Guide · Servo motor repair · Updated October 2026
Servo motor not running after a bearing change? Why encoder alignment matters
A servo motor does not run just because it was taken apart and put back. The encoder or resolver must be aligned to the rotor again, and some encoders must be programmed. If this is skipped, the motor jerks, trips or does not turn.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
Why a serviced motor does not simply run
A servo motor is not like an ordinary motor that turns when you give it power. It runs only when the drive knows exactly where the magnets inside the motor are at every instant. A small device at the back of the motor, the encoder or the resolver, gives the drive that information.
Think of it as a compass. If you take the compass apart and put it back with the needle pointing a few degrees off, it still looks fine, but everything it tells you is wrong. A servo motor behaves the same way: after a bearing change, if the encoder or resolver is not set back at the correct angle, the drive sends current at the wrong moments and the motor will not run properly.
That is why a servo motor needs encoder alignment after it has been opened, and sometimes reprogramming, not just a new bearing.
What you will see when the alignment is wrong
- The motor does not turn, or turns a little and stops
- The motor jerks or vibrates, or runs very noisily
- The motor runs in the wrong direction or runs away
- High current, hot motor or an overcurrent alarm
- An encoder, feedback or overspeed alarm on the drive
- A motor that runs but cannot hold position or produce its normal torque
Related alarm guides: Mitsubishi AL.16 and AL.20, Yaskawa A.C90, Fanuc SV0368 and SV0366 and servo encoder alarms.
What must be done after opening the motor
The right step depends on the feedback device in your motor. The table gives the general picture. Details differ by brand and model, so always confirm for the motor in front of you.
| Feedback device | What it is | What is needed after opening the motor | If it is skipped |
|---|---|---|---|
| Incremental encoder with commutation signals | Optical encoder that also gives the drive the rotor angle through UVW or Hall tracks | The encoder zero and commutation signals are aligned to the rotor magnets and checked against the motor on a test system | Motor jerks, runs the wrong way or draws high current |
| Serial or absolute encoder | Digital encoder that sends position data and often stores motor data in its own memory | Zero offset and stored motor data may need to be set or written again; multi-turn encoders need the battery and the machine origin set again | Encoder alarms, wrong position, or the drive does not recognise the motor |
| Resolver | Analog sensor with no electronics inside; common on older and heavy-duty motors | The resolver stator is turned to its electrical zero against the rotor, then locked; some drives also have a resolver offset setting | Motor will not start, hunts, or runs with poor torque |
Why many repairs end with "this motor cannot be repaired"
A general motor repair shop usually changes the bearing, cleans the motor and tests it like an ordinary motor. The step that makes a servo motor run, aligning and checking the feedback against the rotor, needs the right equipment and the right data for that model. Without them the motor does not run, and the result is called "not repairable".
In most such cases the motor was repairable. The last step was missing. Our page on servo motor repair questions answers this and other common worries.
Alignment and a run test on the Universal Servo Motor Repair Equipment
- Incoming inspection of the bearings, shaft, magnets, winding and feedback
- Repair of the failed part, then reassembly
- Encoder or resolver aligned to the rotor on the Mitchell TI-5000JX, and encoder programming where the model needs it
- Motor run without the original drive, then a no-load and load run, with speed and current compared against the nameplate
- A printed test report with the motor
- Warranty of 3 months to 1 year, stated on each quotation
Read more about the Universal Servo Motor Repair Equipment and what it makes possible.
What to do and not do
- Photograph the motor, the nameplate and the encoder before opening it
- Mark the position of the encoder or resolver housing against the motor body
- Do not force the motor to run again and again after reassembly
- Do not loosen the encoder clamp or turn the resolver to "find" a position
- Send the motor with a note of what was done and the alarm shown
Fitted a new motor and now see a mismatch error? Read why a same-brand, same-kW servo motor can still be rejected by the drive.
Questions
Why does my servo motor not run after I changed the bearing?
Changing the bearing means the motor and its encoder or resolver were taken apart. When they go back together, the position of the feedback device against the rotor magnets is no longer guaranteed. The drive cannot tell where the magnets are, so the motor jerks, runs the wrong way, vibrates, overheats or trips an alarm. The feedback has to be aligned again, and on some motors programmed again.
Is it enough to put the motor back exactly as I found it?
Not always. The encoder or resolver must sit in a precise angle against the rotor, and a small shift is enough to stop the motor running. Marking the parts before opening helps, but it does not replace a proper alignment and a test.
What is encoder alignment?
It is the step that tells the drive where the magnets in the rotor are. The encoder or resolver is set against the rotor at the exact angle the drive expects, then locked in place and checked on a test system.
What is encoder programming?
Some serial and absolute encoders hold motor data or an offset in their own memory. After repair or encoder replacement that data may have to be written again so the drive recognises the motor. We do this with Mitchell equipment.
What if I have already tried to run it a few times?
Stop. Repeated attempts with a wrongly aligned motor can overheat the winding and stress the drive. Send us the motor and tell us what was done. We check the bearings, magnets, winding and feedback before we align it.
Can Star Automations repair it if a local mechanic could not?
In most cases yes. The usual missing step is alignment and a proper run test. We test the motor on our Universal Servo Motor Repair Equipment and confirm repairability before you courier it.
Send the nameplate. We confirm repairability before you courier.
WhatsApp a nameplate photo and the alarm code to +91 97866 22233, or email info@starautomations.com.
Brand names identify the units we repair. Star Automations is an independent multi-brand repair company and is not an OEM service centre.