Troubleshooting
Troubleshooting VFD Faults
The fault code names which protection fired, not why. This page reasons from the fault category to the subsystem and the measurement that confirms it.
Safety. This is a reasoning aid, not a work instruction. Diagnosis and testing are performed by qualified personnel under your site’s LOTO procedures. VFD DC bus capacitors hold a lethal charge after power removal — observe the drive’s marked discharge wait time before touching any terminal. Insulation testing is done de-energized with the drive output disconnected. Measure, don’t guess.
Overview
A variable frequency drive trips to protect itself and the motor. The vendor’s numeric code tells you which protection operated — it does not tell you the cause, and no two manufacturers number them the same way. Always read the actual code out of the drive against your drive’s own fault-code list; this page never reproduces one. What it does instead is reason about the fault CATEGORIES those codes fall into, so the code becomes a starting branch rather than an answer.
The most useful single question is not “what number?” but “when does it trip?” — on decel, on accel, under steady load, after warm-up, or on enable. The correlation with a phase of operation points at the subsystem far more reliably than the code.
Start Here
Before touching anything, observe and note:
- The exact fault text/code and any fault-history entries — from the drive, against the manufacturer’s fault-code list.
- When it trips — power-up, enable, accel, steady run, decel, or coast.
- Whether it correlates with drive/plant load — clean when idle, faults when loaded is a distinct signature (see nuisance trips below).
- What changed — new cable pull, new motor, parameter edit, hot day, added a second drive nearby.
- Whether the bus is discharged before any hands-on work.
Decision Tree
flowchart TD
A[Drive faulted?] -->|No fault, shaft still| NORUN[Enable / reference / STO gap]
A -->|Yes, has a code| B{When does it trip?}
B -->|On decel / overhauling load| OV[Overvoltage - bus]
B -->|Instant on run or accel| OC[Overcurrent / output short]
B -->|After minutes-hours running| OT[Overtemperature]
B -->|On enable, leakage to earth| GF[Ground fault]
B -->|On power events / at power-up| UV[Input / undervoltage]
B -->|Sporadic, only under load| EMC[Nuisance trip - EMC / coupling]
OV --> OVc[Extend decel ramp;<br/>add brake resistor/regen]
OC --> OCc[OFFLINE insulation + ring out<br/>motor & cable; check accel/motor data]
OT --> OTc[Cabinet ambient, airflow,<br/>carrier frequency, derate]
GF --> GFc[OFFLINE megger motor & cable<br/>to ground - drive disconnected]
UV --> UVc[Measure 3 input phases;<br/>lost phase / precharge / terminals]
NORUN --> NRc[Run permissive, reference SOURCE,<br/>direction, STO]
EMC --> EMCc[Shields 360 both ends, segregation,<br/>PE/filter bonding - not parameters]
Likely Causes
Overvoltage (DC bus)
- What it looks like — trips on deceleration or with an overhauling load; high-inertia machine stopped too fast; never on a gentle coast.
- Discriminating check — extend the decel ramp; if the trip moves out or vanishes, regen energy had nowhere to go.
- Fix direction — lengthen decel, or add a braking resistor/chopper or regen unit sized for the duty. Braking-resistor wiring is covered in the VFD wiring guide. A genuinely high input supply is a separate cause — measure it.
Overcurrent / output short
- What it looks like — instant trip on run or accel; may trip the instant the output is enabled if a short is already present. Most likely of all categories to mean real damage.
- Discriminating check — with the drive locked out and the output disconnected, insulation-test and ring out motor and cable phase-to-phase and phase-to-ground.
- Fix direction — repair a real motor/cable fault before re-running. If insulation is good and it only trips on hard accel, the ramp is too aggressive or the motor data/control mode is wrong — hand back to the commissioning workflow.
Overtemperature
- What it looks like — trips after minutes to hours, worse in a hot cabinet or on a hot day; not tied to a specific motion event.
- Discriminating check — measure cabinet ambient; confirm filters, fans, and clearance; note the switching (carrier) frequency, which raises drive losses when increased.
- Fix direction — restore cooling, derate for ambient/altitude per the manual, or lower carrier frequency. Derating curves are vendor-specific.
Ground fault
- What it looks like — trips on enable or accel; can read like an overcurrent, distinguished by the code and by leakage going to earth.
- Discriminating check — OFFLINE insulation test of motor and cable to ground, drive output disconnected. This is the classic megger-through-the-drive trap — see why you never megger through a connected drive.
- Fix direction — repair motor/cable insulation. A long, poorly-shielded cable can raise capacitive earth-leakage enough to nuisance-trip a sensitive detector even with sound insulation — that shades into EMC below.
Input / undervoltage
- What it looks like — trips on power events, sags, or at power-up; or a bus that never charges.
