Alarm guide · Yaskawa · Updated October 2026
Yaskawa A.850 alarm: encoder overspeed
Yaskawa A.850 is an encoder overspeed alarm: the encoder rotated too fast when power came on, or the encoder is faulty.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
What the alarm means
At power-up the encoder must be nearly still. If the shaft is moving, the drive raises this alarm. Alarm numbers and wording can differ between series, so confirm with the manual for your unit.
Common causes
- Shaft moved by the machine at power-up
- Vertical load dropping at power-up
- Encoder fault
- Cable or connector problem
- Power-up sequence too early
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
If the shaft is still at power-up and the alarm remains, the encoder needs testing.
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 Yaskawa servo motor repair, Yaskawa encoder repair and Yaskawa servo drive repair.
Questions
What should I check first?
Whether the machine moves the shaft as the drive powers up.
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.