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

The symptom

A conveyor drive shows an overcurrent fault within a second of the start command: F0001 on a SINAMICS G120, F012 on a PowerFlex 525 (or F013 there, which the drive reports when it sees the problem as a ground fault), OC1 on many Asian drives. You reset it, try again, and it trips the same way.

Safety first: do every check below that touches the motor circuit with the drive isolated, locked out and its DC bus discharged. Wait the time printed on the drive cover and measure the bus before you touch the terminals.

Step 1: find out exactly when it trips

Read the fault history, including the frequency and current at the moment of the trip. Where the trip lands tells you where to look.

If it trips instantly at 0–2 Hz, suspect a short or earth fault in the cable or motor, or wrong motor data, and go to Step 4. If it trips during the ramp, the load needs more current than the ramp allows; go to Step 2. If it only trips when hot, or only after a stop, suspect a mechanical brake that does not release, or a load that is still turning backwards when the drive tries to start it.

Step 2: compare the ramp time with the load inertia

A 0.5 s acceleration ramp on a heavily loaded conveyor demands torque far above the motor's rating. As a test, lengthen the ramp, for example to 5 s. If the trip disappears, pick the shortest ramp the process accepts with some margin, rather than the longest one that happens to work.

Step 3: check the machine turns freely

Uncouple the motor if you can, or try to turn the load by hand. To the drive, a seized bearing, jammed product, an over-tensioned belt or a holding brake that never releases all look the same: overcurrent. Check that the brake output is wired and that the brake actually lifts. You should hear it click.

Step 4: check the motor data and run identification

Compare the drive parameters with 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 in star when the drive expects delta, gives the wrong flux and high current. In vector mode, run the motor identification routine after any motor change.

Step 5: test the insulation and the cable

Disconnect the motor cable from the drive first. Then use an insulation tester between each phase and earth, typically at 500 V or 1000 V DC; check the motor documentation for the test voltage, and never apply it to the drive output. Readings well below the motor manufacturer's minimum point to a damaged winding or cable. Also measure the 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. Look up the drive's maximum cable length and whether it needs an output reactor or filter at the length you have installed.

Step 6: check the boost and the control mode

In V/f mode, too much voltage boost at low speed drives the current up, while too little boost can stall a heavy load. Reset the boost to its default before you tune it. Change one parameter at a time and write each change down.

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. And do not bypass the drive to run the motor direct-on-line as a "test" without thinking through what the load will do when it gets full speed instantly.

For the control side, see the hardware chapter on motion and feedback and the VFD programming article.