Fanuc alarm 90: Unexpected rotation detected

What it means

The αi SPM detected a rotation state in which the torque-command polarity disagrees with the direction of acceleration, issued for BiS synchronous spindle motors. Often follows mechanical shock to the spindle that shifted the sensor-to-rotor phase relationship.

Likely causes

• Motor sensor tooth count / phase parameter setting error
• AMR offset parameters (No. 4084/4085) need readjustment after sensor or shaft slippage
• Motor-sensor count errors due to noise

Replacement parts

We supply the following units for this alarm — tested, warranty-backed:

• Motor sensor ring (if slippage confirmed)

  amplifier · Common cause

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