speed. By following these steps and paying attention to safety, you can effectively diagnose and repair inverter systems without causing further damage.