Key takeaways
Industrial servo motors rarely die without warning, but the warnings are easy to misread, and a healthy motor gets replaced while the real fault stays in the machine. This guide is for maintenance technicians, maintenance managers, and plant engineers who need to diagnose a suspect servo axis, confirm the actual failed component, and stop the fault from coming back.
A servo axis is a loop: drive, motor, feedback, coupling, and driven mechanism. Any element can produce the same alarm, so the first job is isolation, not replacement. A seized ballscrew, a dragging brake, and a failing amplifier can all throw the same following-error fault.
Before condemning the motor, do three cheap checks. With the machine locked out, rotate the axis by hand (or with the brake safely released) and feel for mechanical binding, roughness, or tight spots. Read the drive's alarm history and note whether faults are current-related, feedback-related, or position-related. And check whether the fault follows a pattern: one axis position, one time of day, one part program.
Match the symptom to the most likely cause before touching a meter.
| Symptom | Most likely causes | First check |
|---|---|---|
| Position error / following-error alarm | Encoder or feedback cable fault, mechanical binding, load increase | Rotate axis by hand under lockout; inspect encoder connector |
| Hunting or oscillation at standstill | Loose coupling, backlash, noisy feedback, tuning drift | Inspect coupling and mechanical connection to the load |
| Motor runs hot or thermal alarms | Overload, dragging brake, degrading winding insulation | Compare running current to nameplate; verify brake releases fully |
| Growling, grinding, or new vibration | Bearing wear, contamination in the motor | Spin the uncoupled shaft by hand and feel for roughness |
| Intermittent faults at certain axis positions | Flexing cable fatigue in the cable track | Wiggle-test cable sections while monitoring the fault |
| No movement, no torque, axis dead | Brake not releasing, open winding, drive output failure | Confirm brake voltage present; measure phase resistance |
Alarm numbers help too, but treat them as pointers rather than verdicts. On Fanuc controls, for example, servo alarm 414 indicates the digital servo system has detected a fault on the amplifier or motor side, and the precise meaning varies by control model and software version, so always confirm in the machine documentation. If an alarm code is ambiguous, diagnose from measurements, not from the number.
In rough order of how often they show up on the shop floor:
Apply lockout/tagout before opening cabinets or touching the mechanism. Servo drives hold a charge in their DC bus capacitors after power-off, so wait the manufacturer's stated discharge time and verify zero voltage before touching terminals. Physically block or pin vertical axes before releasing a brake: gravity is stored energy, and a released brake drops the load instantly. Never bypass interlocks or guard switches to speed up a test.
Disconnect the motor leads from the drive first. A megger applied through the drive will destroy the output stage. With the leads free, test each phase to the motor frame ground, typically at 500 V DC for low-voltage machine-tool motors (confirm the rating in the motor documentation). A healthy motor reads very high, usually in the hundreds of megohms; readings drifting down toward single-digit megohms indicate insulation degradation, and near zero means a ground fault. Never connect the megger to the encoder or its cable, which the test voltage will destroy.
With a good milliohm-capable meter, measure U-V, V-W, and W-U. The three readings should be low, per the datasheet, and balanced within a few percent of each other. A noticeably low leg suggests shorted turns; an open-circuit leg means a broken winding or a broken conductor in the cable, so repeat the measurement at the motor terminals to separate cable from motor.
With the motor disconnected and uncoupled, spin the shaft at a steady speed by hand or with a drill and measure AC voltage across each phase pair. All three should produce similar, balanced voltages. Unequal voltages point to winding damage even when resistance readings look acceptable.
Inspect the encoder connector for contamination and bent pins, verify the drive's reported position tracks smooth hand rotation of the shaft, and check the alarm history for feedback-specific faults. Marginal encoders often work cold and fail hot.
With the axis blocked and locked out, apply the brake's rated voltage (commonly 24 V DC, confirm on the nameplate) from a bench supply. You should hear a distinct click and the shaft should turn freely. No click, no release, or a shaft that still drags means the brake is failing or its supply circuit is.
If measurements are inconclusive, swap one component at a time: motor to a known-good drive, known-good motor to the suspect drive, or exchange identical axes where the machine allows it. Change one variable per test and record the result. One critical caution: never connect a replacement drive to an untested motor. A shorted winding can take out the new drive in seconds, turning one failure into two.
A servo fault that gets cleared with a power cycle and no record will return. Log every occurrence as a downtime event with a cause code (encoder, winding, brake, cable, drive, mechanical), even the two-minute resets. Track MTBF and MTTR for the asset: if the same axis faults every few weeks, that is a chronic engineering problem (contamination path, cable routing, undersized motor), not a run of bad luck. Availability losses from repeat servo faults flow straight into your OEE numbers, and the short stops are usually the ones nobody writes down.
Fabrico is computer-vision-verified OEE plus closed-loop maintenance execution: cameras catch the stops and micro-stops that manual logs and sensors miss, and maintenance work orders close the loop from detection to fix. For a recurring servo fault, that means the 90-second alarm resets get counted, coded, and turned into a work order history you can act on instead of vanishing between shifts. If you want to see how that works on your lines, book a Fabrico demo.
When the motor and drive test healthy but the axis still misbehaves, check the mechanics: ball screw wear and backlash mimics several servo symptoms.
Test the motor independently: insulation resistance to ground, phase-to-phase resistance balance, and a back-EMF spin test. If the motor passes all three and the encoder checks out, suspicion shifts to the drive, the cable, or mechanical binding. Confirm by swapping a known-good component one variable at a time.
Yes, but only with the motor leads fully disconnected from the drive, and never on the encoder circuit. Test each phase to frame ground at the voltage specified for the motor (often 500 V DC for low-voltage motors) and compare against the manufacturer's minimum acceptable reading.
The usual causes are mechanical overload or binding, a holding brake that is not fully releasing, degraded winding insulation, excessive duty cycle, and blocked cooling. Check current draw against the nameplate and verify the brake releases before assuming the motor itself is at fault.
Bearing wear produces growling or grinding that changes with speed. Hunting or buzzing at standstill usually indicates feedback noise, loose coupling, or tuning problems rather than the motor. A rhythmic click once per revolution suggests physical damage or debris.
There is no universal figure; life depends on load, duty cycle, temperature, and contamination. Bearings and encoders typically fail before windings. Tracking MTBF per axis in your maintenance system gives you a real number for your machines instead of a guess.
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