Guide · Servo motor repair · Updated October 2026
Servo motor mismatch error after replacing it with the same model: why it happens
A mismatch error after fitting a "same brand, same kW" servo motor usually means the drive does not recognise the motor. The series, encoder type, motor ID, voltage, speed or drive parameters differ, or the encoder is not aligned. Same capacity does not mean same motor.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
Why a "same" motor is not always the same motor
A servo motor and its drive work as a matched pair. The drive is set up for one motor type. At start-up it reads information from the motor, mainly from the encoder, and compares it with what it expects. If the two do not agree, it stops and shows a mismatch or encoder error instead of risking damage.
Think of a car key. Two keys can look identical and be cut for the same make and size, but if the chip inside is different, the car will not start. Capacity (kW) is only the size of the key. The series, the encoder and the motor ID are the chip.
So fitting a new motor of the same brand and the same capacity is not a guarantee. It must match in the details below, and the encoder must be correctly aligned or programmed. For the alignment side of the story, read why a motor does not run after a bearing change.
Same brand, same capacity: what can still be different
| What looks the same | What can differ | Why the drive cares |
|---|---|---|
| Brand and kW | Series or generation (older and newer series of one brand) | Winding, rated speed and encoder often change between series |
| Shaft and flange | Encoder type: incremental, serial absolute or resolver | The drive can only read the type it is set up for |
| Encoder look | Resolution and communication protocol | A different protocol or bit count is read as an error |
| Nameplate power | Voltage class and rated speed | The drive limits and tuning are built around them |
| Model name | Motor ID or model code stored in the encoder | The drive compares it with the motor type parameter |
| Fits mechanically | Brake option, absolute battery data, zero position | Wrong zero or missing data stops the drive or upsets torque |
What the drive checks at start-up
On power-up the drive identifies the feedback device and, on serial encoders, reads the motor data held in it. It then compares this with the motor type, encoder type and rated values set in its parameters. A disagreement raises a motor mismatch, encoder type or parameter error. The alarm number and wording differ by brand and drive family, so read the meaning in the manual of your drive.
Even when the data matches, the commutation angle must be right. With a wrongly aligned encoder the drive applies current at the wrong rotor angle. The motor draws high current, makes little torque, runs hot or vibrates, and a large error can cause runaway or an overcurrent or overspeed trip. That is a separate fault from the mismatch error, and it needs encoder alignment.
Typical reasons for a mismatch error after replacement
- The replacement belongs to a different series even though the brand and kW are the same
- The encoder type or resolution is different from the original
- The motor ID or model code in the encoder does not match the motor type set in the drive
- Drive parameters still describe the old motor
- The encoder of a repaired or replaced motor was never programmed or aligned
- Different voltage class or rated speed
- Absolute encoder data or battery data is missing or lost
- A damaged encoder cable or connector, or a wrong cable type
What to check before you call anyone
- Photograph the nameplate of the old motor and the new motor and compare the full model codes, not only kW.
- Write down the alarm number and the text shown on the drive, and the drive model.
- Compare encoder type, voltage class, rated speed and brake option.
- Check the encoder cable and connector.
- Do not keep switching the motor on and off. Repeated attempts with a wrong match can stress the motor and the drive.
- Send us the details. We tell you what is compatible and what needs repair, programming or alignment.
How Star Automations solves it
Star Automations is an independent multi-brand servo repair company working since 2000, with 180+ servo brands covered on our Mitchell-based Universal Servo Motor Repair Equipment, India's first universal servo motor test system. We read the encoder, check the motor data, programme the encoder where needed, align it to the rotor, and run every motor on our Universal Server Run Test System before it is returned.
Very often the original motor can be repaired, which avoids a mismatch altogether. If you need a replacement, we tell you what is truly compatible. We serve customers across India from Chennai, Puducherry and Bengaluru, with Hyderabad and Delhi support, and by courier from anywhere.
More: Universal Servo Motor Repair Equipment · Servo motor repair FAQ · Encoder repair · Servo motor repair in India · All guides
Questions
Why do I get a mismatch error when I fitted a new servo motor of the same brand and kW?
Because the drive checks more than power. It compares the motor series, encoder type and resolution, motor ID, voltage class and speed with what it has been set up for. A motor with the same brand and kW can still belong to a different series or have a different encoder, so the drive refuses to run it. The fix is to match the full model code, set the correct motor parameters in the drive, and align or program the encoder.
Is the same kW rating enough to replace a servo motor?
No. kW only tells you the size. Rated speed, voltage class, torque, encoder type, shaft, brake and series also have to suit the drive and the machine. Always compare the full model code on the nameplate, not just the capacity.
What does a motor mismatch or encoder type error mean on a servo drive?
It means the information the drive reads from the motor or encoder does not agree with the motor type or encoder setting stored in the drive. The exact alarm number and wording differ by brand, so read the meaning for your drive in its manual.
Can the drive parameters fix a mismatch error?
Often yes, when the new motor is genuinely compatible. The motor type, encoder type and rated values have to be set for the new motor. If the encoder is not aligned or the motor ID does not match, parameters alone will not help.
Do I need to program the encoder of a new or repaired motor?
On many serial and absolute encoders, yes. The motor data and the zero position are stored in the encoder, and the drive reads them at start-up. If they are missing or do not match, the drive raises an error. Star Automations writes this data with Mitchell equipment.
Why does a replacement motor run but with noise, heat or weak torque?
The encoder angle may be slightly off, so the drive applies current at the wrong moment. The motor still turns, but it draws more current and produces less torque. This needs encoder alignment, not a new drive.
Can Star Automations help if I cannot find the exact same motor?
Yes. Send the nameplate of the old motor and the drive details. We check what is compatible, repair the original motor wherever possible, and run it on our Universal Servo Motor Repair Equipment and Universal Server Run Test System before it is returned. We work with 180+ servo brands across India from Chennai, Puducherry and Bengaluru.
Send the nameplate. We tell you what matches.
WhatsApp the 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.