Yaskawa Sigma-7 A.C90 at power-up, what to check first?
I'm getting an A.C90 alarm on a Yaskawa Sigma-7 SERVOPACK every time I power up the machine. This started right after I replaced the servomotor. What should I check first?
Alarm A.C90 on a Yaskawa Sigma-7 SERVOPACK indicates an Encoder Communications Error, meaning the SERVOPACK cannot communicate with the encoder. This is a Gr.1 alarm that requires the underlying cause to be resolved before it can be cleared. Since it occurs at power-up, especially after a motor replacement, the issue likely involves the encoder connection or wiring.
Likely causes and checks
1Faulty contact or wrong wiring at the encoder connector
- Points to it
- The alarm appears at power-up, often after a motor or cable change, suggesting an issue with the physical connection or wiring of the encoder.
- Check
- Check the condition of the encoder connector at both ends, reconnect it, and verify the encoder wiring.
2Encoder Cable open, shorted, or outside specifications
- Points to it
- If the alarm repeats at power-up, it could indicate a problem with the encoder cable itself, such as an open circuit or a short.
- Check
- With power off, check the Encoder Cable for continuity and shorts. Confirm it is a cable within Yaskawa’s specifications (shielded twisted pair, 50 m maximum for rotary motors).
3SERVOPACK or encoder (motor) failure
- Points to it
- If wiring and environmental checks are good and the alarm persists, either the SERVOPACK or the motor's encoder could be faulty.
- Check
- Perform a swap test: connect the Servomotor to another SERVOPACK and turn on control power. If there's no alarm, the original SERVOPACK is faulty; if the alarm follows the motor, the motor encoder is faulty.
Next check: Given the alarm occurs at power-up immediately after a motor replacement, the first step should be to thoroughly inspect and re-seat the encoder connector at both the motor and SERVOPACK ends, and then verify the wiring.
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