When the frequency converter displays the "LU" fault code, it typically indicates a "Low Voltage" fault, meaning the input power supply voltage is below the minimum threshold required for normal operation. This fault can be caused by multiple factors and requires step-by-step troubleshooting based on actual conditions. Below is a detailed analysis and solutions:
I. Fault Cause Analysis
1. Abnormal Power Input
● Unstable grid voltage: Sudden mains voltage drops, three-phase imbalance, or excessive voltage fluctuations may cause the inverter to detect transient low voltage.
● Missing phase in power supply: For three-phase inverters, a phase break or poor contact can trigger the LU alarm due to insufficient input voltage.
● Wiring Issues: Loose power cables, oxidized terminals, or insufficient cable cross-sectional area causing excessive voltage drop.
2. Internal Inverter Problems
● Low DC Bus Voltage: Aging rectifier modules or filter capacitors fail to maintain stable DC voltage.
● Voltage detection circuit failure: Damaged sampling resistors or signal transmission anomalies cause false low-voltage alarms.
● Main control board malfunction: Processor or AD conversion module errors lead to incorrect voltage readings.
3. Load or Parameter Configuration Issues
● Sudden load surge: Excessive instantaneous current during motor startup or acceleration pulls down input voltage (especially in weak grid conditions).
● Improper parameter settings: e.g., low voltage protection threshold (e.g., 80% of rated voltage) set too high, or deceleration time too short causing insufficient feedback voltage.
II. Troubleshooting and Resolution Steps
1. Check Power Input
● Measure input voltage: Use a multimeter to test the three-phase voltage at the inverter input terminals. Normal range should be within ±10% of rated voltage (e.g., 342–418V for a 380V system). If voltage remains persistently low, contact the power utility or install a voltage stabilizer.
● Check for phase loss: Verify balanced three-phase voltage, with any two-phase voltage difference not exceeding 2%. Investigate circuit breakers, contactors, and wiring for phase loss.
● Secure wiring: Retighten power terminals and inspect cables for aging or insufficient cross-sectional area (recommended wire gauge: 1.2 times the inverter's rated current).
2. Inspect Drive Hardware
● DC Bus Voltage: Measure DC bus voltage during shutdown (typically 1.35 times input line voltage; e.g., ~513V for 380V input). If abnormal, inspect rectifier bridges and filter capacitors for leakage or bulging.
● Voltage detection circuit: Have a qualified technician inspect components on the PCB board, such as voltage sampling resistors and optocouplers, for damage.
● Main control board status: Check for burn marks or burst capacitors; replace or repair as necessary.
3. Adjust parameters and load
● Modify Protection Thresholds: Appropriately lower the low-voltage protection value (e.g., from 80% to 70%), ensuring it does not compromise safe operation.
● Extend Acceleration/Deceleration Time: Reduce transient current surges to prevent voltage drops.
● Check Load Inertia: For high-inertia loads (e.g., fans, pumps), enable the "voltage compensation" function or install a braking resistor.
4. Environment and Auxiliary Equipment
● Avoid Shared Power Sources: Separate power supply for high-power equipment (e.g., welders, cranes) and inverters to prevent voltage interference.
● Install Reactors: An AC reactor on the input side suppresses voltage fluctuations, while an output-side reactor reduces harmonic effects.
III. Reference Case Studies
1. Case 1: A factory's VFDs frequently reported LU faults. Inspection revealed grid voltage dropping to 340V (rated 380V) at night. Installation of an automatic voltage regulator resolved the issue.
2. Case 2: A VFD misreported LU after main control board replacement. The fault was ultimately traced to drift in the voltage sampling resistor value. Normal operation resumed after replacement.
3. Case 3: A conveyor belt VFD triggered an alarm during acceleration. Extending the acceleration time from 5 to 10 seconds and enabling the "voltage stall prevention" function eliminated the fault.
IV. Preventive Measures
● Regular Maintenance: Clean cooling fans and inspect capacitor capacity to prevent voltage anomalies caused by component aging.
● Real-time Monitoring: Install voltage monitors to record historical voltage fluctuation data.
● Redundancy Design: Implement dual power supply switching or UPS backup power for critical equipment.
If the above steps fail to resolve the issue, contact the VFD manufacturer or professional maintenance personnel to avoid exacerbating the fault through improper handling. LU codes may vary slightly across different VFD brands (e.g., ABB defines it as "Undervoltage"). Refer to the specific model's manual for accurate interpretation.