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Fault explained · Allen-Bradley servo motor
Allen-Bradley servo motor not working after bearing replacement and encoder refit: why it happens
A common pattern: the bearings of an Allen-Bradley servo motor are replaced, the encoder is refitted, and the motor then does not run, trips on a feedback or commutation fault, or runs roughly. The bearings are usually not the problem. When the encoder is removed and refitted, its position relative to the rotor magnets is lost, so the drive no longer knows the rotor angle. The encoder has to be aligned and reprogrammed to the motor. This page describes the typical pattern, not a specific customer job. Star Automations is an independent repairer and is not a Rockwell Automation authorised service provider.
- 25+years of servo repair, since 2000
- 3,000+customers across India
- 100+servo brands covered by our universal servo motor test equipment
- 5sites: Puducherry, Chennai, Bengaluru, Hyderabad, Delhi
- Same dayemergency repair option*
*Where the fault and parts allow. Every repaired unit returns with a printed test report.
Repair turnaround you can plan around
Send a photo of the nameplate first. We confirm whether the motor is repairable, and the quote, the same working day.
Urgent: 3 working days
Repair, encoder alignment and test, counted from the day the motor reaches the bench and the quote is approved.
Standard: 7 working days
Full repair, encoder alignment, run test under load and a printed test report.
- Free repairability check from a nameplate photo
- Printed test report with every motor
- Written warranty, terms stated on every quotation
- Pickup can be arranged; parts that must be imported are quoted with their own timeline
01Symptom
What the plant sees
Motor does not run
The drive enables but the shaft does not turn, or it twitches and stops.
Drive trips
Feedback, commutation or following-error faults appear soon after enable.
Rough running or high current
The motor turns but vibrates, runs hot or draws high current for the load.
Wrong direction or runaway
The motor moves the wrong way or accelerates because the drive reads the wrong rotor angle.
02Cause
Why the motor stops working after the encoder is refitted
A brushless servo motor needs the drive to know where the rotor magnets are at all times. The encoder provides that position.
- The encoder is tied to the rotor position. The encoder is set to a fixed angle relative to the rotor magnets. Removing it to change the bearings breaks that relationship.
- Refitting by eye or by the old marks is not enough. A small angular error changes how much torque the motor makes for a given current, and a large one stops it from running.
- The drive then commutates the wrong phases. The result is no torque, a trip, heat or vibration.
- Encoder data may need to match the motor. A replaced or disturbed feedback device may also need to be programmed for that motor.
- Bearings are rarely the cause. Once the bearing job is correct, the fault sits in the feedback alignment.
03Repair
What fixes it
- Read the nameplate and confirm the feedback type.
- Mount the motor on the universal servo test bench and read the feedback signals.
- Reprogram the feedback device for the motor where needed.
- Align the encoder to the rotor so that the electrical angle is correct.
- Run the motor and check it before it goes back to the plant.
04Prevention
How to avoid it
- Do not remove the encoder unless the repair needs it.
- If bearings must be changed, plan the encoder alignment as part of the job.
- Send the motor for bearing replacement and alignment together rather than refitting the encoder on site.
- Keep the nameplate details and the drive fault text with the motor.
→How it works
From nameplate photo to a tested motor in 7 steps
One route for servo motors, encoders and drives. You see the price before any work starts, and the report before the unit leaves our bench.
- 1 · Nameplate photoSend it on WhatsApp or the form
- 2 · Repairable? QuoteUsually the same working day
- 3 · Courier or pickupThe unit reaches our bench
- 4 · Bench diagnosisInsulation, windings, brake, encoder
- 5 · RepairFailed parts, bearings, seals, brake
- 6 · Align and run-testEncoder aligned; no-load and load run
- 7 · Report and dispatchPrinted test report with the unit
- Line down?Servo Swap ships a tested unit first. How Servo Swap works
EmergencySame day*
Urgent3 working days
Standard7 working days
*Where the fault and parts allow. Turnaround counts from when the unit reaches our bench and the quote is approved.
06Questions
Questions
Why does my Allen-Bradley servo motor not run after the bearings were changed?
The encoder was probably removed and refitted without alignment. The drive then does not know the rotor angle. The encoder needs to be aligned and reprogrammed to the motor.
Is the new bearing faulty?
Usually not. If the motor ran before the job and the fault started after the encoder was refitted, check the encoder alignment first.
Can the motor be damaged by running it like this?
Running with wrong feedback can cause trips, heat and high current. Stop the axis and send the motor for inspection.
Can you fix a motor that was already repaired elsewhere?
Yes. Send the motor with the nameplate and a description of what was done. We check the feedback and align the encoder.
What should I send?
The motor with its nameplate, the drive fault text, and a note of what was replaced.
Send the nameplate. We confirm repairability before you courier.
WhatsApp a nameplate photo and the fault to +91 97866 22233, or email info@starautomations.com.