Servo Drive and Amplifier Alarms: A Structured Troubleshooting Method

Classify the alarm before you touch anything

Drive alarms fall into four families, and each family has a different investigation path: power-stage faults (overcurrent, IPM/IGBT fault, short circuit), thermal/duty faults (overload, motor or drive overheat), feedback faults (encoder disconnect, count error, pulse missing), and bus faults (DC-link under/overvoltage, regeneration overload, communication loss).

Recording the exact alarm code with the axis, the operation in progress, and whether the fault repeats on power-up is worth more than any measurement. A power-stage fault that appears only during rapid deceleration is a regeneration or brake-resistor issue; the same code on a stationary axis at power-on is a genuine short in the motor or cable.

Power-stage and overcurrent faults

With the drive isolated and the DC link discharged, measure motor phase-to-phase resistance and phase-to-ground insulation resistance at the motor connector, then repeat at the drive end of the cable. Balanced phase resistance with low insulation resistance points at a motor winding fault; a difference between the two measurement points isolates the fault to the cable, which is the more common failure in a drag chain.

If the motor and cable are healthy, the remaining candidates are the drive power stage itself and mechanical binding that demands current the axis was never sized for. Rotate the axis by hand with the brake released — an axis that feels notchy or requires markedly more torque in one direction is a mechanical problem masquerading as an electrical alarm.

• Unbalanced phase resistance: motor winding failure.
• Low insulation resistance only at drive end: cable or connector water ingress.
• Fault only on deceleration: braking/regeneration path — check resistor and its thermal contact.
• Fault on power-up with no motion: hard short or failed power module.

Feedback and encoder faults

Feedback alarms are dominated by connector and shield problems, not by encoder electronics. Inspect the feedback connector for bent pins and corrosion, confirm the shield is terminated at the specified end only, and verify that the feedback cable is routed away from motor power cables — a parallel run of more than about 300 mm inside the same duct is enough to inject noise that reads as a count error.

For absolute encoders with battery-backed position, a battery alarm after a power-down usually requires a re-reference of the axis, not a new encoder. Always record the machine's reference offsets before replacing an absolute encoder, or commissioning becomes a full axis calibration.

Thermal and duty-cycle faults

Overload alarms that appear only in summer, or only after several hours of production, are almost always cabinet cooling problems. Check the filter fan and heat exchanger before the drive: a blocked filter mat raises internal cabinet temperature by 10–15 °C, which is enough to push a correctly sized drive into thermal derating.

Persistent overload on one axis with clean cooling means the duty cycle exceeds the drive's continuous rating. That is a sizing or process problem — reduce acceleration, reduce cutting load, or move up one drive frame size. Replacing the same drive repeatedly will simply repeat the failure.

Documenting the repair

Before returning the machine to production, record the alarm code, the measurements taken, the part numbers fitted, the parameter set before and after, and the axis reference offsets. On a fleet of machines this record is what turns a two-day diagnosis into a two-hour one the second time the fault appears.