Fanuc spindle alarm 90 means an unexpected rotation state was detected — the polarity of the torque command differs from the polarity of the actual acceleration. Issued on BiS-series synchronous spindle motors: the drive pushed one way and the motor accelerated the other — which on a permanent-magnet motor means its knowledge of the rotor's magnetic position is wrong. Fanuc's triage: at first start, causes (a)–(d) all apply; on a machine that has been operating normally, it is highly likely (c) or (d).
Commissioning / first-start causes
- (a) Sensor/motor configuration parameters wrong — number of sensor teeth, number of motor phases: verify against B-65280EN (the same parameter set as AL-65: 4011 bits, 4334, 4368).
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(b) AMR offset parameters wrong — the BiS-specific one:
- Parameter 4084 (AMR offset) stores the phase relationship between the rotor's magnetic 0° position and the sensor's one-rotation signal — adjusted individually for each machine. It must be re-adjusted whenever that relationship changes: sensor replaced, shaft/sensor slippage, or — the classic — the parameter was loaded from another machine. A sister machine's 4084 is wrong for this rotor by definition.
- Parameter 4085 exists for the AMR offset adjustment procedure and must normally be 0 — a non-zero 4085 left behind after an adjustment session is itself a fault.
Any-time causes — the running-machine suspects
- (c) After the spindle is banged (crash): Fanuc's own wording — impact can slip the motor sensor or the rotor on its shaft, changing the sensor-to-rotor phase relationship. If AL-90 follows a crash, the mechanical mounting must be inspected/corrected and the AMR offset (4084) re-established. Do not simply re-adjust 4084 over a slipping ring — fix the mount, then adjust.
- (d) Count errors from noise: intermittent AL-90 with no crash history → feedback signal noise — shielding continuity, frame-ground clamps, separation from power leads (method per AL-47).
Tip: Wrong-way torque on a synchronous motor is violent — jumps, growls, instant trips. Stop retrying after two attempts; every start with wrong pole data hammers the mechanics. The forensic shortcuts: after a crash → (c), after a parameter restore → 4084 came from the wrong machine, after sensor work → re-adjust 4084 by procedure. And record your machine's adjusted 4084 value in the machine file — it is machine-specific gold that no backup from anywhere else can replace.
Diagnosis checklist (in order)
- First start or veteran machine? Veteran → crash history and noise first.
- Verify sensor/motor configuration parameters (4011/4334/4368).
- Verify 4084 is THIS machine's adjusted value and 4085 = 0; re-run the AMR adjustment after any sensor/rotor disturbance.
- After impact → inspect sensor ring and rotor mounting for slip before re-adjusting.
- Intermittent → shielding/grounding/routing of the feedback path.
- Persists with all verified → amplifier/SSM electronics: bench diagnosis.
Related guides
- Fanuc spindle alarm 65 (magnetic pole detection error)
- Fanuc spindle alarm 91 (magnetic pole count error)
- Fanuc spindle alarm 89 (SSM abnormal)
AL-90 after a crash or restore? JJ Automation performs AMR offset adjustment, corrects sensor/rotor mounting, and repairs BiS spindle amplifiers at component level — on site in NCR. WhatsApp us or call +91 88022 12394.