Inrush current limit circuit fault

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This fault stops the inverter output because the resistor of the inrush current limit circuit has overheated. Frequent power ON/OFF cycling is a common cause, stressing the components. Overheating can lead to component failure and damage to the inverter.

• Excessive frequency of main circuit power cycling (ON/OFF) within a short period, not allowing the inrush current limiting resistors to cool down.
• Failure of the bypass contactor (or SCRs) that shunts the inrush current limiting resistors after startup, causing continuous current flow through them.
• An internal short circuit or ground fault in the DC bus or output stage drawing excessive current during power-up, overloading the inrush resistors.
• Ambient temperature surrounding the inverter exceeding its specified maximum, reducing the cooling capacity of internal components including the inrush resistors.
• Degradation or incorrect sizing of the inrush current limiting resistors themselves, making them prone to overheating under normal power-up conditions.

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1. 1

   1. Reduce the frequency of main circuit power cycling. Implement a minimum delay (e.g., 5 minutes) between power OFF and power ON cycles to allow the inrush resistors to cool.

2. 2

   2. With power disconnected and safety procedures followed, use a multimeter to check the resistance of the inrush current limiting resistors (often visible on the DC bus side) for signs of open circuit or significant deviation from specified values.

3. 3

   3. Inspect and test the bypass contactor (if present) for proper operation. With power ON, verify that the contactor pulls in within 1-2 seconds after startup and that its main contacts are closed by measuring voltage drop across them (should be near 0V).

4. 4

   4. Measure the resistance between DC bus terminals P and N, and between each output terminal (U, V, W) and ground with power OFF to detect any potential short circuits or ground faults.

5. 5

   5. Check the inverter's internal cooling fans for proper operation and ensure that air vents are clear and not obstructed, maintaining adequate airflow over internal components.