Alarm guide · Mitsubishi · Updated October 2026
Mitsubishi AL.10 alarm: undervoltage
Mitsubishi AL.10 is an undervoltage alarm: the supply voltage to the servo amplifier dropped below the allowed level. Check the incoming power and the main circuit before suspecting the motor.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
What the alarm means
The amplifier saw its power supply or bus voltage fall too low, often for a moment. Alarm numbers differ between MELSERVO series, so confirm with the manual for your drive. Alarm numbers and wording can differ between series, so confirm with the manual for your unit.
Common causes
- Low or unstable incoming supply, or a momentary power dip
- Loose or burnt power terminal
- Failed main contactor or inrush circuit
- Aged bus capacitors in the amplifier
- Another large load starting on the same supply
Safe first checks
General guidance only. Follow the drive manual and your site's electrical safety rules. Power off, and let the DC bus discharge, before touching connectors.
- Note the exact alarm code and when it appears: at power-up, at start, or under load.
- Power off and turn the shaft by hand if safe: it should turn smoothly.
- Check the motor cable, encoder cable and connectors for damage and loose contacts.
- Check what changed recently: load, speed, settings, cables or a replaced part.
When to send it for repair
The motor is rarely the cause. If the alarm returns on a stable supply, the amplifier is the likely fault; a motor insulation test is still worth doing.
We test every motor on our Universal Servo Motor Repair Equipment, which reads and aligns the feedback and runs the motor under load without the original drive.
See Mitsubishi servo motor repair, Mitsubishi encoder repair and Mitsubishi servo drive repair.
Questions
Is AL.10 a motor fault?
Usually not. It points to the supply or the amplifier. A motor with damaged insulation can pull the supply down, so test insulation if it repeats.
How long does repair take?
Emergency repair is done the same day where the fault and parts allow, urgent in 3 working days and standard in 7 working days, counted from when the unit reaches our bench and the quote is approved. Every unit returns with a printed test report.
Can I get an exchange unit?
If we hold a matching unit, Servo Swap ships a tested exchange unit while yours is repaired. Mark "line down" on the request form and add the part number.
Related: AL.25 · AL.50 · AL.16 and AL.20
Brand names identify the units we repair. Star Automations is an independent multi-brand repair company and is not an OEM service centre.