Servo Drive Repair: Common Alarms, Root Causes and When to Replace

A servo axis that stops with an alarm can bring a whole line to a halt. The good news is that many servo alarms are caused by something outside the amplifier, such as a damaged encoder cable or a mechanical problem, and can be fixed without replacing expensive parts. This article explains the most common alarm groups, how to trace the root cause, and when servo drive repair makes more sense than replacement.

1. Encoder and feedback alarms

On Mitsubishi MELSERVO amplifiers these appear as AL.16 (encoder initial communication error) and AL.20 (encoder normal communication error); other brands have equivalent codes. They are the most common servo alarms we see.

Typical causes:

• Encoder cable damaged by flexing in a cable chain, or crushed in the machine
• Loose or contaminated connector, often with oil or coolant
• Electrical noise from the encoder cable running alongside power cables
• A failed encoder inside the motor

How to isolate: swap in a known-good encoder cable, then a known-good motor if available. If the alarm moves with the cable or motor, the amplifier is fine.

2. Overcurrent alarms

For example AL.32 on MELSERVO. The amplifier detected current beyond its limit. Check for a short circuit or insulation failure in the motor or power cable, a wrong phase sequence (U, V, W) after maintenance, or tuning gains set far too high. If the alarm appears with the motor cable disconnected, the power stage of the amplifier is damaged and needs repair.

3. Overload and excessive error alarms

Overload alarms (AL.50 and AL.51 on MELSERVO) and error excessive alarms (AL.52) usually mean the motor cannot follow the command. Common reasons:

• Mechanical binding: seized bearing, misaligned coupling, worn ball screw or guide
• A holding brake that is not releasing (check the brake supply voltage)
• Collision or jammed product
• Acceleration too high for the motor size, or a motor that was undersized from the start

Rotate the axis by hand with power off. If it is stiff, the problem is mechanical, not electrical.

4. Regenerative and overvoltage alarms

Regenerative error (AL.30) and overvoltage (AL.33) occur when the energy returned by a decelerating or vertical load exceeds what the amplifier can absorb. The fix is often an external regenerative resistor of the correct rating, a longer deceleration time, or checking that the existing resistor and its wiring are intact.

5. Undervoltage alarms

Undervoltage (AL.10 on MELSERVO) points to supply dips, loose power terminals or a failing contactor. Check the incoming supply before suspecting the amplifier.

Repair or replace?

What good servo repair includes

• Component-level diagnosis, not just board swapping
• Replacement of ageing parts such as DC bus capacitors and cooling fans
• Testing under load with a motor, not just a power-on check
• Encoder calibration and motor checks where needed
• A clear report of what failed and why, so the root cause can be fixed on the machine

Keep a backup of your parameters

Before any repair, save the amplifier parameters with MR Configurator2 or the equivalent software. After repair or replacement, restoring parameters and verifying tuning gets the axis running again in minutes.

Servo drive repair in Surat and across Gujarat

AB Automation provides servo drive and motor repair for Mitsubishi and other major brands, with on-site field service and new, refurbished and standby drives. If you are considering an upgrade, see our MELSERVO-J5 vs JE comparison. Call +91 90331 37026 or send us the alarm code and model.