Why Does My VFD Trip? Overcurrent, Overvoltage, Overtemp
Why does a VFD trip? A variable frequency drive trips when its internal protection detects output current, DC bus voltage, or heatsink/IGBT temperature outside the limit set by its overload class (per IEC 61800-2), and it opens the output stage before the power semiconductors take damage. The trip code names the symptom, not the root cause — an overcurrent fault can come from a jammed conveyor, too fast a ramp, or a nicked motor cable, and each needs a different fix. This article separates the three headline trip families (overcurrent, overvoltage, overtemperature) from ground fault, undervoltage, and external faults, and shows how the fault log narrows down which one you actually have.
How VFD Fault Protection Actually Works
Every VFD runs two layers of protection on top of the rectifier-DC bus-inverter power stage. The first is instantaneous: a hardware comparator watches for a hard short or an IGBT desaturation event and opens the gate drive in microseconds, with no software delay involved. The second is thermal, or I²t: a software counter integrates output current over time against the drive's normal-duty or heavy-duty overload curve and trips once the accumulated heating exceeds the class limit. Both write to the same place — a fault log that records the trip code plus a snapshot of current, voltage, output frequency, and heatsink temperature at the instant of the trip.
Instantaneous vs Thermal Protection
Instantaneous overcurrent protection exists to save the IGBTs from a phase-to-phase or phase-to-ground short at the output terminals; it does not care about duty rating, only peak current. Thermal overload protection is the one that actually enforces the ND/HD class — it is why a drive can carry 110% for 60 seconds on a pump but only 150% for 60 seconds is guaranteed on a conveyor, and why running a normal-duty drive on a heavy-duty load nuisance-trips on thermal overload long before the instantaneous limit is ever reached.
Overcurrent Trips: Causes and Fixes
Overcurrent is the most common trip because it has the most causes. A jammed or unusually heavy load draws more torque current than the motor nameplate assumes. An acceleration ramp set for a light load overcurrent-trips the moment real load inertia is coupled in, because the drive has to supply more current to accelerate the same mass in the same time. A drive sized for normal duty but installed on a heavy-duty job — conveyors, crushers, positive-displacement pumps — will thermal-trip well inside its rated life. Autotune skipped at commissioning leaves the vector current regulator guessing at motor parameters, which shows up as current overshoot on load steps. A nicked or moisture-contaminated motor cable can short phase-to-phase or phase-to-ground and reads on some drives as an instantaneous overcurrent rather than a dedicated ground fault.
Formula: Overload Trip Current — Source: IEC 61800-2, dual overload rating classes
IOL = k × Irated
| Symbol | Description | Unit |
|---|---|---|
| IOL | Overload trip current threshold | A |
| Irated | Drive rated continuous output current | A |
| k | Duty multiplier: 1.10 (normal duty, 60 s) or 1.50 (heavy duty, 60 s) | dimensionless |
| t | Permitted duration at k before trip | s |
Before replacing anything, confirm which side of that formula is wrong: is the load drawing more current than the class allows, or was the wrong class specified for the job? See normal duty vs heavy duty overload ratings for how to check duty class against a specific application, and sizing a VFD to the motor and load for the sizing steps that catch this before commissioning.
Overvoltage Trips: DC Bus and Regenerative Energy
A decelerating motor with real inertia behind it feeds energy back into the drive rather than drawing it. On a diode rectifier front end that energy has nowhere to go except the bus capacitors, so bus voltage climbs until the drive trips to protect them. The usual triggers: a decel ramp set shorter than the load's kinetic energy can dissipate through motor and drive losses alone; an overhauling load — a lowering hoist, a coasting fan, a decelerating centrifuge — that regenerates continuously rather than just on stop; a braking resistor sized for occasional stops but not the actual duty cycle; or, less often, a line-side switching transient from a capacitor bank on the same feeder.
What we see in the field: overvoltage trips on hoists and centrifuges get blamed on the drive itself, when the brake chopper and resistor were undersized for the real duty cycle from day one — the fix is a resistor rated for continuous regen, or a regenerative/active front end, not a bigger drive.
Extending the decel ramp is the free first move, since it spreads the same kinetic energy over more time. For loads that regenerate as part of normal operation rather than only on stop, a properly sized brake chopper and resistor — see dynamic and regenerative braking options — is the permanent fix, not a workaround.
Overtemperature Trips: Cooling, Carrier Frequency, and Derating
A drive that overtemp-trips has too much heat coming in, too little heat going out, or both. Coming in: switching losses rise with carrier frequency — running at 12-16 kHz for a quieter motor generates markedly more IGBT loss than 2-4 kHz, and most manufacturers derate rated current at the higher settings. Going out: a stalled or dust-caked cooling fan, a blocked heatsink, an enclosure with no forced ventilation in a hot process area, or a drive mounted above another heat source all reduce the heat the drive can shed. This depends heavily on cabinet layout and ambient temperature, so the same drive model can run all day in one panel and overtemp-trip in another built for a hotter mechanical room.
Not always the fan. On a conveyor or compressor duty running near its heavy-duty limit at high ambient, dropping the carrier frequency one step is a legitimate — and free — first troubleshooting move, before opening the cabinet to check cooling hardware.
