29.05.2026 by Viktor Siebert
Repair of a Yaskawa SGMAH-08A1A-SE31 AC Servo Motor with Defective Encoder and Broken Power Cable
A Yaskawa SGMAH-08A1A-SE31 AC servo motor from a STAR machining system was sent in for repair. The motor operates together with a Yaskawa SGDK-101010AEA-VY24 Servopack and is equipped with a UTSAH-B16DC-E encoder in Siemens/Yaskawa version.
The motor did not run on our test bench. During the first inspection, the power cable was found to be heavily hardened and mechanically damaged. The motor bearings also showed clear signs of wear.
Diagnosis on the Test Bench
First, we checked whether the fault was caused by the encoder cable. For this purpose, we used a verified reference cable. The fault remained unchanged.
This confirmed that the cause was not only the external wiring. The encoder itself had to be inspected in detail.
The uploaded alarm table contains several relevant faults that directly match a motor with encoder failure, cable break or feedback problems. The most relevant entries are position detecting signal wire break, INC signal error, PG communication error and wire break in encoder A/B or C phase.
Actual Root Cause
After disassembling the motor, several faults were found:
- Defective UTSAH-B16DC-E encoder
- Broken and hardened power cable
- Worn bearings
- Aged seals
- Mechanically stressed cable entry
The encoder was the main cause of the failure. Due to incorrect feedback, the servopack could no longer evaluate the rotor position reliably. At the same time, the power cable had already aged so much that repairing only the encoder would not have been sufficient.
Repair Process
The motor was completely overhauled. All wear related assemblies were inspected and repaired.
Work performed:
- Complete motor disassembly
- Internal cleaning
- Bearing replacement
- Seal replacement
- Power cable replacement
- UTSAH-B16DC-E encoder repair
- Connector inspection
- Insulation test
- Mechanical inspection
- Final test on Yaskawa test bench
This preventive repair approach is especially important for older servo motors. If only the visibly defective encoder is repaired, bearings, seals and cables often remain as the next failure points in the system.
Final Test
After repair, the motor was tested again with the Yaskawa SGDK-101010AEA-VY24 Servopack.
The following checks were performed:
- Encoder feedback
- Direction of rotation
- Smooth running
- Low speed operation
- Higher speed operation
- Thermal behaviour
- Repeated start and stop cycles
- Feedback stability
After the overhaul, the motor ran reliably again and was returned to the customer.
Conclusion
This repair case clearly shows why the complete system must always be checked when servo motors show encoder related faults. In this case, not only the encoder was defective. The power cable, bearings and seals had also reached the end of their service life.
The complete overhaul restored the Yaskawa SGMAH-08A1A-SE31 to a reliable operating condition.
Information about the mentioned Servopack and Servomotor:
More information about our Yaskawa repairs can be found here.
📞 Feel free to contact us if you have any questions regarding your Yaskawa drive technology.
Technical Specifications
| Parameter | Value |
|---|
| Manufacturer | Yaskawa |
| Model | SGMAH-08A1A-SE31 |
| Device Type | AC Servo Motor |
| Encoder | UTSAH-B16DC-E |
| Servopack | Yaskawa SGDK-101010AEA-VY24 |
| Machine | STAR machining system |
| Rated Output | 750 W |
| Rated Voltage | 200 V |
| Rated Current | 4.4 A |
| Rated Torque | 2.39 Nm |
| Rated Speed | 3000 min⁻¹ |
| Insulation Class | B |
Alarm Codes and Troubleshooting
| CNC Code | Drive Code | Alarm | Meaning | Corrective Action |
|---|
| 100 | 0x64 | Position detecting signal wire break | Position signal interruption | Check encoder cable and connector |
| 101 | 0x65 | INC signal error when encoder is used | Incremental encoder signal fault | Inspect and repair encoder |
| 129 | 0x81 | PG backup error | Encoder feedback fault | Check encoder and supply |
| 130 | 0x82 | PG sum check error | Encoder checksum error | Repair or replace encoder |
| 132 | 0x84 | PG absolute error | Absolute encoder fault | Check and reinitialize encoder |
| 133 | 0x85 | PG over speed | Invalid speed feedback | Check encoder and mechanics |
| 134 | 0x86 | PG over heat | Encoder area overheating | Check temperature and encoder |
| 195 | 0xC3 | Wire break in pulse encoder A and B phases | Wire break in A/B phase | Replace encoder cable |
| 196 | 0xC4 | Wire break in pulse encoder C phase | Wire break in C phase | Check encoder wiring |
| 197 | 0xC5 | Motor magnetic pole detection error | Rotor position cannot be detected correctly | Check encoder and adjustment |
| 201 | 0xC9 | PG communication error | Encoder communication fault | Check encoder, connector and cable |
Assemblies
| Assembly | Function | Condition Found | Action |
|---|
| UTSAH-B16DC-E encoder | Position feedback | Defective | Repaired |
| Power cable | Motor power supply | Hardened and broken | Replaced |
| Bearings | Rotor support | Worn | Replaced |
| Seals | Protection against contamination | Aged | Replaced |
| Stator | Torque generation | Inspected | Insulation tested |
| Connectors | Signal and power connection | Inspected | Contacts checked |