The initial situation seemed unspectacular but quickly developed into a significant machine downtime for the customer. In his spare parts inventory there was a used Mitsubishi HC102T-SZ which had been purchased two years earlier from an online seller as “fully functional, tested, with six months warranty”. When the identical motor in the machine failed and production stopped, the customer decided to install the stored motor. A few seconds after switching on the machine the unexpected happened: the corresponding servo drive MDS-B-SVJ2-10 burned on the encoder output section. The motor had destroyed the drive.

The maintenance technician then tested both motors. Insulation levels were within limits and there were no shorts in the power paths. The cause therefore had to be in the encoder system. Both motors and the damaged servo drive were shipped to us for diagnosis.

The incoming analysis revealed a clear picture. On the supposedly functional spare motor, the OSA107 encoder showed visible burn marks. The 5-volt supply inside the encoder was internally shorted which, when the system was powered up, forced a massive overcurrent into the communication section of the drive. This effect caused severe damage inside the MDS-B-SVJ2-10. Melted tracks and components on the communication board confirmed the typical damage of an encoder short circuit.

The motor from the machine also showed an encoder failure but in a different form. In this motor the position signals were unstable and communication was repeatedly interrupted, triggering alarms 18 and 22. Both motors were fully disassembled, cleaned, inspected and technically overhauled. The bearings showed no visible wear but were replaced preventively. The windings were thermally stable and showed no abnormalities.

The encoders of both motors had to be completely replaced. We always use new OSA107 encoder blanks and program them according to Mitsubishi standards to ensure full compatibility. The motors were then tested on our Mitsubishi test bench with simulated load conditions and were subjected to several hours of thermal and dynamic testing.

The servo drive showed that the encoder interface was irreparably damaged. The communication board had been destroyed so severely by the short circuit that restoration would not have been economical. Since we stock tens of thousands of spare boards, we installed a tested donor board. After calibration and final testing, the drive was restored to full function.

The customer received both motors and the drive fully tested and ready for use. Additionally he received clear recommendations for storing servo motors: absolute encoders are sensitive to humidity, shocks and electrostatic discharge. The originally stored motor had definitely not been functional at the time of purchase, which highlights the risks of buying used components without a test report.

With the repair completed the customer now has a reliable replacement motor and a tested servo drive for future emergencies. The risk of unplanned downtime has been significantly reduced and the machine returned to operation without further delay.

Preventive Measures for the Customer

• Store motors dry and vibration free
• Annual encoder diagnostics when motors are stored for long periods
• Regular inspection of seals and connectors
• Never connect drives to unknown encoders
• Perform insulation tests before installation
• Replace bearings every 5–7 years preventively

Conclusion

This case clearly demonstrates how critical the encoder unit of a Mitsubishi servo motor is. Through timely inspection, proper storage and professional refurbishment, expensive secondary damage to the drive can be avoided.

To mentioned Mitsubishi Motor:

Mitsubishi HC102T-SZ / HC102T-A47 AC Servo Motor
Mitsubishi MDS-B-SVJ2-10 AC Servo Drive Unit

More details about our Mitsubishi repair services can be found here:
Mitsubishi motor Repair by Industrypart

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Technical Specifications

Operating Environment and Compatible Devices

The HC102T-SZ belongs to the robust Mitsubishi AC servo motors commonly used in machine tools with MELDAS controllers. Typical applications include feed axes in CNC milling machines, lathes and handling systems. The motor works with drive units of the MDS-B-SVJ2 series, in this case the MDS-B-SVJ2-10.

The OSA107 encoder provides absolute position data and forms a closed-loop positioning system together with the drive.

Functional Description

The HC102T-SZ converts electrical energy into precise rotary motion. The motor uses sinusoidal commutation with an absolute position feedback system. The OSA107 encoder transmits position, speed and reference data through the serial CN2 interface to the servo drive. The drive additionally monitors temperature, overcurrent, overspeed and the communication quality of the encoder.

A defect inside the encoder or a short circuit in the 5-volt supply leads immediately to serious faults inside the servo drive because the communication is permanently monitored. In such cases alarms like 18, 22, 2B or overcurrent error 32 typically occur.

Alarms and Troubleshooting

Components