Mitsubishi MDS alarm 3A means overcurrent — excessive current was detected in the motor drive current. Note the difference from alarm 32: 32 is the power module's hardware protection tripping on a hard event (short, IPM failure); 3A is the measured motor current running too high — which is why Mitsubishi's investigation starts not with a megger but with tuning: an unstable, vibrating servo loop draws violent currents with nothing broken at all.
Steps 1–2 — Vibration and gain (the tuning side)
- Is the table or spindle vibrating — or vibrating with load fluctuation? Yes → set a (notch/vibration) filter, or lower the speed loop gain (SV005 servo / SP005 spindle). An oscillating loop hammers current at the resonance frequency — the classic after mechanical changes (new fixture, worn ways altering the machine's stiffness) or over-ambitious tuning.
- Does the alarm repeat at rapid traverse (servo) or accel/decel (spindle)? Yes → lower SV005/SP005 to the level where it stops — the loop is too hot for the mechanics as they are today.
Step 3 — Current values at maximum demand
Run repeated rapids (servo) or check the load meter at unloaded max speed (spindle):
- Displayed maximum current high → increase the current loop gain — servo: SV009–SV012; spindle: SP077–SP080 and SP081–SP084 — per the setup procedure for the fitted motor (these are motor-specific values from the parameter list, not tuning knobs to freestyle).
- Values appropriate → the electrical side; continue.
Steps 4–5 — The insulation checks (with Mitsubishi's exact criteria)
Disconnect the power cable (U/V/W) at the terminal block and the cannon plug at the motor; test with a tester:
- Cable phase resistance not ‘∞’ (conductor-to-conductor) → shorted cable → replace the power cable.
- Motor terminal to unit (shaft/frame) 1 MΩ or less → replace the motor — and Mitsubishi's own note: on motors with absolute position encoders, the zero point must be re-established afterwards (our zero point guide).
- Cable = ∞ and motor ≥ 1 MΩ → check insulation between the power cable and FG — ground fault → replace the cable; clean → continue.
Step 6 — Environment
Ambient temperature and — Mitsubishi names it directly — cutting water: coolant reaching the motor or connectors is the background cause behind a large share of recurring 3A cases. Fix the ingress path, not just the failed part.
Diagnosis checklist (in order)
- Vibration present? → filter / lower SV005 (SP005).
- Repeats at rapids/accel? → lower the speed loop gain to stability.
- Max current high? → set current loop gains (SV009–012 / SP077–084) per the motor spec.
- Disconnect and test: cable phases = ∞; motor-to-frame ≥ 1 MΩ; cable-to-FG clean — replace as found (zero point after absolute-encoder motor swaps).
- Recurring → find and stop the coolant/heat ingress.
Related guides
- Mitsubishi MDS alarm 32 (power module overcurrent — the hardware trip)
- Mitsubishi MDS alarm 24 (grounding fault)
- Zero point initialization after motor replacement