Common Inverter Faults and Solutions

1. Overcurrent Fault​

Overcurrent is one of the most frequent faults encountered during inverter operation. To better protect the inverter, multi-level protection against overcurrent is generally implemented. Based on the severity of the overcurrent, it can be categorized into the following situations: power module overcurrent, hardware overcurrent, and software overcurrent. Typically, power module overcurrent is the highest-level fault. The hardware overcurrent threshold is significantly lower than the power module overcurrent threshold but higher than the software overcurrent threshold. In terms of response speed, hardware blocking is faster than software.

The reporting mechanism for ​power module overcurrent​ is generally as follows: The hardware design triggers the FAULT signal on the primary side of the optocoupler to flip when the IGBT conduction current greatly exceeds the hardware overcurrent threshold (generally no more than 6 times the IGBT rated current). The hardware circuit then blocks the PWM wave output and simultaneously transmits this signal to the control chip's pin. The software responds to this signal via an interrupt, immediately shutting down and blocking further operation.

The reporting mechanism for ​hardware overcurrent​ is generally as follows: Using a hardware comparator circuit, when current exceeding the hardware overcurrent threshold is detected, the hardware circuit blocks the PWM wave output and transmits the fault signal to the control chip's pin. The software responds via an interrupt, immediately shutting down.

The reporting mechanism for ​software overcurrent​ is generally as follows: After sampling the three-phase currents, the software calculates the RMS value. This RMS value is compared to the software overcurrent threshold. If it exceeds the threshold, a software overcurrent fault is reported, and the inverter shuts down.

Generally, troubleshooting and resolving overcurrent faults can involve the following steps:

1. If the inverter has been running normally and occasionally reports a power module overcurrent fault, first try resetting the fault. If reset fails, the power module may be damaged and require replacement.
2. If reset is successful, consider if operating conditions have changed (e.g., momentary overload/stall causing a sudden high current). If caused by an external anomaly, eliminate the cause to maintain stable operation. If the change is intentional (e.g., increased load demand or impact load), reduce current surges by extending acceleration time, adjust speed/current loop PI parameters to optimize control performance, or enable the overcurrent stall prevention function.
3. If reset is successful with no changes to external conditions, check the inverter output circuit for ground faults or short circuits. Eliminate any found. If none exist, observe the current magnitude throughout the operating cycle. If stable with no significant surges, consider electrical noise interference and inspect wiring/grounding.
4. During commissioning, if overcurrent faults occur easily, first verify correct settings for both inverter and motor parameters, including matching inverter and motor power ratings. If settings are correct and power is matched but the fault persists, perform dynamic parameter identification to ensure motor parameter accuracy.
5. If an overcurrent occurs during startup under V/f control, check if the torque boost setting is too high and reduce it if necessary. Also, check if the V/f curve settings are unreasonable and adjust accordingly.
6. If starting while the motor is coasting/spinning freely, overcurrent might occur. Wait for the motor to stop completely before starting, or set the starting method to flying start / spin tracking start.

​II. Overvoltage Fault​

Overvoltage is also one of the most common inverter faults. To protect the inverter, multi-level overvoltage protection is usually implemented. Based on severity, it is generally categorized into hardware overvoltage and software overvoltage.

Typically, the hardware overvoltage threshold is higher than the software overvoltage threshold, and hardware blocking is faster. The ​hardware overvoltage​ reporting mechanism is generally: Using a hardware comparator circuit, when the DC bus voltage exceeds the hardware threshold, the hardware circuit blocks the PWM output and signals the control chip. Software responds via interrupt, shutting down.

The ​software overvoltage​ reporting mechanism is generally: After sampling the DC bus voltage, software compares it to the software threshold. If exceeded, a software overvoltage fault is reported, and the inverter shuts down.

Troubleshooting and resolving overvoltage faults typically involve:

1. If significant energy is being regenerated to the grid, check if a braking resistor unit (BRU) is installed and suitably sized.
2. If regenerative energy is modest, try extending deceleration time to reduce regeneration, or adjust speed/current loop PI parameters to improve control performance.
3. If modest regeneration with momentary voltage spikes occurs (e.g., sudden loss of heavy load) and stopping position/time is not critical, enable the overvoltage stall prevention function. Use with caution as it can prevent timely shutdown; do not use where stopping position is critical.
4. If regenerative energy is very low, check if the three-phase input voltage is excessively high.
5. Check if the motor is being driven by an external force (e.g., overhauling load). If so, eliminate this force.

​III. Input Phase Loss​

Input phase loss is another relatively common inverter fault. Reporting mechanisms vary by manufacturer/model but generally fall into two types:

1. Software-based detection: Two line voltages are sampled and converted to phase voltages. Phase unbalance is calculated to determine if phase loss conditions are met.
2. Hardware-based detection: A dedicated circuit detects phase loss and signals the control chip via a pin. Software monitors this pin's state to determine phase loss.

If phase loss is detected, a fault is reported, and the inverter shuts down (or generates an alarm in some cases).

Troubleshooting and resolution usually involve:

1. Check the integrity and security of the three-phase input power connections.
2. Verify that all input power phases are present (no fuses blown, breakers tripped).
3. If both 1 & 2 are confirmed OK, monitor the input power and check the control logic for any automatic disconnection/reconnection sequences.

​IV. Inverter Overload​

Inverter overload is an occasionally reported fault. Reporting mechanisms vary but generally are:

1. ​Thermal accumulation method:​​ Software calculates a thermal accumulation value based on current (and possibly other factors) over time, comparing it to a design threshold. Exceeding the threshold triggers an overload fault and shutdown.
2. ​Inverse-time characteristic:​​ Based on the inverter's designed overload curve, software calculates how long a specific overcurrent magnitude is allowed. Timing starts when overcurrent occurs; exceeding the allowed time triggers the fault and shutdown.

Troubleshooting and resolution typically involve:

1. Check if the load's duty cycle (ON/OFF times) adheres to the inverter's overload curve. Adjust or reduce the load current to prevent exceeding the curve's duration limits.
2. Check if the motor power exceeds the inverter's continuous load rating. If the load is genuinely large, select a higher-power inverter.

​V. Motor Stall​

Motor stall is another fault occasionally reported by inverters. Essentially, the inverter commands the motor to reach a certain speed and outputs significant torque, but the motor fails to rotate properly, remaining in a stalled condition.

Conditions typically required to trigger a motor stall fault:

1. The feedback torque current exceeds the set stall current threshold and this condition lasts longer than the set stall time.
2. During this period, the actual motor speed is below the set stall frequency threshold.
3. The inverter is not operating in V/f control mode (since V/f lacks speed feedback, stall detection isn't possible).

Troubleshooting and resolving motor stall faults generally involve:

1. Check if an external force physically prevents motor rotation. Eliminate the cause.
2. Adjust the stall frequency and stall current threshold parameters according to the application's needs.
3. Check if the motor/load power exceeds the inverter's capability. If so, select an appropriately sized inverter.