VFD Trips on Overcurrent at Start: Causes, Checks and Fixes

The symptom

A conveyor drive shows an overcurrent fault (F0001 on a SINAMICS G120, F012/F013 on a PowerFlex 525, OC1 on many Asian drives) within a second of the start command. Reset, try again, same trip.

Safety first: every check below that touches the motor circuit is done with the drive isolated, locked out and its DC bus discharged — wait the time printed on the drive cover and measure the bus before touching terminals.

Step 1 — When exactly does it trip?

Read the fault history, including the frequency and current at the moment of the trip.

Step 2 — Ramp time versus load inertia

A 0.5 s acceleration ramp on a heavy loaded conveyor demands torque far above rating. Lengthen the ramp (for example to 5 s) as a test. If the trip disappears, choose the shortest ramp the process accepts with margin, rather than the longest one that works.

Step 3 — Is the machine free?

Uncouple the motor if practical, or try to turn the load by hand. A seized bearing, a jammed product, an over-tensioned belt or a holding brake that never releases all look like overcurrent to the drive. Check that the brake output is wired and that the brake actually lifts — listen for the click.

Step 4 — Motor data and identification

Compare the drive parameters to the motor nameplate: voltage, current, frequency, speed, power and the star/delta connection. A 400 V delta motor entered as 230 V, or a motor wired star when the drive expects delta, gives wrong flux and high current. In vector mode, run the motor identification routine after any motor change.

Step 5 — Insulation and cable test

With the motor cable disconnected from the drive, use an insulation tester (typically 500 V or 1000 V DC — check the motor documentation, and never apply it to the drive output) between each phase and earth. Readings well below the motor manufacturer's minimum indicate a damaged winding or cable. Also check phase-to-phase winding resistance: the three readings should be close to each other.

Long motor cables add capacitance, which can cause earth-fault or overcurrent trips at start. Check the drive's maximum cable length and whether an output reactor or filter is required.

Step 6 — Boost and control mode

In V/f mode, excessive voltage boost at low speed drives current up. Too little boost can stall a heavy load. Reset boost to default before tuning it, and change one parameter at a time, recording each change.

What not to do

Do not raise the current limit above the motor rating to "get it running". You will hide a mechanical fault and overheat the motor. Do not bypass the drive to run the motor direct-on-line to "test" it without considering what the load will do at full speed instantly.

See the hardware chapter on motion and feedback and the VFD programming article for the control side.