Other Common Trips: Ground Fault, Undervoltage, External Fault
Ground fault trips when current returning to the source through earth rather than the intended phase conductors exceeds the drive's imbalance threshold — usually motor winding insulation breakdown or feeder cable damage. On a newly commissioned drive it is more often a wiring or termination defect: a pinched cable, a stray strand, moisture in a gland. On a drive with months or years of service it is more often insulation aging, and meggering the motor and feeder cable independently is the fastest way to localize it. Long, unfiltered runs are part of the same picture — see dV/dt output filters and motor cable length limits for how reflected-wave stress accelerates insulation breakdown over a cable's service life.
Undervoltage trips when the DC bus falls below the level needed to hold the motor at its commanded torque — a line sag or brownout, a loose incoming terminal, or an upstream contactor chattering under load. Because it is a supply-side symptom rather than a drive defect, checking incoming voltage at the terminals under load, not just at no load, is the first diagnostic step; see VFD voltage and current ratings for how supply tolerance bands are defined. External fault is not an internal drive fault at all — a digital input wired to an e-stop circuit, a motor thermistor (PTC), or a safety relay has opened, and the drive is reporting exactly what it was told to report.
Diagnosing a Trip: Fault Log, Reset Strategy, and When It's Really the Load
Every drive worth specifying keeps a fault history — the trip code plus the current, voltage, frequency, and temperature snapshot at the moment it happened — and that snapshot, not the trip name alone, is what separates these causes. ABB ACS580, Schneider Altivar ATV340, and Siemens SINAMICS G120 label the same physical faults with different codes, but the underlying causes covered here are identical across the collection of variable frequency drives regardless of brand.
Reset strategy matters as much as diagnosis. A limited auto-restart on 1-2 attempts is reasonable for undervoltage tied to momentary line dips, since the cause is external and transient. For overcurrent, overvoltage, or ground fault, repeated auto-reset without inspection risks turning one trip into a burned-out motor or a damaged IGBT, because the underlying condition (a jam, an undersized resistor, a degrading winding) is still there on the next attempt. A trip that repeats at the same speed on the same job points to the load or the sizing; a trip that only appears after months of unchanged operation points to component aging — a bearing, a capacitor, or insulation — not a new external cause. Working through the fault families in this order, rather than swapping the drive first, is covered in more depth in the VFD engineering guide.
Frequently Asked Questions
What does an overcurrent (OC) fault mean on a VFD?
The drive's output current exceeded the instantaneous or timed overload limit set by its duty class — 110% for 60 s on normal duty, 150% for 60 s on heavy duty, per IEC 61800-2. It flags a symptom, not one single cause: a jammed load, too fast a ramp, a cable short, or a skipped autotune can all produce it, so check the fault log's current and frequency snapshot before replacing hardware.
Why does my VFD only trip during acceleration, not at constant speed?
Accel-only overcurrent trips point to inertia, not motor rating: the load needs more torque current to speed up than to hold speed, and a ramp tuned for a lighter load will overcurrent-trip once real load inertia is coupled in. Lengthening the accel ramp, or verifying the duty class against the actual load, usually clears it without any hardware change.
Can a long motor cable cause an overvoltage trip?
Not directly. A long cable causes reflected-wave voltage spikes at the motor terminals — approaching twice the DC bus voltage past roughly 15-50 m of cable — which is a separate problem from DC bus overvoltage inside the drive. The fixes differ: a dV/dt reactor or sine-wave filter addresses cable spikes, while a brake chopper and resistor address bus overvoltage from regeneration.
Does a higher carrier frequency cause more overtemperature trips?
Yes. Raising carrier frequency from roughly 4 kHz to 12-16 kHz increases IGBT switching losses and pushes the drive's current derating, so a unit already running warm — high ambient, a dusty heatsink, an undersized enclosure — trips on overtemp sooner at the higher setting. Dropping one carrier step is a legitimate first check before inspecting cooling hardware.
Is a ground fault trip always a wiring problem?
On a new installation, usually yes — a pinched cable or moisture in a gland. On a drive that has run for months or years, it is more often insulation aging in the motor winding or feeder cable. Meggering the motor and cable separately localizes which one is failing before the drive itself is suspected.
Should automatic fault reset be enabled on a VFD?
For nuisance faults tied to momentary line dips (undervoltage), a limited auto-restart of one or two attempts is standard practice. For overcurrent, overvoltage, or ground fault, repeated auto-reset without inspection can mask a developing mechanical or insulation problem, so most integrators cap auto-restart at two to three attempts and require a manual reset with the fault log open beyond that.
Conclusion
A VFD trip is a protection response, and the fastest way to resolve one is to classify it before touching the drive: overcurrent points to load, ramp, or duty-class mismatch; overvoltage points to regenerated energy and decel/braking sizing; overtemperature points to cooling and carrier frequency; ground fault, undervoltage, and external fault each have their own separate signature in the log. Reading the current/voltage/temperature snapshot at the moment of trip, rather than the trip name alone, is what turns a repeat nuisance trip into a fixed one.
