Troubleshooting guide

Why Does My VFD Trip on Undervoltage?

A drive trips on undervoltage when its DC bus falls below the limit. Covers supply dips, other starts, connections and fault-history timing.

Electrical & Power · Last reviewed

A variable frequency drive rectifies the supply into a DC bus and builds its output from that. If the bus voltage falls below the limit, usually because the supply dipped or a connection opened, the drive trips to protect itself. The fault log shows when it happened and what the bus voltage was.

Drives keep dangerous voltage on the DC bus after the power is removed. Wait for the bus to discharge and verify it with a meter before opening the drive. Settings, measurements and repairs belong with qualified personnel following lockout/tagout.

What you observeLikely causeWhat to check
Trips when a large motor or load starts elsewhereVoltage dip on the shared supply during the startVoltage recording at the drive input during the other start. Starting method of the other load
Trips at random times, mostly during the dayUtility voltage sags, or a loaded transformer or feederUtility disturbance records and the supply voltage trend
Trips on starting under a heavy loadWeak supply, undersized conductors or a long feeder that drops under loadVoltage at the drive input while accelerating
Trips with a flickering or bouncing displayLoose connection, a bad fuse, or a worn input contactorInput terminals, fuses and the contactor, with the drive isolated and verified dead
Trips after a power outage and returnThe drive detects a sag or a loss during the restart sequenceRide-through or auto-restart settings, and the timing of other equipment
Trips at constant intervalsA cycling load, a capacitor bank or a power factor controllerTime the trips against other equipment

Use the fault-history timing

Most drives record the time of the fault, the bus voltage, the output frequency and current at that moment. These details separate the causes:

  • A trip with a sudden bus voltage drop and no change in output current points to a supply event.
  • A trip during acceleration with a high current points to a supply that cannot support the load: a weak source or a high feeder voltage drop.
  • Trips that line up with other plant events (a compressor starting, a welder, the end of a shift) point to a shared supply.
  • Trips at random, with no event, suggest an intermittent connection or the utility.

What the numbers look like

A drive on a 480 V supply has a DC bus of about √2 × 480 = 679 V at no load (typically a little lower under load). The undervoltage trip is set well below that, at a value given in the drive manual, and the exact threshold varies with the model and the voltage class. The supply therefore has to dip a substantial amount to cause a trip, which is why a deep sag or a lost phase is more likely than a small one. A drive that trips on small dips is more often a connection problem, or has a threshold set too high.

Write a useful problem statement

  • Drive model, rating, supply voltage and the programmed undervoltage and ride-through settings.
  • Fault log entries: time, DC bus voltage, frequency and current at the trip.
  • What else runs on the same supply and what happened at the time of the trip.
  • Recent changes: new loads, work on the feeder, changes at the utility.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Can I lower the undervoltage threshold?

Some drives allow adjustment within limits. Lowering it hides the symptom, can allow operation at low voltage that overheats the drive, and should follow the manual.

What is ride-through?

A drive function that keeps the drive running briefly through a short supply loss by using the energy stored in the bus and in the load inertia.

Would a line reactor help?

A reactor reduces harmonics and limits input current peaks, but it does not fix a supply that dips. It is not a cure for undervoltage.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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