FANUC SP9091 means that the spindle amplifier detected an error in the calculated magnetic pole position of a BiS Series synchronous spindle motor. The CNC displays SP9091, while the spindle amplifier displays alarm 91.
FANUC describes the fault as a pole position count miss. The alarm is generated when the magnetic pole position detected at the motor sensor's one-rotation signal differs greatly from the value expected from parameter No. 4084, the AMR offset.
| Alarm item | Display or condition | Meaning |
|---|---|---|
| CNC alarm | SP9091 / SERIAL SPINDLE ALARM | The spindle system reported amplifier alarm 91 |
| Spindle amplifier | 91 | A magnetic pole position count error was detected |
| Applicable motor | BiS Series synchronous spindle motor | The alarm is associated with magnetic pole position recognition |
| Comparison used | One-rotation signal position versus No. 4084 | The detected relationship differs substantially from the stored AMR offset |
| Official diagnostic direction | Parameters, sensor-ring slippage, and noise | Correct the cause of the invalid one-rotation or feedback count |
For the surrounding alarm numbers, see the FANUC Spindle Alarm Codes SP9034–SP9092.
What Does FANUC SP9091 Mean?
A synchronous spindle motor must maintain a known relationship between the rotor magnetic poles and the motor-sensor feedback. The spindle amplifier uses the A/B feedback pulses and the once-per-revolution reference signal to track that relationship.
For alarm 91, the amplifier compares the magnetic pole position calculated at the one-rotation reference with the AMR offset stored in parameter No. 4084. If the difference becomes abnormally large, the amplifier can no longer trust the pole-position count and issues SP9091.
The alarm does not automatically prove that the spindle amplifier is defective. FANUC's official procedure first identifies:
- An incorrect motor-sensor tooth-count or motor pole/phase setting
- An unnecessary or false one-rotation signal
- Feedback pulse count errors caused by sensor-ring slippage
- Electrical noise affecting the motor-sensor signal
SP9091 Applies to BiS Synchronous Spindle Motors
FANUC specifically associates alarm 91 with BiS Series synchronous spindle motors. Do not apply this procedure to an induction spindle motor or to a different feedback alarm solely because the CNC message contains the words “serial spindle alarm.”
Before troubleshooting, confirm the exact spindle motor model, spindle amplifier model and suffix, motor-sensor type, spindle software series, and machine-builder wiring configuration.
Why Parameter No. 4084 Appears in the Alarm Definition
Parameter No. 4084 stores the AMR offset used to represent the phase relationship between the rotor's magnetic pole reference and the motor sensor's one-rotation signal. Alarm 91 is detected by comparing the observed relationship with this stored value.
This does not mean that No. 4084 should be changed immediately whenever SP9091 appears. In the official alarm-91 procedure, the first parameter checks concern the number of motor-sensor teeth and the motor pole/phase setting. On a machine that previously operated normally, FANUC directs technicians to investigate sensor-ring slippage and noise.
Changing No. 4084 to match a mechanically shifted or noisy signal can conceal the actual fault. Adjust an AMR offset only under the applicable FANUC and machine-builder procedure and only after the mechanical relationship and feedback signal have been verified.
1. Confirm SP9091 and Amplifier Alarm 91
Before changing parameters or disturbing the sensor, confirm:
- The CNC alarm screen shows SP9091 or identifies spindle alarm 91.
- The spindle amplifier status display shows 91.
- The installed spindle motor is a FANUC BiS Series synchronous spindle motor.
- The alarm occurs at initial commissioning, immediately after maintenance, during acceleration, at a repeatable speed, or intermittently.
- No different motor-sensor, one-rotation, overspeed, or magnetic-pole alarm is being mistaken for SP9091.
Record the alarm screen before switching off the machine. Also record:
- Complete CNC series and model
- Exact spindle amplifier model and all suffixes
- Exact BiS spindle motor model
- Motor-sensor model and connector location
- Current spindle parameters and a verified backup
- Whether the motor, sensor, sensor ring, cable, amplifier, or parameters were recently changed
- Whether the spindle was struck, rebuilt, or subjected to a crash before the alarm appeared
2. Use the Alarm Timing to Set the Diagnostic Priority
FANUC separates the likely causes according to when the alarm began.
| When SP9091 first appeared | Checks to prioritize |
|---|---|
| Initial installation or commissioning | Motor-sensor tooth count, motor pole/phase settings, motor data, and feedback configuration |
| After parameter restoration or amplifier replacement | Compare the restored values with the exact motor data and original machine backup |
| After motor-sensor, bearing, rotor, or spindle work | Sensor-ring position, mounting security, mechanical phase relationship, and cable connection |
| Machine had operated normally | Sensor-ring slippage, false one-rotation signal, feedback count instability, shielding, grounding, and noise |
| Alarm is intermittent or speed-dependent | Cable movement, connector condition, ring runout, vibration, and electrical interference |
The timing is diagnostic evidence. A start-up alarm on a newly configured machine is more likely to involve data or installation, while a new alarm on a previously stable machine deserves immediate attention to sensor movement and signal integrity.
