How to Diagnose a VFD Undervoltage Fault
An undervoltage fault means the drive's internal DC bus fell below the level it needs to run, and the drive tripped to protect itself and the motor. The cause can be outside the drive, a sagging or interrupted supply, or inside it, a loose connection or a tiring capacitor bank. This page is for the technician facing a drive that keeps tripping on undervoltage. It works the likely causes in order, gives a test for each, and separates a genuine supply problem from a drive that is starting to fail, so the fix lands on the real fault.
VFD Undervoltage Fault in one line: A VFD undervoltage fault means the DC bus dropped below the run threshold, so work the causes in order: a supply voltage sag or brownout, a lost or weak input phase, a loose or corroded input connection, and finally a failing DC bus capacitor bank inside the drive. Measure the incoming line voltage on all phases first, because a supply problem is common and external; confirm the DC bus reading against the line; and only after ruling out the supply and connections should you suspect the drive's own components.
First Checks: Measure the Incoming Supply
The DC bus is fed from the incoming line, so measure the supply first. Read the voltage on all input phases at the drive terminals, ideally while the fault occurs or under the load that triggers it, because a supply that reads fine at rest can sag under load. A supply sitting low, or dipping when a large load elsewhere starts, feeds a low DC bus and trips the drive on undervoltage even though the drive is healthy. The bus level itself and how to read it is covered in the guide on interpreting VFD DC bus voltage readings.
Look for a sag tied to an event rather than a constant low. Undervoltage that appears only when a big motor across the plant starts, when a utility event passes, or at a certain time of day points to a supply that dips transiently, not a steady deficiency. A brief dip can trip a drive without power-loss ride-through configured, so note whether the fault is momentary and event-driven or a persistent low, because the two point to different fixes: ride-through and supply stiffening versus a wiring or transformer problem.
Check all three phases against each other, because a low or lost phase looks like undervoltage. If one input phase reads low or is missing, the drive cannot maintain the bus and trips, and a lost input phase also stresses the drive. A meaningful imbalance between the phase voltages is itself a fault to chase, using the same balance-checking method as in the guide on verifying three-phase voltage balance at a VFD. A phase that comes and goes explains an intermittent undervoltage trip.
Check the Connections and Input Path
A loose or corroded connection mimics a supply problem and is one of the most common real causes. A slack input terminal, an oxidized lug, a tired disconnect contact, or a fuse holder with high resistance all drop voltage under load, starving the bus at exactly the moment current is drawn. With the drive safely isolated, inspect and re-torque the input terminations and check the disconnect and any upstream contactor for signs of heat or pitting. A connection that is tight and clean at rest can still fail under the load that triggers the trip, so look for discoloration and heat damage.
Trace the input path back through the protection. An undervoltage that only appears under load can come from a marginal upstream device, a fuse, breaker, or contactor, that adds resistance or drops out momentarily. A control-power supply that dips can also cause a drive to fault in ways that read as undervoltage, so confirm both the power circuit and the control-power circuit are solid. Follow the path from the drive terminals back toward the source, checking each connection and device for the voltage drop that starves the bus.
Distinguish a wiring fault from a supply fault by where the voltage drops. If the line is solid at the source but low at the drive terminals under load, the drop is in the wiring or a device between them, and that is your fault. If the voltage is already low back at the source, the problem is the supply itself, a transformer, a utility issue, or another load pulling it down, and no amount of work at the drive will fix it. Measuring at both ends localizes the fault to the right section.
Suspect the Drive Only After the Supply Is Clear
Once the supply and connections check out, the drive's own front end and DC bus become suspect. The DC bus capacitors that smooth the rectified supply age over service life, and as they weaken the bus can no longer hold its level under load, tripping on undervoltage. A drive that has been in service a long time, or one stored de-energized for a long period, is a candidate for tired capacitors. This is a repair for qualified personnel, because the bus holds a dangerous charge even after power is removed and must be verified discharged before any work.
Consider the input rectifier and any pre-charge circuit. A failing rectifier diode or a pre-charge resistor and its bypass contactor that is not doing its job can leave the bus low or unstable, reading as undervoltage. These are internal drive faults that show up after the external supply is proven good, and they point to a drive repair or replacement rather than anything in the field wiring. Do not chase the field endlessly when every external measurement is clean and the pattern points inward.
Rule the drive in or out by comparison where you can. If a known-good drive on the same supply does not fault, or the suspect drive faults on a supply that feeds other equipment cleanly, the evidence points to the drive. Because the DC bus and trip events are values a monitoring system can trend, a bus that has been slowly reading lower over months, or undervoltage trips that are becoming more frequent, is a data signature of a drive front end that is aging toward failure, letting you plan the replacement before it strands the process.
Frequently Asked Questions
Is a VFD undervoltage fault usually the drive or the supply?
More often the supply or the wiring than the drive itself. Measure the incoming line voltage on all phases first, ideally under the load that triggers the trip, because a sag, a lost phase, or a loose connection starves the DC bus and trips the drive even though it is healthy. Only after the supply reads solid at the drive terminals and the input connections are proven tight and clean should you suspect the drive's own capacitors or rectifier.
Can a loose input connection cause an undervoltage trip?
Yes, and it is one of the most common real causes. A loose or corroded input terminal, a tired disconnect, or a high-resistance fuse holder drops voltage under load, starving the DC bus exactly when current is drawn, so the drive trips on undervoltage. The connection often reads fine at rest and fails only under load, which is why the fault can be intermittent. Isolate the drive, inspect and re-torque the input terminations, and look for heat discoloration on connections and devices.
Do failing DC bus capacitors cause undervoltage faults?
They can, especially in an older drive or one stored de-energized for a long time. The bus capacitors smooth the rectified supply, and as they age they lose the ability to hold the bus level under load, so the drive trips on undervoltage. Suspect them only after the external supply and connections are proven good. This is a repair for qualified personnel, because the bus holds a dangerous charge after power removal and must be confirmed discharged before any work is done.
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