25.05.2026 by Viktor Siebert
CNC System Interruption Caused by Encoder Failure on Yaskawa SGMGH-30DCA6F-OY Servo Motor
Introduction as a Real Service Case.
An industrial customer reported repeated interruptions on a CNC axis equipped with a Yaskawa servo system. The installed Yaskawa SGMGH-30DCA6F-OY AC servo motor occasionally generated encoder faults during operation.
The critical issue was that the fault did not occur continuously. In some cases, the axis operated normally for extended periods before feedback errors reappeared.
The motor was therefore tested together with a Yaskawa SGDH-30DE Servo Drive in the test field.
The first analysis already revealed a typical behavior of older CNC servo axes. In cold condition, the system initially operated without problems. Only under movement, temperature increase or mechanical load did disturbances in encoder feedback appear.
Initial Diagnosis
| Inspection Points | Result |
|---|
| Motor | Yaskawa SGMGH-30DCA6F-OY |
| Encoder | Yaskawa UTSIH-B17CK |
| Test System | Yaskawa SGDH-30DE |
| Fault Pattern | sporadic encoder fault |
| Main Observation | unstable feedback under load |
The investigation focused on the complete motor feedback circuit.
Inspection focus:
- encoder connection
- PG signals
- connectors
- encoder cable
- thermal stability
- mechanical load
- bearing condition
- seals
- insulation condition
The Yaskawa documentation describes PG signal transmission through phases A, B and C and the feedback via the 2CN connector of the SERVOPACK. Faulty PG signals can cause Overrun Prevention, Overspeed or communication faults according to the manual.
Actual Root Cause
The technical analysis revealed several age related issues inside the motor.
Detected causes:
- sporadic interruptions in encoder feedback
- unstable feedback signals during temperature increase
- wear of the UTSIH-B17CK encoder
- aged ball bearings with beginning mechanical instability
- hardened seals
- mechanical stress in the encoder area
- aged housing surface with beginning corrosion
The fault did not occur permanently but increased under thermal load and longer operating cycles. This behavior is typical for older servo axes in CNC applications.
Repair Measures
The motor was completely disassembled and technically overhauled.
Performed work:
| Measure | Description |
|---|
| Encoder replaced | replacement of Yaskawa UTSIH-B17CK |
| Ball bearings replaced | complete bearing replacement |
| Seals replaced | replacement of all relevant seals |
| Motor disassembled | complete technical disassembly |
| Cleaning performed | removal of oil, dirt and deposits |
| Old coating removed | complete stripping of old paint |
| Repainted | industrial repainting of housing |
| Connectors inspected | inspection of all connections |
| Insulation test performed | winding insulation inspection |
| Load test performed | operational load test |
| Thermal test performed | thermal monitoring under load |
The repainting process was carried out after complete preparation of the housing surfaces. Besides the optical restoration, corrosion protection of the motor housing was also restored.
Preventive Overhaul
In addition to fault repair, the motor was preventively overhauled to avoid future failures.
| Preventive Measure | Benefit |
|---|
| Bearing replacement | reduced vibration |
| Seal replacement | protection against oil and contamination |
| New encoder | stable feedback signals |
| Housing refurbishment | improved corrosion protection |
| Cleaning of cooling surfaces | improved heat dissipation |
| Connector inspection | increased operational reliability |
| Shielding inspection | improved signal stability |
Final Inspection and Functional Test
After completion of all work, the motor was tested again on the Yaskawa SGDH-30DE.
The following points were checked:
- encoder stability
- PG signal quality
- bearing noise
- thermal behavior
- smooth operation
- start stop behavior
- signal stability during continuous operation
The motor was only approved for return shipment after successful long term testing.
Customer Feedback
“The fault was previously difficult to reproduce. The complete technical overhaul allowed the root cause to be clearly identified and permanently eliminated.”
