Common Servo Motor Alarm Causes and How to Troubleshoot Them

1. Introduction — Every Automation Engineer Has Faced Servo Alarms

Servo drive alarms are among the most common issues in automation systems.

Typical symptoms include:

Servo refusing to enable
Unexpected motion stop
Position errors
Communication faults
Encoder failures

Troubleshooting must be systematic, because servo systems include mechanical, electrical, and control layers.

2. Types of Servo Alarms

2.1 Overload Alarm

Occurs when:

Mechanical load is too heavy
Friction is too high
Motor is jammed

2.2 Overcurrent Alarm

Triggered by:

Sudden acceleration
Short circuit
Mechanical collision

2.3 Encoder Error

Caused by:

Damaged encoder cable
Loose connector
Electrical noise
Misaligned encoder

2.4 Communication Error

Common communication protocols:

RS485
CANopen
EtherCAT

Issues arise from wrong parameters, wiring, or EMI.

2.5 Servo Enable Error

If servo drive does not receive proper enable signals, it cannot operate.

3. Signal Flow of a Servo System

PLC → Servo Enable

PLC outputs:

Enable
Run
Direction
Speed commands

Servo → PLC Feedback

Feedback includes:

Alarm status
Servo ready
Position reached

Encoder → Servo Drive

Real-time high-speed feedback for:

Position
Velocity
Direction

If this loop breaks, servo will immediately alarm.

4. Troubleshooting Process for Servo Alarms

4.1 Check Alarm Code

Every servo displays an alarm ID such as:

AL-01 Overload
AL-12 Encoder Fault
AL-30 Communication Error

Always start with the code.

4.2 Inspect Mechanical System

Check for:

Mechanical jam
Misalignment
Excess friction
Loose couplings

4.3 Measure Power Supply Output

Voltage dips will cause:

Servo resets
Overcurrent alarms

4.4 Check Encoder Cable and Connectors

Look for:

Broken pins
Bent connectors
Loose plugs
Oil contamination

4.5 Verify PLC Logic

Incorrect PLC enable logic may:

Fail to release the brake
Send unstable commands
Cause motion error

5. Common Field Problems

5.1 Mechanical Load Too High

Servo overheats and enters overload alarm.

5.2 Shaft Misalignment

Causes encoder errors and vibration alarms.

5.3 Encoder Signal Interruption

Especially common with long encoder cables.

5.4 Servo Gain Too High

Motor vibrates heavily → triggers overcurrent.

5.5 Communication Errors on RS485 / EtherCAT

Often caused by:

EMI
Wrong baud rate
Mismatched node address

6. Real Engineering Cases

Case 1: Robot Z-Axis Cannot Lift

Cause: mechanical binding
Fix: alignment and lubrication

Case 2: Tool Changer Cannot Rotate

Cause: servo gain too high → oscillation
Fix: adjust servo tuning parameters

7. Best Practices

✔ Always Align Servo + Mechanical Axis Properly

Prevents encoder errors and excessive load.

✔ Regularly Inspect Encoder Cables

One of the MOST common failure points.

✔ Use Damping or Gain Tuning

Reduces vibration and improves stability.