Initial Situation.

The customer sent a Yaskawa SGMG-44A2AAG motor together with the matching SGDB-30VDY1 servo amplifier. The axis triggered an overload alarm at irregular intervals. The machine frequently stopped during operation without showing a clear pattern.
The operator attempted to solve the issue himself and replaced the servo amplifier with a used device of the same type. However, the replacement amplifier did not operate with the motor at all. The axis did not move and sometimes the system did not even start correctly. This marked the beginning of the diagnostic process.

First Inspection

After delivery we performed full insulation measurements, winding tests and a visual inspection. Externally the motor was in good condition, only a slight oil film was visible. The OGURA RNB4.4G-03 brake worked correctly, the coil was electrically stable and released properly.
Nothing indicated a typical motor defect. The SGDB-30VDY1 also showed no basic faults on our test bench. This made the case interesting.

Systematic Failure Analysis

We simulated the customer’s operating conditions on our Yaskawa test stand. The motor ran smoothly up to certain speeds. However, once it reached a critical point, the SGDB went into overload.
This behavior is typical for worn encoder bearings. When the encoder no longer runs round, it produces mechanical imbalance and unstable feedback pulses. The control loop receives distorted data and the amplifier interprets this noise as overload.

The fact that the replacement SGDB purchased by the customer did not run with the motor confirmed the suspicion. Missing parameter transfer and incorrect jumper settings often prevent SGDB devices from working with absolute encoders. Two identical amplifiers only work together when parameters match exactly. Many operators struggle with this, especially with older SGDB models.

Disassembly and Damage Pattern

We disassembled the motor completely. When opening the encoder, the actual problem became clear.
The bearings of the old UTOPH-81AWF were worn and no longer running smoothly. They produced mechanical vibration and electrical noise within the signal electronics. The internal circuit was driven outside its specified range and caused sporadic false pulses.
The rest of the motor was technically flawless. Neither the windings nor the insulation nor the brake showed any problems.

Repair Process

The defective encoder was completely replaced. We installed our newly developed UTOPH-81AiF:
Stabilized electronics, improved signal quality, better temperature stability and more precise bearings.
We also replaced all seals, cleaned the motor housing, renewed the shaft seal and added fresh grease. Afterwards the motor was repainted and tested in a long-term run.

In parallel we overhauled the SGDB-30VDY1 preventively: checked the power section, replaced intermediate circuit capacitors, renewed solder joints and optimized the thermal path of the output stage. The amplifier passed the extended load test without any issues.

Test Result

Afterwards the motor ran absolutely smooth, stable and without overload. No encoder signal showed any disturbance. The brake released cleanly.
Together with the overhauled SGDB amplifier the system was released and later tested again together with the customer.

Preventive Measures

• Replace encoders every 15 to 20 years of service
• Regular seal and bearing inspections
• Keep the motor dry and protect from oil
• Save SGDB parameter sets to avoid misconfiguration after device replacement
• Inspect machine fans and filters routinely
• Perform regular full-load tests

Conclusion

The defect was not located in the motor or the drive but in the encoder. It was a classic age-related failure after more than two decades of operation.
The combination of precise diagnostics, full overhaul and real-world load testing ensures that the customer receives a system capable of running reliably for many more years.

nformation about the mentioned Servopack and Servomotor:

• Yaskawa SGMG-44A2AAG AC Servo Motor
• Encoder Yaskawa UTOPH-81AWF -> UTOPH-81AiF
• Yaskawa Servopack SGDB-30VDY1

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

Operating Environment and Compatible Equipment

The motor is used in machine tools, portal systems and heavy feed axes, typically as an X or Z axis.
The combination SGMG-44A2AAG with SGDB-30VDY1 is a robust solution of the older Sigma generations and is frequently installed in lathes, machining centers and transfer machines.
The OGURA RNB4.4G-03 brake is suitable for vertical loads, and the encoder UTOPH-81AWF or the modernized UTOPH-81AiF provides position feedback for the SGDB drive.

Functional Description

The SGDB-30VDY1 controls current, speed and position of the motor. The encoder provides feedback signals for the closed loop. Disturbances in the encoder signal lead directly to overload, because the control loop receives incorrect data. The internal monitoring of the SGDB reacts very sensitively to inconsistent signals.
The motor operates with absolute torque control and uses internal temperature monitoring, intermediate circuit monitoring and a multi-stage current limitation.

Alarm Messages and Troubleshooting (SGDB Series)

Components