When the Power Drops: Troubleshooting DC Link Undervoltage
Is your VFD tripping on DC Link Undervoltage? Learn how to identify input power drops, blown fuses, and internal rectifier issues to keep your motors running.
When the Power Drops: Troubleshooting DC Link Undervoltage
While "Overvoltage" is usually caused by the motor pushing back, DC Link Undervoltage (often coded as UV, F0003, or F30003) is usually a "supply side" problem. It means the VFD’s internal DC reservoir has dropped below the level required to safely operate the IGBTs and the motor.
When the DC bus voltage sags, the drive can no longer maintain the correct torque, and continuing to run could damage the internal electronics due to high current draw.
What is DC Link Undervoltage?
The DC Link is the heart of the VFD. It stores energy from the incoming AC line.
For a 480V AC drive, the DC link should sit around 650V DC.
If that voltage drops below a specific threshold (typically around 400V–450V DC for a 480V unit), the drive will trip to protect itself.
The Common Culprits
1. Input Power Sag (Brownouts)
The most common cause is a dip in the incoming AC voltage. This could be caused by a heavy machine (like a massive compressor or another large motor) starting up elsewhere in the factory and "pulling down" the local grid.
2. Input Phase Loss (The "Missing Leg")
If you lose one of your three phases (due to a blown fuse or a loose lug), the DC bus can no longer stay "filled." It will work fine at no load, but the moment the motor draws power, the DC voltage will crash.
3. Blown Fuses or Tripped Breakers
Check the high-speed semiconductor fuses protecting the drive. If one fuse is blown, the drive is effectively running on single-phase power, which isn't enough to maintain the DC link under load.
4. Faulty Pre-charge Circuit
VFDs have a "pre-charge" resistor that limits current when you first turn the power on. Once the capacitors are full, a relay or thyristor bypasses that resistor. If that relay fails to close, the drive will try to pull all its running power through a tiny resistor, causing the DC voltage to plummet instantly.
Step-by-Step Troubleshooting
1. Check Input Voltage
Use a multimeter to measure the AC voltage at the VFD input terminals (L1, L2, L3).
Check L1-L2, L2-L3, and L3-L1.
They should be balanced. If one is significantly lower, move upstream to the disconnect or the main breaker.
2. Monitor the DC Bus (Software Check)
Go to the drive's monitoring menu and look for the actual DC Link Voltage (e.g., r0026 on Siemens or dnu on Altivar).
Watch this value as you give the motor a "Run" command.
If the voltage is healthy at standstill but drops 100V+ the moment the motor turns, you likely have a loose connection or a missing input phase.
3. Inspect the Pre-charge Relay
If the drive trips on Undervoltage the exact moment it finishes its power-up sequence (even without a run command), the internal pre-charge circuit may be damaged. This often requires professional repair or drive replacement.
Summary Checklist
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