Mitsubishi MR-J2S-350B4: low bus voltage and cooling fan failure

This Mitsubishi AC Servo Drive MR-J2S-350B4 had been diagnosed with low bus voltage. Our workshop found a defective internal power supply and a heavily worn power output stage. A failed, contaminated fan was identified as the cause. We carried out a preventive overhaul and subsequently tested the unit.

Customer report and initial situation

The supplied initial diagnosis was “bus voltage too low”. No specific alarm number or exact time of occurrence was provided for this case.

Bus voltage here refers to the DC link voltage. It is distinct from communication over the control bus.

Technical data

Electrical ratings are taken from the photographed nameplate. The applicable B4 manufacturer documentation covers this 400 V version.

Function: supply, output stage and cooling

The servo amplifier supplies and controls the connected servo motor. The DC link provides energy to the power output stage, which switches current to the motor phases according to the control requirements. The internal power supply powers the electronics.

The fan and heat sink dissipate the heat generated during operation. Fan failure and contaminated airflow paths impair cooling. In this case, inadequate cooling led to severe component wear and failure of the unit.

Workshop findings and repair

We performed a preventive overhaul of the Mitsubishi MR-J2S-350B4, treating the unit as an interconnected assembly. The photographs at the end of this report show its condition before and after repair.

Troubleshooting and alarm information

Low bus voltage initially describes an electrical condition. Troubleshooting considers the machine supply, the condition of the servo amplifier, the complete display indication and when the fault occurs. The findings in this unit cannot be applied to every undervoltage indication.

Display 10 is provided solely as general information from the applicable manufacturer documentation. It was not reported as an actual alarm in this repair case.

Testing and prevention

The unit was tested after its preventive overhaul. The following video includes original footage from this test. Test duration, load profile and measurement values were not supplied for this report.

Preventive maintenance should include regular checks of fan operation, airflow paths and cabinet ventilation. Contamination, unusual fan noise or a stationary fan warrant professional inspection.

Sources

Case-specific sources: supplied diagnosis, workshop findings, repair information and nameplate. Technical references: Mitsubishi MR-J2S-A4/B4 Supplementary Instruction Manual, SH(NA)030026-F, Chapter 3 and MR-J2S-B Instruction Manual, SH(NA)030007-G. For the 400 V version, consult the supplement alongside the main manual.

Repair case video

Photographs before and after repair

Request a repair

Send us the complete model designation, a photograph of the nameplate and the exact fault indication. Please also describe when the fault occurs. industrypart can assist with assessment, repair and overhaul of your servo amplifier.

Pricing and available services are provided on the corresponding product page: Mitsubishi MR-J2S-350B4 AC Servo Drive, item 41469 (German). Enquire about lead time and scope for your specific job.