3. Verify the Motor-Sensor and Motor Parameters
The pole-position calculation depends on correct motor and sensor data. Compare the current settings with the parameter table for the exact installed motor.
| Parameter item | Common FANUC parameter reference | What to verify |
|---|---|---|
| Motor-sensor tooth selection | No. 4011 bits 2, 1, and 0 | Bit selection agrees with the installed sensor |
| Number of motor-sensor teeth | No. 4334 | Numerical value matches the exact motor-sensor specification |
| Motor pole selection | No. 4011 bits 7 and 3 | Selection agrees with the installed synchronous motor |
| Number of motor poles | No. 4368 | Value matches the exact BiS motor specification |
| AMR offset used for comparison | No. 4084 | Preserve the machine-specific value unless the approved adjustment procedure requires a change |
Parameter assignments can vary with the CNC generation and spindle software. Always confirm them in the applicable FANUC AC Spindle Motor αi Series Parameter Manual B-65280EN and the machine builder's original data.
Before changing any value:
- Back up the CNC, PMC, and spindle parameters.
- Photograph or export the existing settings.
- Confirm the complete motor and sensor model numbers.
- Compare both bit settings and numerical values with the correct motor table.
- Change only a value that can be proven incorrect.
Do not initialize all spindle parameters or copy values from a similar machine. A similar BiS motor can use different sensor, pole, AMR, orientation, or machine-specific settings.
4. Check the Motor Sensor and Sensor Ring for Slippage
FANUC identifies sensor-ring slippage as a direct cause of alarm 91. If the ring moves relative to the rotor, the one-rotation reference no longer occurs at the expected magnetic pole position.
With power isolated and the required discharge time observed, inspect:
- Sensor-ring mounting and any locking, clamping, keying, or heat-fit features
- Witness marks showing movement between the ring, hub, shaft, or rotor
- Loose sensor brackets or mounting screws
- Ring eccentricity, runout, cracks, impact damage, contamination, or worn teeth
- Sensor-head alignment and the detector-specific clearance through a full revolution
- Evidence of movement after a crash, spindle impact, bearing replacement, or motor repair
- Any mechanical component that can change the sensor-to-rotor phase relationship
Do not merely retighten a visibly shifted ring and restart the machine. Restore the correct mechanical position and mounting method according to the applicable motor and machine procedure, then verify the feedback and magnetic pole relationship.
5. Inspect the Motor-Sensor Cable and Connectors
A damaged or intermittent feedback cable can corrupt the one-rotation signal or the A/B pulse count and produce the same detected inconsistency.
Inspect the complete feedback path for:
- Loose or incompletely locked connectors
- Bent, recessed, contaminated, or corroded contacts
- Oil or coolant inside a connector or cable jacket
- Crushed, abraded, stretched, or sharply bent cable sections
- Damage near the motor, clamps, cabinet entry, cable carrier, and moving machine sections
- Incorrect repairs, splices, extensions, or pin assignments
- Intermittent behavior when the cable moves or vibrates
Replace the feedback cable if testing identifies an open circuit, intermittent conductor, incorrect pin assignment, damaged shield, or physical deterioration. Use the exact compatible FANUC cable specification; do not substitute a visually similar cable.
6. Check Shielding, Grounding, and Electrical Noise
FANUC states that noise can generate an unnecessary one-rotation signal or cause a feedback pulse count error. Check the installation before replacing the sensor.
- Confirm that the sensor cable shield is continuous and terminated as specified.
- Inspect shield clamps and connector shells for secure low-impedance contact.
- Verify the documented machine and amplifier frame grounding.
- Keep sensor wiring separated from spindle-motor, servo-motor, braking, contactor, transformer, and other high-power wiring.
- Check whether the cable is bundled with power leads or routed in parallel with them over a long distance.
- Check whether SP9091 coincides with contactor switching, another axis moving, a coolant pump starting, or another high-noise event.
- Inspect any intermediate connector, junction, or relay section that can interrupt the shield or feedback pair.
Restore the documented wiring and shielding arrangement. Do not add arbitrary ground wires or connect the shield at extra points without the applicable FANUC and machine wiring instructions.
7. Evaluate the Motor Sensor Only After the Installation Checks
If the parameters are correct, the sensor ring is secure, and the cable installation is verified, inspect the motor sensor itself.
Look for:
- Physical wear or impact damage at the sensor head
- Metal debris or contamination affecting the sensing surface
- Unstable A/B feedback pulses
- A false, duplicated, or missing one-rotation signal
- Signal amplitude or waveform outside the detector-specific limits
- Excessive gap variation caused by ring runout or loose mounting
Use the approved FANUC adjustment and measurement procedure for the exact detector. If the signal cannot be adjusted or remains unstable with a known-good compatible cable, replace the sensor and complete the required alignment and commissioning procedure.
8. Do Not Replace the Spindle Amplifier First
FANUC's alarm-91 procedure does not identify spindle-amplifier replacement as the primary corrective action. The official checks focus on parameter data, sensor-ring slippage, false one-rotation signals, feedback counting, and noise.
