Alarm 10 on a Mitsubishi MDS drive means undervoltage — the DC bus (PN bus) or the incoming supply has dropped below the drive's working limit. It is the mirror image of alarm 33 (overvoltage) and applies across the family — MDS-A/B, MDS-C1, MDS-D/DH, MDS-D2/DH2 and MDS-E/EH servo and spindle units — shown on the unit's LED and as servo alarm 0010 on the M60/M64/M70/M80 screen. Unlike alarm 33, which usually points at the regeneration path, alarm 10 is most often a supply-side problem — and sometimes not a fault at all, just a weak factory feeder telling on itself. Here is how to find the cause, in bench order.
What is happening
The power supply / converter unit rectifies the incoming 3-phase supply to roughly 280–310 V DC (200 V class) on the PN bus. If the bus sags below the trip threshold (about 190–200 V DC) while the servos are under load, every drive on that bus can trip alarm 10 together. The drive is protecting itself: running IGBTs at low bus voltage and high current overheats them.
Cause 1 — Incoming supply problems (most common)
- Genuine mains sag: measure L1-L2-L3 at the drive input terminals — not at the incomer — while the machine runs a heavy cycle. Below ~180 V AC line-to-line (200 V class) under load will trip. Log whether trips coincide with a compressor, furnace or welding set starting on the same feeder, or with grid dips at fixed times of day.
- One weak phase: a failing contactor pole, loose lug or undersized cable shows up as one low phase under load. Check all three, and feel/thermal-scan the input contactor and terminals for heating.
- Step-down transformer undersized or tapped wrong: machines imported and run through a local transformer often sit at the bottom edge of tolerance; a tap change fixes it permanently.
Cause 2 — Power supply / converter unit faults
- Aged DC bus capacitors: on 15–25-year-old MDS-A/B and C1 systems this is the classic cause — capacitance drops, the bus sags under every acceleration, and alarm 10 appears under load but never at idle. Confirmed on the bench by capacitance/ESR measurement; repairable at component level.
- Rectifier or charging circuit fault: failed diode module or a stuck charging (soft-start) resistor/relay leaves the bus low from power-on — alarm 10 immediately at servo-on.
- Bus voltage sensing fault: the bus is actually fine but the sensing divider or A/D on the control board reads low — the alarm follows the same unit even when swapped to another position. Board-level repair.
Cause 3 — External contactor and sequence issues
- Main magnetic contactor (MC) dropping out: a worn contactor, weak coil, or a chattering interlock in the E-stop string momentarily opens the drive supply — the bus collapses and alarm 10 trips. Watch the contactor during a trip, and check the coil circuit for loose terminals.
- External emergency-stop / power-off sequence mis-timed: if alarm 10 appears at every normal power-down but the machine otherwise runs fine, the MC is opening before the servos are off — a sequence/ladder timing issue, not a drive fault.
Diagnosis checklist (in order)
- Check whether one unit or all units on the bus trip — alarm 10 on everything simultaneously is supply-side until proven otherwise.
- Measure incoming 3-phase at the drive terminals, idle AND under the heaviest cycle, all three phase pairs.
- Watch the main contactor and listen for chatter during a trip; inspect and torque input terminals.
- Trip at first servo-on every day → suspect the charging circuit; trip only under load → suspect bus capacitors or feeder sag.
- If the supply measures healthy at the terminals during a trip, the power supply/converter unit needs bench testing — capacitor bank, rectifier and sensing cannot be judged from outside.
Can you keep running?
If the cause is a genuine mains dip, the alarm is doing its job — reset and continue, but fix the feeder before it costs you scrapped parts mid-cut. If trips are load-related on an old system, treat it as a capacitor bank warning: capacitors do not recover, and the end stage is a rectifier failure that takes the fuses — and sometimes the IGBTs — with it.
Related guides
- MDS Alarm 33 (overvoltage) — causes & fixes
- Mitsubishi M70 / M80 alarm list
- MELDAS M64 backup & restore procedure