Mitsubishi FR-D700G Fault Uv or Uu (Undervoltage)

Getting the "Uv" error on your Mitsubishi FR-D700G inverter? Learn what causes main circuit undervoltage, how to check your power supply, and steps to fix the fault.

If you are working with a Mitsubishi FR-D700G series inverter and the display starts flashing Uv, your drive has detected an Undervoltage condition. This isn't necessarily a sign that the drive is broken, but it is a major warning that the "fuel" (the electrical power) reaching the drive's internal circuit is insufficient.

When the DC bus voltage (the main circuit) drops below a specific threshold, the drive trips to protect the internal electronics and to prevent the motor from running at an unstable torque. Here is a human-made, step-by-step guide to diagnosing and fixing the Uv fault.

What Exactly is the Uv Fault?

The FR-D700G monitors the voltage across its internal DC bus capacitors. If this voltage falls too low—usually due to a dip in the incoming AC power—the Uv alarm is triggered. If the drive were to continue running during an undervoltage event, it could lead to excessive heat in the rectifier or failure of the control logic.

Common Causes of the Uv Fault

• Main Supply Power Drop: The incoming AC voltage from the grid or transformer has sagged or dropped significantly.
• Missing Input Phase: One of the three input phases (L1, L2, or L3) has been lost due to a blown fuse or tripped breaker.
• Loose Wiring: Vibration in the cabinet has caused a power lead at the R/L1, S/L2, or T/L3 terminals to become loose.
• Large Capacity Loads: Starting another massive motor on the same electrical line can cause a momentary "brownout" that trips the VFD.
• Internal Pre-charge Failure: The internal resistor or relay responsible for charging the capacitors has failed.

Step-by-Step Troubleshooting Guide

1. Investigate the Power Supply Line

The first step is to look outside the drive. Use a multimeter to measure the incoming AC voltage at the inverter terminals (R/L1, S/L2, T/L3).

• Balanced Voltage: Ensure all three phases are present and balanced. If one is significantly lower than the others, you have a phase loss issue.
• Rated Range: Check the measured voltage against the inverter's nameplate. For a 400V class drive, if your input is dipping below 340V-360V, the Uv fault is likely to occur.

2. Check Upstream Protective Devices

Check the devices feeding the drive:

• Fuses and Breakers: Look for blown high-speed fuses or partially tripped circuit breakers.
• Contactors: If you use an input contactor, inspect the contacts for pitting or carbon buildup, which creates high resistance and voltage drops.

3. Inspect Physical Connections

Power down the drive and follow Lock-Out/Tag-Out procedures. Ensure all terminal screws on the input power block are tightened to the manufacturer's torque specifications. A loose wire can create enough of a voltage drop under load to trigger the Uv alarm.

4. Analyze the Environment (Starting Dips)

If the fault happens only when the motor starts or when another machine in the plant turns on:

• Your facility's power system may be "soft" (high impedance).
• Solution: Consider installing an AC Line Reactor. This helps buffer the drive against incoming voltage dips and protects it from transients.

Summary: When to Suspect Drive Hardware

If your incoming AC power is perfectly stable and all three phases measure correctly at the drive terminals, but the Uv fault refuses to clear after a reset:

1. The internal rectifier bridge or DC bus capacitors may be reaching the end of their service life.
2. The internal voltage sensing circuit on the control board may have failed.

In these cases, the drive will likely need to be sent to an authorized Mitsubishi service center for repair or replaced entirely.

How to Reset the Uv Fault

Once you have verified the power supply is healthy:

• Press the [STOP/RESET] key on the operation panel.
• Alternatively, cycle the power (Turn OFF, wait for the display to go blank, and turn back ON).