A servo fault can stop a machine long before the motor itself has failed. A following-error alarm, encoder fault or unexpected overload trip may originate in the drive, feedback cable, brake supply, mechanics or programme parameters. Effective servo motor troubleshooting starts by separating those possibilities before a replacement is ordered.
For maintenance teams, the objective is not simply to clear an alarm. It is to establish which part number is genuinely required, protect personnel and equipment, and return the axis to service without creating a repeat failure on the next production run.
Start servo motor troubleshooting with the alarm history
Record the exact alarm code from the servo drive, motion controller or HMI before cycling power. Include the machine state, axis name, commanded movement, load condition and any recent work on the equipment. A vague report such as “motor not running” is rarely enough to identify the correct spare.
Alarm descriptions vary between Siemens, Allen-Bradley, Mitsubishi, Schneider and Omron systems, but the broad fault groups are familiar: overcurrent, overvoltage, undervoltage, overload, following error, encoder or resolver feedback loss, brake fault and thermal protection. The drive manual gives the manufacturer-specific meaning, while the fault timing often points to the physical cause.
A fault at power-up suggests a wiring, feedback, brake or drive issue. A fault only during acceleration may indicate excessive load, poor tuning, binding mechanics or a motor that is undersized for the application. A fault after the axis has run for some time is more likely to involve heat, intermittent feedback, brake drag or a deteriorating cable.
Do not repeatedly reset a servo fault to keep production moving. Repeated reset attempts can damage a drive, conceal an intermittent connection and make the original alarm history harder to interpret.
Make the axis safe before testing
Servo systems can retain hazardous voltage after mains isolation. Follow the machine’s lockout procedure, isolate all relevant supplies and allow the DC bus to discharge for the period specified by the drive manufacturer. Confirm absence of voltage with appropriate test equipment before touching motor terminals, connectors or brake wiring.
Also consider stored mechanical energy. A vertical axis may drop if its holding brake is released, while a loaded rotary axis can move unexpectedly when the coupling is disconnected. Support or secure the load before testing, and use qualified personnel for live measurements or parameter work.
Separate motor, cable, drive and mechanical faults
The quickest diagnostic route is usually controlled substitution and isolation. If two identical axes are available, compare the faulting axis with a healthy one. Check parameters first so that the axes are genuinely configured alike, then inspect connectors, cable routing and mechanical load before moving parts.
Check the mechanical load
With power safely removed and the machine secured, inspect the gearbox, coupling, belt, ballscrew, bearings and driven load. Look for a seized bearing, damaged coupling insert, misalignment, contamination or a brake that has not released. Rotate the mechanism by hand only where the machine design permits it.
A motor that becomes hot, draws excessive current or trips on overload may be working against mechanical resistance rather than suffering an electrical failure. Replacing the servo motor without correcting a tight gearbox or contaminated ballscrew can result in another failure within hours.
Pay attention to changes made before the fault appeared. A heavier tooling fixture, altered acceleration rate or revised cam profile can push a previously adequate axis beyond its torque or inertia limits. In that case, the right remedy may be mechanical correction or retuning, not a like-for-like motor.
Inspect power and feedback connections
Servo feedback faults are frequently caused by damaged cables and connectors. Examine both ends of the motor power and encoder or resolver cable for bent pins, oil ingress, loose locking rings, crushed insulation and poor shielding termination. Cables routed alongside high-current conductors or variable-speed drives are more exposed to electrical noise, particularly where shielding or earthing has been disturbed.
Check continuity and insulation only with equipment and procedures suitable for the servo system. Do not use a standard insulation resistance tester on encoder, resolver or brake circuits. These low-voltage circuits can be damaged by inappropriate test voltage. Motor winding tests should also follow the OEM limits, as connected drives must be isolated before any resistance or insulation testing.
If the fault follows a cable when exchanged with an identical known-good cable, the cable is the likely cause. If it remains with the motor, investigate the motor feedback device, windings, brake and connector assembly. If it stays with the drive channel, the drive, parameter set or incoming supply deserves closer attention.