- Discriminating check — measure the three input phases under load; look for a lost phase, a loose input terminal, or a failed precharge (relay clicks but the drive never comes ready).
- Fix direction — correct the supply, tighten terminations, service the precharge circuit. A single lost input phase both undervolts the bus and overheats the remaining path — treat as urgent.
Motor won’t run — but no fault is shown
- What it looks like — “Ready” or “Stopped”, no fault, no output.
- Discriminating check — walk the enable chain: run/enable input, the active reference SOURCE (keypad vs terminal vs fieldbus — a drive commanded from the wrong source sits at zero), direction, and STO. An unsatisfied STO holds the output off, sometimes with only a status bit.
- Fix direction — satisfy the missing permissive; select the intended reference source; confirm STO is de-asserted through the safety function.
Nuisance trips under load (EMC / coupling)
- What it looks like — clean at commissioning (drive idle), then trips or corrupts signals once the plant loads up; correlated with drive state, not a mechanical event.
- Discriminating check — scope the quiet plant, then the running plant; the difference is the coupling. Check shield terminations, segregation, and bonding.
- Fix direction — fix the installation, not the parameters. This is EMC-mitigation territory, worked end-to-end in the intermittent I/O case study.
What to Measure
Every category resolves to a measurement, not a guess:
- DC bus voltage — from the drive readout, or across the marked DC terminals after the discharge wait time; separates over- from under-voltage.
- Insulation resistance — OFFLINE, drive output disconnected — motor and cable, phase-to-phase and phase-to-ground. The one non-negotiable rule; the wiring reasoning is in the VFD wiring guide and panel grounding & bonding.
- Output phase-current balance — the three output currents should be roughly equal; a large imbalance points at a motor/cable fault or a lost output phase.
- Thermal — cabinet ambient, airflow, heatsink temperature; correlate with run time.
- Coupling / noise — when trips track drive load, capture analog and comms baselines idle vs loaded; the oscilloscope practice is on the RS-485 physical layer page, and the Wireshark methodology covers network symptoms.
Common Root Causes
| Symptom | Likely cause | First check | Typical fix |
|---|---|---|---|
| Trips on decel | Regen bus overvoltage; decel too fast | Extend decel ramp — does it clear? | Longer decel or brake resistor/regen |
| Instant trip on run/accel | Output short / motor-cable fault, or accel too aggressive | OFFLINE insulation + ring out | Repair insulation; soften accel/fix motor data |
| Trips after warm-up | Cooling / ambient / carrier frequency | Cabinet ambient + airflow | Restore cooling, derate, lower carrier |
| Trips on enable, leakage to earth | Ground fault in motor or cable | OFFLINE megger to ground | Repair motor/cable insulation |
| Trips on power events / won’t charge | Supply, lost input phase, precharge | Measure 3 input phases | Correct supply; service precharge/terminals |
| Ready but shaft still, no fault | Enable/reference-source/direction/STO gap | Walk the permissive chain | Satisfy permissive; select right reference |
| Sporadic trips only under load | EMC coupling, shield/bonding/segregation | Scope idle vs loaded | Fix shielding/segregation/bonding |
When It’s Not What It Looks Like
- A ground fault that is really EMC. A sound but long, poorly-terminated cable can leak enough capacitive current to trip a sensitive ground-fault detector. Insulation tests clean; the trip is a shielding/termination problem, not a damaged motor.
- An “overcurrent” that is a lost input phase. Single-phasing the input starves the bus and stresses the output; the drive may report overcurrent or undervoltage depending on where the protection catches it. Measure all three input phases before condemning the output stage.
- A “dead” drive that is a permissive. No fault, no motion reads like a hardware failure but is usually an open run permissive, the wrong reference source, or an unsatisfied STO — no protection has tripped at all.
- A decel overvoltage blamed on the supply. The bus is high because the load is regenerating, not because the mains are high — confirm by ramp, then measure the supply.
- The fault that only appears months later. Long unfiltered motor leads double the voltage stress at the motor terminals; the winding fails later and the drive gets blamed. The remedy lives in the VFD wiring guide.
Related Pages
- How to Wire a VFD — the wiring behind most of these faults, and the megger warning
- Noise & EMC Mitigation — segregation, shielding, and coupling
- Panel Grounding & Bonding — PE and shield termination practice
- VFD Commissioning Workflow — motor data, first power-up, functional checks
- Motor Troubleshooting Decision Tree — the broader motor/drive routing page
- Troubleshooting: Motor Won’t Start — for starter-driven (non-VFD) motors
- Intermittent I/O case study — a noise-coupling investigation end to end
Trust Boundary — Engineering Judgment Required
This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.
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