Mitsubishi FR-F800 Error E.UVT (Undervoltage)

Getting the Mitsubishi FR-F800 "E.UVT" error? This undervoltage fault means your supply power is too low. Learn how to check your line voltage, verify wiring, and stabilize your power supply.

 

The Mitsubishi FR-F800 is an advanced inverter designed for precise control, but it requires a stable, consistent input voltage to function correctly. If your drive trips with the E.UVT (Undervoltage) error, the drive has detected that the internal DC bus voltage has dropped below the operational threshold.

When this happens, the inverter shuts down to protect its internal logic and to prevent the motor from operating with insufficient torque, which could otherwise lead to motor overheating or unstable process control.

Why Does the E.UVT Error Trigger?

The E.UVT error is a warning that the "fuel" (DC bus voltage) is too low to run the drive. This is usually caused by:

• Input Power Sag: The incoming AC line voltage has dropped due to grid fluctuations or facility-wide power issues.
• Missing Phase: A blown fuse, tripped circuit breaker, or loose wire has resulted in a loss of one or more phases (L1, L2, or L3).
• Sudden Load Spikes: If multiple large machines start at the same time in your facility, it can cause a "brownout" effect that triggers the drive's low-voltage protection.
• Internal Charger Failure: In rare cases, the drive’s internal pre-charging relay or resistors have failed, preventing the DC bus from reaching full charge.

Step-by-Step Troubleshooting Guide

Safety Warning: Always disconnect main power and wait at least 10 minutes for the internal capacitors to discharge before opening the drive cover. Use a multimeter to verify zero voltage before touching any wiring.

1. Check Incoming Line Voltage

Before assuming the drive is faulty, verify the power quality:

• Measure the voltage across the input terminals (R/L1, S/L2, T/L3) while the drive is running.
• Compare your readings to the drive's nameplate input voltage (e.g., 400V or 480V). If your measured voltage is consistently 10% lower than the rated voltage, you have an upstream power quality issue.
• Check for phase-to-phase balance. All three phases should be within 1–2% of each other.

2. Inspect Power Wiring and Connections

High-resistance connections cause voltage drops during motor operation.

• Tighten all screws on the input terminal block.
• Check upstream contactors and disconnect switches. Look for signs of "pitting" or blackening on the contacts, which indicates arching and high resistance.
• Inspect input fuses; ensure they are seated tightly in their holders.

3. Analyze the Timing of the Fault

• Does it happen during startup? If E.UVT occurs only when the motor starts, your power supply might be too weak to handle the inrush current, or your acceleration time (Pr. 7) may be too aggressive.
• Does it happen during running? If it trips while running, check if other heavy machinery in the plant is cycling on and off at the same time.

4. Resetting the Inverter

Once you have addressed the supply issue:

• Press the [STOP/RESET] button on the keypad.
• If the drive continues to fault immediately upon reset, do not repeatedly cycle the power, as this puts extreme stress on the drive’s internal charging components.

Maintenance Tip

If you are frequently seeing E.UVT faults despite having solid input power, consider checking the DC bus voltage monitoring parameter (usually found in the drive’s display monitor mode). If the DC bus voltage is unstable even when the input AC voltage is perfect, the drive’s internal rectifier bridge may be failing.