20.11.2025 by Viktor Siebert
Repair of a Moisture-Damaged Yaskawa SGMKS-20A5A-MU11 with Encoder Failure UTSAH-B17BE
Initial Situation.
The Yaskawa SGMKS-20A5A-MU11 was removed from a machining center using a Siemens–Yaskawa hybrid control system. The customer initially observed a motor short circuit, followed by encoder faults. The drive repeatedly shut down during startup, and the axis exhibited irregular pulsing. These symptoms strongly indicated two simultaneous faults: moisture ingress and resulting encoder failure.
Findings During Incoming Inspection
The external inspection showed heavy contamination, coolant residue and oxidation at the terminal box. The cooling fins were clogged. Rotating the shaft manually revealed slight resistance, indicating early bearing corrosion.
Insulation measurements showed significantly reduced values phase-to-phase and phase-to-ground. Moisture had clearly penetrated the motor.
Inside the encoder, condensation exposure was visible. The UTSAH-B17BE showed corrosion on multiple tracks, including the incremental channel. This explained the drive’s immediate shutdown with encoder fault messages.
Disassembly and Deep Cleaning
After full disassembly, all components underwent multi-stage cleaning. The stator was dried in a heated chamber. The rotor was cleaned and magnet surfaces inspected. Bearing seats were lightly corroded and reconditioned. New bearings were installed, as refurbishment would not ensure long-term reliability.
The housing was chemically cleaned, rinsed and protected. The terminal box was resealed with new gaskets, as the original sealing had hardened and cracked, allowing coolant to penetrate.
Encoder Replacement and Programming
The UTSAH-B17BE encoder was irreparably damaged. Moisture had corroded internal tracks and electronics. Repair was not feasible.
A factory new encoder was installed and programmed to match the motor. Rotor alignment and zero-point calibration were carried out on our Yaskawa test stand. Any deviation here would cause jitter, overspeed alarms or improper positioning later.
Electrical and Mechanical Testing
After reassembly, insulation tests and resistance checks were performed, all within specification.
Mechanical checks included:
- Shaft runout
- Bearing noise
- Vibration
- Thermal behavior under load
The motor then underwent a full dynamic test on our Yaskawa load test bench with the SGDK-2020AEA-V drive:
- Continuous duty at rated torque
- Dynamic reversals
- Low-speed feedback verification
- Thermal endurance cycle
All tests were passed without issue.
Final Quality Assurance
Special attention was given to encoder stability. The new UTSAH-B17BE delivered clean, stable signals with perfect UVW commutation tracks. All diagnostics were logged and stored.
Result
The motor was restored to full working condition. Encoder replacement, bearing renewal, complete drying and resealing ensured long-term reliability. Root cause: coolant ingress through worn seals and a degraded terminal box gasket.
Information about the mentioned Servopack and Servomotor:
More information about our Yaskawa repairs can be found here.
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Technical Data of the AC Servo Motor (SGMGS‑05A5W‑SE12)
| Specification | Value | Note |
|---|
| Manufacturer | Yaskawa Electric Corporation | Japan |
| Motor Type | AC Servomotor, M Series | Precision servo |
| Model | SGMKS-20A5A-MU11 | Nameplate |
| Rated Power | 1.8 kW | Nameplate |
| Rated Torque | 11.5 Nm | Nameplate |
| Rated Current | 15.7 A | Nameplate |
| Rated Voltage | 200 V | Nameplate |
| Rated Speed | 1500 rpm | Nameplate |
| Protection Class | Insulation Class B | Nameplate |
| Encoder | UTSAH-B17BE | Root cause of failure |
| Mounting Style | Flange mounted | Standard |
| Operating Environment | Machine tools / machining centers | Application-based |
| Compatible Drives | SGDK-2020AEA-V | Confirmed by customer |
| O/N | 532168-037-001 | Nameplate |
| S/N | B0056A534210001 | Nameplate |
| Cooling | Self-cooled / cooling fins | Visible |
| Production Date | 0506 | Nameplate |
| Manual Reference | Yaskawa AC Servo Drives M Series | Internal reference |
Operating Environment & Compatible Equipment
The Yaskawa SGMKS-20A5A-MU11 belongs to the M Series and is typically used in high-precision axis applications of industrial CNC machines.
Common operating environments:
- Vertical axes and feed drives in machining centers
- Siemens–Yaskawa hybrid control systems
- High-dynamic positioning systems in machine tools
- Applications with continuous duty and frequent load changes
Compatible servo drives:
- Yaskawa SGDK-2020AEA-V (verified)
- SGDK-xx20A-x variations in the same power class
- Other Yaskawa M-Series compatible Sigma drives
Typical environmental influences:
- Coolant mist and emulsions
- Oil exposure and metal dust
- Moisture accumulation inside the motor
- Frequent start-stop cycles increasing encoder stress
Functional Description
The SGMKS-20A5A-MU11 is a three-phase AC servomotor with a permanent magnet rotor and a high-resolution optical encoder. Key functions:
- Providing precise dynamic motion for axis positioning
- Returning rotor position and speed to the servo drive
- Supporting closed-loop control with high accuracy
- Compensating load peaks with high overload capability
- Ensuring smooth rotation due to low rotor inertia
The UTSAH-B17BE encoder delivers both incremental and absolute feedback. These signals serve:
- Position feedback
- Speed feedback
- Zero-point detection
- Reliable startup referencing
Faulty encoder signals lead to:
- Jitter at low speed
- Position deviation
- Sudden shutdown by the drive
- Motor pulsing instead of running smoothly
Moisture inside the encoder housing is especially destructive.
Alarm Messages & Troubleshooting
| Code | Error Description | Cause | Solution |
|---|
| A.20 | Encoder communication fault | Moisture, corrosion, short circuit | Replace encoder, clean connectors |
| A.81 | Position feedback error | Incremental track disturbed | Recalibrate or replace encoder |
| A.90 | Overspeed | Incorrect position signals | Check signal quality, replace encoder |
| A.C0 | Absolute data error | Absolute track damaged | Replace encoder |
| A.D1 | UVW track error | Commutation track defective | Encoder electronics damaged |
| Motor Overcurrent | Motor short circuit | Moisture, insulation failure | Dry motor, check insulation |
| Startup malfunction | Motor only vibrates | No valid encoder signal | Replace encoder |
| No feedback | Drive blocks motor | Encoder electronics defective | Replace encoder |
| Jitter | Bearing damage in encoder | Misaligned or worn encoder bearings | Replace encoder |
Components
| Component | Description | Function | Notes |
|---|
| Stator Assembly | 3-phase winding | Creates rotating field | Check insulation |
| Rotor | Permanent magnet | Produces torque | Check for cracks |
| Encoder | UTSAH-B17BE | Position & speed feedback | Moisture-sensitive |
| Front/Rear Bearings | Precision bearings | Shaft support | Replace when worn |
| Motor Housing | Cooling fins | Heat dissipation | Clean regularly |
| Terminal Box | Power & signal interface | Electrical connection | Check sealing |
| Shaft Seal | Oil seal | Prevents moisture entry | Replace regularly |
| Cooling Ribs | Passive cooling | Heat dissipation | Keep clean |