Overcurrent protection in an inverter is essential because it safeguards the system from damage caused by excessive current. This typically occurs when the peak current exceeds the inverter's rated capacity. Since inverters are not designed for long-term overload, effective overcurrent protection is critical to ensure system reliability and longevity.
1. Causes of Overcurrent
Overcurrent can occur during normal operation or under specific conditions:
1.1 Operating Overcurrent
This happens when the motor experiences sudden load changes or mechanical issues. Common causes include:
- Motor encountering an impact load or the drive mechanism being "stuck," leading to a sharp increase in current.
- Short circuits on the inverter output side, such as between the motor terminals or within the motor itself.
- Internal failure in the inverter, like both devices in the same bridge arm turning on simultaneously due to temperature issues or component aging, causing a short circuit between the DC bus positive and negative terminals.
1.2 Overcurrent During Acceleration
If the load has high inertia and the acceleration time is too short, the inverter may struggle to keep up with the required speed increase. The motor’s rotor cannot match the rapid rise in synchronous speed, resulting in a surge in current.
1.3 Overcurrent During Deceleration
A similar issue occurs during deceleration if the time setting is too short. The motor’s rotor continues to spin at a high speed due to inertia, while the synchronous speed drops quickly. This mismatch leads to excessive induced current, causing overcurrent conditions.
2. Troubleshooting and Solutions
2.1 Immediate Tripping at Start
If the inverter trips immediately when starting, consider the following:
- Is the mechanical system stuck? Check for any physical obstructions.
- Is there a short circuit on the load side? Use a megohmmeter to test for ground faults.
- Is the inverter power module damaged? Inspect for signs of overheating or failure.
- Is the motor’s starting torque insufficient? Ensure the drive system can rotate properly.
2.2 Tripping During Operation
If the inverter doesn’t trip at start but fails during operation, check these factors:
- Adjust the acceleration time to allow for a smoother speed-up process.
- Extend the deceleration time to prevent sudden speed drops that could cause current spikes.
- Review the torque compensation (U/F ratio) settings. Too much compensation at low frequencies can lead to high no-load current.
- Verify the electronic thermal relay settings. If the current threshold is set too low, it may trigger unnecessary trips.
By understanding the root causes of overcurrent and implementing proper diagnostic and adjustment methods, you can significantly improve the performance and stability of your inverter system.
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