VFD fault codes: what the drive is telling you
Drives display a code; most people read it as a model-specific mystery. Grouped by what the drive is actually measuring, almost every fault resolves into one of six physical conditions — and the diagnosis is the same whatever the label says.
OC, E-OC, OC1 or Err 04 depending on make,
model and firmware. Look up what your own drive calls each condition in its manual, then use this page
to work out what to check.The six conditions
| Condition | The drive measured | Check, in order |
|---|---|---|
| Overcurrent | Output current past the limit | Acceleration time too short · rated voltage or current parameter wrong · a short or earth fault on the output · a contactor switching the output |
| Overvoltage | DC bus too high | Deceleration time too short · no brake resistor or one too high in ohms · supply voltage genuinely high |
| Undervoltage | DC bus too low | Supply missing or a phase missing · loose terminal · undersized supply cable · supply dip on acceleration |
| Overload | Sustained current above the motor's rating for long enough | Rated current parameter too low · genuine mechanical overload · duty too aggressive · cooling failed |
| Overheat | Drive's own heatsink temperature | Fan failed · no clearance around the drive · cabinet too hot · carrier frequency set high |
| Earth fault | Current leaving on the earth conductor | A winding earthed · cable damaged · moisture in the connector · screen incorrectly bonded |
The classic: trips with nothing on the spindle
This one gets its own section because it is the most common first-power-up experience and it is almost never a hardware fault.
If the drive trips the moment you command a run, with no tool fitted and the spindle spinning freely, then mechanical overload is impossible by definition. What is left:
- Rated voltage parameter wrong — too low and the motor cannot make flux and draws excessive current; too high and it saturates.
- Rated current parameter wrong — too low and the protection fires on normal magnetising current.
- Acceleration time far too short — the spindle has inertia even with nothing in the collet.
- Output frequency far above what the parameters expect, putting the motor into a region the V/f curve was not set up for.
The second classic: overvoltage every time you stop
Energy has nowhere to go. Your options, in the order to try them:
- Lengthen the deceleration time. Free, and often sufficient.
- Fit a braking resistor sized against the drive's minimum resistance.
- If it still trips, the demanded stop is faster than the resistor can absorb — lengthen the deceleration time rather than reducing the resistance.
Before you blame the drive
Rule these out first; they cost nothing and they account for a large share of “faults”:
- A parameter that was changed and forgotten. Restoring defaults is the cheapest diagnostic there is.
- A loose terminal. Re-torque the input, output and earth terminals. A drive input that has been running warm will have relaxed.
- The supply. Measure it under load, not at rest.
- The spindle, disconnected. Measure winding resistance balance and phase-to-earth with the drive unplugged — the procedure is here.
Resetting is not fixing
Most drives will let you define an automatic restart after a fault. That is a useful feature on a production machine with a known intermittent cause and a bad idea on a machine you are still commissioning. A drive that silently restarts into a fault condition will eventually restart into the one that was a real earth fault.
Find the cause first. Automate the restart afterwards, if you still want to.
Parts
Control and test parts
Questions
My drive trips immediately with nothing on the spindle. What does that mean?
Most often it is not the spindle: a short or earth fault in the cable, a brake resistor on the wrong terminals, or a control input not in the state the drive expects.
Why does it trip on overvoltage every time I stop?
The spindle is regenerating into the DC bus with nowhere for the energy to go. Lengthen the deceleration time or fit a braking resistor.
Is resetting the fault a fix?
No. A fault code tells you which protection operated, not what caused it. Clearing it without finding the cause just means it will come back, usually at a worse moment.
Sources and method
Three kinds of statement appear on this page, and each is labelled where it occurs:
- Derived — faults are grouped by symptom and by what you measure next, not by code number — the numbers are maker-specific.
- Rule of thumb — workshop practice, not a standard. Given with the range it is meant to hold over, and it loses to your manual wherever the two disagree.
- Maker-specific — parameter numbers, connector pin numbers, minimum brake resistance, maximum cable length, bearing speed limits. Not published here. They move between firmware revisions of the same model, and a wrong one is worse than none.
Standards and documents
- IEC 61800-5-1 — adjustable speed drives: safety requirements
- IEC 61000-6-2 — EMC immunity, industrial environments
- IEC 60204-1 — electrical equipment of machines
- The manual for your own drive and spindle — the only valid source for maker-specific values