55kW Inverter Overcurrent Fault on Boiler Induced Draft Fan – A Case Study

Date: December 4, 2025

Source: ANYHZ Technical Support Team

In a recent boiler induced draft fan retrofit project, a 55kW inverter from ANYHZ was installed to drive a 55kW motor. During the commissioning phase, the system triggered a “software overcurrent” fault when the frequency reached approximately 5Hz.

Initial Analysis:

The motor was connected to the inverter via a cable several dozen meters long. Both the power cables and control lines were routed through the same conduit, raising suspicion of electromagnetic interference (EMI) as the root cause.

Initial Troubleshooting:

To address the suspected EMI issue, the control and power cables were separated, and magnetic rings were installed on the inverter’s input and output lines. However, the fault persisted even after several EMI mitigation measures were applied.

Re-evaluation:

Upon closer observation, engineers noticed that after the inverter stopped output, the fan impeller continued to rotate for over 30 minutes. This indicated a high rotational inertia, suggesting that the overcurrent fault might be due to the large inertial load rather than EMI.

Solution:

High-inertia loads require longer acceleration times during startup. Additionally, fan-specific inverters often use a square-law torque curve (U/f curve), which provides lower torque at low frequencies. To improve starting torque, the startup frequency was increased from the default to 3Hz, and low-frequency torque compensation was enabled. After these adjustments, the inverter started successfully without any overcurrent faults.

Conclusion:

This case highlights the importance of correctly identifying the root cause of inverter faults. What initially appeared to be an EMI issue was, in fact, a mechanical inertia problem. By adjusting the inverter’s startup parameters, the issue was resolved efficiently—without unnecessary hardware changes.

Key Takeaway:

Not all overcurrent faults are caused by electrical interference. For high-inertia loads like induced draft fans, proper torque compensation and startup frequency settings are crucial for reliable inverter performance.