Mitsubishi HF-H154BS Servo Motor Repair Case Study

Initial Customer Report

• Motor unreliable after coolant leakage.
• Brake wouldn’t hold vertical axis (drifted down).
• Alarm AL.118 (encoder error) triggered randomly.

Incoming Inspection

1. Visual: Coolant residue on encoder, brake, and windings; seals hardened.
2. Insulation Test: 0.3MΩ (well below 1MΩ minimum).
3. Encoder: Contaminated lens; weak signal.
4. Brake: Coil shorted; friction pads worn.
5. Bearings: Excessive play + grinding noise.

Repair Steps

1. Disassembly: Carefully remove encoder, brake, and stator.
2. Stator Rewind: Strip old windings; rewind with class H insulation; bake at 120°C for 4h; vacuum impregnate.
3. Encoder Service: Clean lens/PCB; replace damaged connector; re-calibrate.
4. Brake Overhaul: Replace coil, friction pads, and springs; adjust air gap.
5. Bearing Replacement: Install new NSK bearings; re-grease.
6. Seal Replacement: New Viton seals for oil/coolant resistance.
7. Reassembly & Testing:
   • Insulation resistance: >5MΩ (passed).
   • Encoder signal: Stable (passed).
   • Brake hold/release: Perfect (passed).
   • 2-hour run test: No noise, no vibration (passed).

Outcome

Key Takeaways

• Coolant ingress is the #1 killer of HF-H series motors.
• Early seal replacement prevents major damage.
• Professional repair is cheaper than OEM replacement (60–70% cost saving).