07.03.2026 by Viktor Siebert
Yaskawa AC Servo Motor SGMGH-03A5A-SE16 with Encoder Failure due to Oil Contamination Repair and Functional Test
Initial situation and fault description
The submitted servo motor Yaskawa SGMGH-03A5A-SE16 originates from a Star CNC lathe and operates together with a Yaskawa Servopack SGDK-050505AEA-V. The customer reported an encoder fault that appeared immediately after switching on the machine.
The control system blocked the axis because no valid feedback signals from the motor were detected.
The fault was reproducible at every machine start. Because the feedback signal is essential for servo control, the drive could not be enabled.
This behaviour typically indicates a failure in the feedback system of the motor.
Incoming inspection and initial diagnosis
After arrival the motor was visually inspected.
During disassembly of the encoder cover, oil leaked from the encoder housing. The entire encoder interior was contaminated with oil.
Oil contamination in feedback systems causes several problems simultaneously. Sensors lose signal quality and electronic components may fail due to contamination.
The encoder was completely damaged.
Repair of the original encoder was not possible. The feedback unit had to be replaced.
Technical analysis
Servo motors of this type use high resolution feedback systems to measure speed and position.
If oil enters the encoder the typical failure chain is:
Damaged shaft seal
→ Oil enters the encoder housing
→ Sensors and electronics become contaminated
→ Feedback signals become unstable or disappear
→ Servo controller detects encoder fault and blocks the drive
This failure pattern was clearly visible in this motor.
Servo systems in machine tools require continuous and accurate feedback signals to maintain precise motion control
Repair measures and overhaul
Because the encoder was beyond repair, the feedback system was replaced with a new unit.
Additionally the motor underwent a full standard overhaul including:
complete disassembly
cleaning of all internal parts
replacement of worn seals and bearings
inspection of rotor and stator
electrical insulation testing
installation of the new encoder
reassembly and mechanical adjustment
Final function test
After repair the motor was tested on an original Yaskawa Servopack SGDK-050505AEA-V.
Testing conditions were comparable to operation in a Star CNC machine.
The test procedure included:
power-up behaviour
encoder initialization
low speed operation
medium speed operation
high speed operation
feedback signal monitoring
thermal stability test
The motor operated stable and without faults throughout the entire test.
Conclusion
The failure of the servo motor was caused by oil entering the encoder housing.
Replacing the encoder and performing a complete overhaul restored full functionality.
The final test on the original servopack confirmed stable operation of the drive.
Regular inspection of seals and environmental conditions can prevent similar failures and reduce machine downtime.
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. Our experienced team is always ready to assist you.
Technical Specifications
| Parameter | Value |
|---|
| Manufacturer | Yaskawa Electric |
| Device type | AC Servo Motor |
| Model | SGMGH-03A5A-SE16 |
| Series | Sigma |
| Power | 300 W |
| Input voltage | approx. 200 V |
| Rated current | approx. 2.6 A |
| Control method | Servo control |
| Feedback | Incremental encoder UTSAH-B17BE |
| Cooling | Self cooled |
| Protection class | approx. IP65 |
| Ambient temperature | approx. 0 to 40 °C |
| Mounting | Flange mounting |
| Origin | Japan |
| Product status | discontinued |
Operating environment and applications
These servo motors are typically used in CNC machine tools such as lathes, milling machines and automated production systems.
Typical applications include precision axes in machining centres and industrial automation systems.
Functional description
The servo motor works together with a servo drive which controls motor currents and processes encoder signals.
Main functions include:
precise speed control
accurate position control
dynamic acceleration control
overcurrent protection
thermal monitoring
feedback monitoring
Alarm Messages and Troubleshooting
| Alarm Code | Description | Possible Cause | Recommended Action |
|---|
| 0x63 | Encoder pulse count error | Incorrect pulse counting due to a contaminated or damaged encoder | Check encoder signals and replace encoder if necessary |
| 0x64 | Position detecting signal wire break | Interruption in the position feedback signal | Inspect encoder cable and connectors |
| 0x65 | Incremental signal error when encoder is used | Fault in the incremental encoder signal | Check encoder wiring and signal integrity |
| 0x79 | Motor overheat | Thermal overload of the motor or insufficient cooling | Reduce motor load and check cooling conditions |
| 0x7A | Heat sink overheat | Excessive temperature in the drive or motor cooling system | Inspect cooling system and ambient temperature |
| 0x84 | PG absolute error | Failure in the absolute position encoder | Inspect or replace encoder feedback unit |
| 0x85 | PG overspeed | Encoder detects speed above the permissible limit | Verify parameter settings and mechanical load |
| 0xC3 | Wire break in pulse encoder A and B phases | Cable break in encoder channels A or B | Inspect encoder cable and connections |
| 0xC4 | Wire break in pulse encoder C phase | Cable break in encoder index signal | Check encoder index signal wiring |
| 0xC9 | PG communication error | Communication failure between encoder and servo drive | Inspect encoder cable, connectors, and feedback unit |
Assembly overview
| Assembly | Functional name | Function | Inspection notes |
|---|
| Stator | Motor winding | Magnetic field generation | Insulation test |
| Rotor | Permanent magnet rotor | Mechanical rotation | Bearing inspection |
| Encoder | Feedback system | Position and speed detection | common failure point |
| Bearings | Shaft bearings | Mechanical guidance | wear inspection |
| Seal system | Shaft seal | Oil protection | critical in oil environments |