Mitsubishi MDS alarm 24 means a grounding fault — the motor power cable (or motor winding) is in contact with FG, frame ground. Current is leaking from the U/V/W power path to earth, and the drive caught it. Applies to both servo and spindle units. Mitsubishi's four-step investigation is a clean megger-based elimination — with exact thresholds worth quoting, and one crucial warning at the end.
Step 1 — Megger the power path (threshold: 1 MΩ)
Power down, lock out, and disconnect the wires from the drive unit (Mitsubishi's own note — megger voltage into a connected drive damages it). Measure insulation from each power conductor (U, V, W, with the motor still connected at the far end) to ground:
- Less than 1 MΩ → the motor or power cable is ground-faulted. Continue to Step 2 to find which and why.
- 1 MΩ or more → the power path is clean; jump to Step 4 (the drive side).
Step 2 — The oil question
Mitsubishi gives coolant/oil contamination its own step because it is that common: has oil come in contact with the motor or power cable?
- Yes → check the motor's cannon connector and the inside of the terminal box, and clean as necessary — oil wicking into the connector or terminal box is the classic recoverable ground fault. Then fix the source: re-route the cable, repair the guard, add the cover — or the fault returns.
- No visible oil → continue.
Step 3 — Megger again after cleaning
- Still less than 1 MΩ → replace the motor or cable — split them at the motor connector and megger each separately to decide which: cable faults are drag-chain crush and oil-rotted jackets; motor faults mean rewind or replacement (re-test warm if borderline — moisture passes cold and fails hot).
- 1 MΩ or more after cleaning → the contamination was the fault; monitor.
Step 4 — The drive side (threshold: 100 kΩ — and a warning)
With the power path proven clean, measure with an ordinary tester (NOT a megger — Mitsubishi's explicit note: do not measure insulation on the unit or it will be damaged) across the U/V/W terminals of the drive unit and ground:
- Less than 100 kΩ → replace (repair) the drive unit — its output stage is leaking to ground.
- 100 kΩ or more → the drive is clean too → the power supply unit is the remaining suspect — replace/repair it.
Diagnosis checklist (in order)
- Disconnect at the drive → megger U/V/W to ground: <1 MΩ = cable/motor side.
- Inspect for oil at the cannon connector and terminal box; clean; fix the oil source.
- Re-megger: still low → split cable from motor, megger each, replace the faulty one.
- Power path clean → tester (not megger!) across drive terminals to ground: <100 kΩ = drive; otherwise power supply unit.
Related guides
- Mitsubishi MDS alarm 32 (overcurrent — the short-circuit cousin)
- Mitsubishi MDS alarm 10 (undervoltage)
- Mitsubishi M70/M80 alarm list