Confirm brake operation on vertical and holding axes
A holding brake is not normally intended to stop a moving axis. Its job is to hold the load when servo torque is removed. A missing or low brake release supply can leave the brake partially engaged, creating high current, heat and poor motion. Conversely, a brake that does not engage can allow a vertical load to drift when the machine is stopped.
Measure the brake supply and verify the drive output sequence against the machine documentation. Listen for brake engagement only as a basic indication - an audible click does not prove adequate release force. Where a motor brake is faulty, confirm the complete motor part number and brake specification before ordering. Similar frame sizes can have different feedback, shaft, seal and brake options.
Diagnose feedback and position faults carefully
Following errors do not automatically mean the encoder has failed. The drive compares commanded and actual position, so a following error can result from a blocked mechanism, insufficient torque, poor tuning, excessive acceleration or lost feedback.
Start with the drive diagnostics. Compare commanded position, actual position, speed demand, actual speed, torque demand and following error during a controlled movement. An actual position that drops out or jumps erratically points towards feedback, cable or connector issues. A steadily rising following error with high torque demand points more towards a mechanical restriction or inadequate axis capacity.
Absolute encoders may also generate battery, initialisation or position-loss alarms. Replace batteries according to the machine procedure and preserve reference data where required. Do not assume that fitting a replacement motor will retain absolute position information. Some systems require encoder set-up, commutation alignment, reference return or parameter restoration after motor replacement.
Know when the servo drive is the probable cause
Drive failures often present as persistent output-stage faults, DC bus faults, internal hardware alarms or a fault that remains on the same drive channel after the motor and cable have been ruled out. Check incoming supply balance, fuses, contactors, line reactors and regenerative components where fitted. A supply problem can look like a motor problem at the HMI.
Before substituting a drive, capture its parameters, firmware level, option cards and network configuration. A physically compatible drive is not necessarily a functional replacement. Motor data, feedback interface, safety configuration, fieldbus settings and tuning values may all be specific to the machine.
For legacy equipment, part-number accuracy matters more than product family naming. Record the full code from the motor and drive labels, including suffixes for voltage, feedback type, brake, shaft arrangement, environmental rating and revision. A photo of the nameplate and connectors is useful when labels are worn, but it should support rather than replace a readable part number.
Decide whether repair, replacement or a spare is the practical route
A damaged connector, cable or external brake supply may be repaired quickly on site. A motor with contaminated bearings, failed feedback electronics or winding damage will usually require specialist assessment or replacement. The decision depends on the production risk, availability of a known-compatible unit and the cost of extended downtime.
When sourcing a replacement, match the complete manufacturer part number wherever possible. If an exact unit is unavailable, compare electrical rating, feedback protocol, brake voltage, mounting dimensions, shaft type and drive compatibility before accepting an alternative. Do not interchange motors across brands or series solely because the flange and power rating appear similar.
For critical axes, holding an identified spare can be cheaper than an emergency search during a shutdown. New and sealed stock may suit sites with strict lifecycle policies, while refurbished stock can be a sensible route for supported legacy machinery when the condition is clearly stated and the part number is verified. Automation Planet UK can assist with sourcing multi-brand industrial automation parts by exact reference, including hard-to-find legacy units.
Before returning the machine to service
After repair or replacement, restore parameters only from a verified backup and check the motor direction at low speed with the load secured. Confirm brake release, home or reference operation, feedback stability and axis travel limits before returning to automatic production. Run the axis through a controlled cycle, monitoring current, following error and temperature under realistic load.
Keep the failed part, alarm record and test results with the maintenance history. That evidence makes the next fault faster to diagnose and helps procurement hold the right spare rather than a part that only looks compatible. The best outcome is not merely a cleared alarm, but an axis whose cause of failure has been removed and whose replacement details are ready before downtime returns.