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 Data
| Specification | Value |
|---|
| Manufacturer | Yaskawa Electric |
| Model | SGMGH-30DCA6F-OY |
| Device Type | AC Servo Motor |
| Power | 2900 W |
| Torque | 18.6 N·m |
| Speed | 1500 r/min |
| Current | 11.9 A |
| Voltage | 400 V |
| Insulation Class | F |
| Encoder | UTSIH-B17CK |
| Test Drive | SGDH-30DE |
| Production Date | 0505 |
Operating Conditions
| Condition | Value |
|---|
| Ambient Temperature | 0 to +40 °C |
| Storage Temperature | -20 to +60 °C |
| Humidity | 20 to 80 % |
| Insulation Test | 1500 VAC |
| Insulation Resistance | 10 MΩ or higher |
Functional Description
The Yaskawa SGMGH-30DCA6F-OY operates as a controlled AC servo motor in CNC applications. Feedback is provided by the UTSIH-B17CK encoder. The servo drive processes PG signals for speed and position control.
The Yaskawa manual describes protection functions such as:
- Overcurrent
- Overvoltage
- Overspeed
- Overrun Prevention
- Heat Sink Overheat
- Open Phase
- Regeneration Trouble
SGDH Sigma II Alarm and Fault Codes
| Alarm | Fault Name | Meaning | Reset | Corrective Action |
|---|
| A.02 | Parameter Breakdown | EEPROM data inside servo amplifier faulty | Power reset after correction | Check parameters and amplifier |
| A.03 | Main Circuit Encoder Detection Error | Main circuit detection data faulty | Power reset | Check servo amplifier |
| A.04 | Parameter Setting Error | Parameter outside valid range | Power reset after correction | Check parameters |
| A.05 | Servomotor and Amplifier Combination Error | Motor and SGDH amplifier mismatch | Power reset | Check motor and drive combination |
| A.10 | Overcurrent or Heat Sink Overheated | Overcurrent at IGBT or heatsink overheated | After cooling and repair | Check power stage, motor cable and cooling |
| A.30 | Regeneration Error Detected | Regenerative circuit or resistor faulty | After inspection | Check regenerative resistor and circuit |
| A.32 | Regenerative Overload | Regenerative energy exceeds capacity | After cooling | Check load cycle and braking energy |
| A.40 | Overvoltage | DC bus voltage too high | After voltage drops | Check power supply and brake circuit |
| A.41 | Undervoltage | DC bus voltage too low | After correction | Check supply voltage |
| A.51 | Overspeed | Motor speed too high | After stop | Check encoder, parameters and mechanics |
| A.71 | Overload High Load | High load over several seconds | After cooling | Check mechanics, bearings and load |
| A.72 | Overload Low Load | Continuous overload | After cooling | Check motor load and operating cycle |
| A.73 | Dynamic Brake Overload | Rotational energy exceeds brake capacity | After cooling | Reduce braking cycles |
| A.74 | Inrush Current Limiting Resistor Overload | Main voltage switched on/off too frequently | After cooling | Check switching cycles |
| A.80 | Absolute Encoder Error | Absolute encoder fault | After repair | Check encoder and cable |
| A.81 | Absolute Encoder Backup Error | Backup fault inside encoder | After inspection | Check battery, encoder and cable |
| A.82 | Absolute Encoder Checksum Error | Encoder checksum error | After inspection | Replace encoder |
| A.83 | Absolute Encoder Battery Error | Encoder battery voltage faulty | After battery replacement | Check battery and cable |
| A.84 | Absolute Encoder Data Error | Invalid absolute encoder data | After inspection | Check or replace encoder |
| A.85 | Absolute Encoder Overspeed | Encoder detected excessive speed | After stop | Check encoder, mechanics and parameters |
| A.86 | Encoder Overheated | Encoder temperature too high | After cooling | Check motor load and ambient conditions |
| A.C1 | Servomotor Runaway Detected | Motor operation implausible due to feedback issue | Power reset after repair | Check phase sequence, encoder and motor cable |
Technical Block
| Component | Description |
|---|
| Servo Motor | Yaskawa SGMGH-30DCA6F-OY |
| Encoder | Yaskawa UTSIH-B17CK |
| Servo Drive | Yaskawa SGDH-30DE |
| Repair Scope | Complete preventive overhaul |
| Bearings | Replaced |
| Seals | Replaced |
| Encoder | Replaced |
| Surface Treatment | Complete paint stripping and repainting |
| Electrical Testing | Insulation and feedback testing |
| Mechanical Testing | Bearing and vibration inspection |
| Final Testing | Continuous load and thermal testing |