VFD Troubleshooting Guides

The thing about VFD troubleshooting is that the drive is usually not the problem. It is the messenger. An OC trip on a PowerFlex 755 is almost always telling you that something downstream is drawing too much current. A bearing starting to seize. A conveyor with a jammed pinch point. A motor that lost its cooling fan. The drive is doing its job by stopping.

Faults that look like drive problems but are not

Most drive alarms send you downstream, not into the cabinet. Read the fault code, then walk to the motor.

OC1, OL, or F7 on any brand: load side. Check bearings, alignment, a jammed mechanism, or a parameter set for the wrong motor.
DC bus OV or F2: the machine is decelerating faster than the ramp allows. Stretch the ramp or size a brake resistor for the duty cycle.
Ground fault, GF, F8: almost always the motor cable or a winding shorted to ground. Rarely the drive.
Over-temp on a drive that used to be fine: cabinet filter is blocked, or room cooling failed. Not the drive.

Faults that are actually the drive

When the drive really is at fault, the evidence is usually hard to miss. A bang and smoke. All three output phases shorted to each other on a meter. A drive that came back from a brown-out with no parameters.

Capacitor aging on a 10-plus year old drive. Unexplained OV trips after short power cycles. Replace the bus caps or the drive.
Gate driver or IGBT failure: output-stage short. Almost always a sign that the motor or cable failed first.
Firmware parameter corruption after a brown-out or a failed update. Restore from a known backup, not the last auto-save.

Do not skip the bus cap wait

Industrial VFDs store lethal energy in their capacitors for 5 to 15 minutes after power-off. Open the cabinet too early and the caps can hurt you. Verify zero voltage across DC+ and DC-minus with a meter, not a lamp. Lamps can hide residual bus voltage. Never hot-plug a motor onto a running drive. The inrush will destroy the output stage before the drive's internal protection can react. And wear the arc-flash PPE your cabinet label specifies. The transformer upstream sets the incident energy, not the drive.

1 machines covered in this category

Allen-Bradley

PowerFlex 525

Compact AC Drive (Rockwell Automation)

7 guides

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Images on this page sourced from Freepik. Credits: eleonimages, George Fontana.

VFDs & Drives