From an occasional dropout to a fault every five minutes

The Mitsubishi HA43NC-SS AC Servo Motor arrived at industrypart with an increasingly frequent fault. According to the customer, the encoder repeatedly lost its connection to the drive. Initially this occurred occasionally. The intervals then became shorter until the fault occurred roughly every five minutes. Our inspection found a contaminated and partly damaged optical code disc inside the Mitsubishi OSE104 encoder. This case explains why a feedback problem can persist after cables have already been replaced and another drive has been tried.

Why cables and the drive were tried first

Before the motor was removed, cables had been replaced and the drive substituted for a test. This approach is understandable from a maintenance perspective. Some servo motors sit deep inside the machine mechanism. Removal may take considerable time and require mechanical adjustment and alignment afterwards. Checking accessible components first can therefore be sensible. Each substitution must use a verified configuration and be documented. If the fault remains unchanged, diagnosis needs to extend to the motor and its feedback system.

What we found inside the encoder

Contamination had entered the encoder and soiled the optical code glass. Dust and abrasive debris had also attacked the disc, partly wearing away its fine pattern. The encoder could no longer count reliably. It was striking that the machine still operated between faults despite this damage. Intermittent operation does not establish that the detector is in a dependable condition. The precise entry path for the contamination was not established in this repair.

Contamination and pattern damage are different problems

Deposits on an optical code disc can interfere with signal evaluation. If the pattern itself is damaged, cleaning alone is insufficient. Neither a new cable nor a different drive can restore the missing structure. The increasing fault frequency reported by the customer is consistent with progressive impairment. On its own, however, it reveals neither an exact alarm code nor whether an electrical communication problem also existed. The customer description and workshop findings therefore remain distinct in this report.

Encoder replacement and complete motor overhaul

We replaced the defective encoder and completely overhauled the motor. This included new ball bearings and seals, cleaning and repainting. Bearings support the shaft, while seals limit contaminant entry. No individual seal was documented as the original entry point. We therefore do not claim a proven entry route. Nor do we infer winding or brake damage from the encoder findings.

Testing with the Mitsubishi MDS-C1-V1-20

After the work, the motor was tested on our bench with a Mitsubishi MDS-C1-V1-20 servo drive. The original recording shows the motor, test setup and shaft rotation. The test illustrates the interaction between the overhauled motor, replacement encoder and connected servo drive.

What this case means for your own machine

An occasional detector fault deserves attention before the machine starts stopping every few minutes. Useful information includes the exact alarm display, operating conditions and changes that affect the symptoms. A fault linked to cable movement calls for different checks from implausible feedback with stable communication. The technical section below explains these distinctions and puts relevant MDS-C1 alarms in context.

Price, Lead Time and Further Information

Please request an individual repair assessment using the complete HA43NC-SS motor identification and a nameplate photograph. Information about Mitsubishi repair services is available in our Mitsubishi service section.

Technical specifications and variant identification

These nameplate values describe the photographed version. The 10 Nm and 17 A entries are maximum values, not continuous ratings. The motor label alone does not establish which replacement drive or feedback version a machine accepts. Dimensions, weight and encoder resolution are not specified here.

Application and compatibility

A servo motor converts controlled current into torque for a machine axis. Typical applications include positioning and feed movements. The customer machine model is not documented in this case. The confirmed workshop combination is HA43NC-SS, OSE104 and MDS-C1-V1-20. This test combination is not a blanket replacement approval for other machines. Full suffixes, connector assignments, feedback type, motor parameters and machine configuration must agree.

How the feedback loop works

The CNC supplies the movement command. The servo drive controls the motor current, while the encoder reports shaft movement. Reliable feedback lets the system compare the intended movement with the measured movement. Loss of the communication link and incorrect position information are different faults, although a customer may describe both as a lost connection. An alarm history and the exact operating conditions help distinguish them.

In an optical encoder, a fine pattern on a code disc modulates the light reaching the sensing system. Deposits can impair the contrast. A damaged pattern also removes measurement information. Cleaning can remove contamination but cannot restore a partly abraded pattern. That distinction explains why replacing the encoder was necessary in this repair.

Assemblies and diagnostic relevance

Selected MDS-C1 servo drive alarm references

The following is a manufacturer reference for the drive family, not a record of alarms observed on this motor. No exact customer alarm number was supplied. In particular, 2F is a communication fault; it does not alone prove optical disc damage. Codes 20 and 21 must not be used as generic OSE104 communication alarms.

Scope limits: alarm 25 concerns an absolute position system. The OSE104 feedback in this test setup is displayed as incremental. In a servo application, alarm 43 concerns a comparison of motor-side and machine-side position feedback. Neither code automatically explains this repair case. Fault group interpretation: 18 and 2F direct attention to the motor feedback connection, whereas 34 and 36 concern the NC-to-drive path. Position errors require feedback and mechanics to be considered together. Overcurrent and overload require a separate assessment of power wiring, motor condition and mechanical demand. A stored code guides diagnosis; it is not a component replacement instruction.

Practical fault isolation

These are diagnostic approaches, not additional tests claimed for this repair. Work on an opened encoder requires suitable equipment and alignment knowledge. Rubbing or polishing an optical disc can damage its pattern further. An insulation test must follow the manufacturer procedure with sensitive electronics isolated as required.

Prevention with a practical purpose

Inspect seals, connector housings and cable entries for damage and contamination during planned access. Keep cleaning fluids and compressed air away from seal gaps and encoder interiors. IP54 does not mean immunity to every coolant jet, ageing seal or damaged cable entry. Changes in bearing noise, vibration and fault frequency are reasons to investigate. Recording the first occasional faults can support a planned intervention before repeated production stoppages occur.

Sources and scope

Case facts: customer/workshop report, motor nameplate, encoder identification and the supplied 22.6-second test clip. Alarm reference: Mitsubishi Electric, MDS-C1 Series Instruction Manual, BNP-B2365(ENG)B, section 6, printed pages 6-4 to 6-6 and 6-17. Machine manufacturer instructions and the actual drive version take precedence. Mitsubishi MDS-C1 Instruction Manual

Test video

Photographs of the repair

Repair enquiry

Please send the full motor and encoder identification, nameplate photos, the exact alarm and a description of when the fault occurs. industrypart supports you with diagnosis, repair and preventive overhaul of industrial servo motors.

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