Consider amplifier input-circuit testing only after all external causes have been eliminated and separate measurements indicate that valid sensor signals reach the correct amplifier input but are processed incorrectly. Use a known-compatible unit with the exact part number, suffix, hardware revision, supported detector, and software compatibility.
Changing the amplifier without preserving the original parameter and AMR data can introduce a second problem and make the original fault harder to diagnose.
Difference Between SP9090 and SP9091
| CNC alarm | Amplifier display | FANUC description | Main diagnostic distinction |
|---|---|---|---|
| SP9090 | 90 | Unexpected rotation state detected | Includes motor/sensor parameters, AMR offset, mechanical phase shift, and noise checks |
| SP9091 | 91 | Magnetic pole position count miss | Focuses on tooth/pole data, sensor-ring slippage, false one-rotation signals, and feedback count errors |
Both alarms apply to BiS synchronous spindle systems and can involve related parameters or hardware. However, SP9091 specifically means that the pole position counted at the one-rotation signal differs greatly from the reference represented by No. 4084.
Difference Between SP9065 and SP9091
| CNC alarm | Amplifier display | Detected condition |
|---|---|---|
| SP9065 | 65 | Magnetic pole detection did not complete normally |
| SP9091 | 91 | The established magnetic pole position count became inconsistent with the one-rotation reference and AMR offset |
Do not treat SP9091 as a generic pole-detection failure. Confirm the exact amplifier display and follow the procedure for alarm 91.
FANUC SP9091 Diagnostic Table
| Finding | Likely interpretation | Recommended action |
|---|---|---|
| Alarm appears during initial commissioning | Motor-sensor tooth count or motor pole setting may be incorrect | Verify the exact motor data and applicable parameters |
| Alarm appears after restoring parameters | Data from another machine or an incomplete backup may have been loaded | Restore verified machine-specific values; do not guess No. 4084 |
| Alarm appears after sensor or motor work | Sensor-to-rotor phase relationship or ring position may have changed | Inspect ring mounting, sensor alignment, and mechanical reference |
| Alarm began after a spindle impact | Sensor ring, sensor bracket, or rotor relationship may have shifted | Inspect mechanically before changing parameters |
| Alarm occurs when the feedback cable moves | A conductor, connector, or shield may be intermittent | Test and replace the correct cable if defective |
| Alarm occurs when another high-power device operates | Electrical noise may be creating false pulses | Correct shielding, grounding, and cable routing |
| Sensor gap changes through one revolution | Ring runout or mounting is abnormal | Correct the mechanical cause to the detector-specific tolerance |
| Stable signals reach the amplifier but the count is still wrong | Amplifier input processing may require further testing | Test with approved equipment and replace hardware only when confirmed |
Recommended Troubleshooting Order
- Confirm SP9091 on the CNC and alarm 91 on the spindle amplifier.
- Verify that the installed motor is a BiS Series synchronous spindle motor.
- Record the alarm timing, maintenance history, complete equipment models, and current parameters.
- Back up all CNC, PMC, and spindle data.
- Verify the motor-sensor tooth count and motor pole/phase settings against the exact motor parameter table.
- Preserve No. 4084 unless the approved AMR-offset adjustment procedure proves that recalibration is required.
- Inspect the motor sensor, sensor ring, mounting security, runout, and sensor-to-rotor relationship.
- Inspect the feedback cable, connectors, pin assignments, and movement-related intermittency.
- Verify shielding, grounding, and separation from power wiring.
- Observe the A/B feedback and one-rotation signal using the applicable FANUC procedure.
- Replace a confirmed defective cable or sensor and complete the required adjustment.
- Investigate the spindle amplifier only after valid external signals and installation have been confirmed.
- Retest the spindle at the specified low speed before returning the machine to normal operation.
What Not to Do With Alarm 91
- Do not change No. 4084 merely to make the alarm disappear.
- Do not copy motor or AMR parameters from another machine.
- Do not initialize all spindle parameters without a verified backup.
- Do not overlook sensor-ring slippage after impact or motor repair.
- Do not route the sensor cable together with spindle or servo power wiring.
- Do not replace the sensor before checking the cable, ring, shielding, and parameters.
- Do not replace the spindle amplifier before completing the official external checks.
- Do not continue high-speed spindle operation while the magnetic pole position feedback is unreliable.
Technical References
- FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Alarm Code 91
- FANUC AC Spindle Motor αi Series Parameter Manual, B-65280EN, BiS motor, motor-sensor, pole, and AMR-offset parameters
- FANUC Servo Amplifier αi Series Descriptions, B-65282EN, detector connections, cable shielding, grounding, and installation
Always use the manual edition that applies to the exact CNC, spindle amplifier, spindle software, BiS motor, motor sensor, and machine.
Need Help With FANUC SP9091 Alarm?
REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers, BiS spindle motors, feedback cables, motor sensors, and related hardware. Send us the CNC alarm screen, amplifier display, complete CNC and spindle amplifier models, exact motor and sensor models, parameter backup, connector photos, and details of when SP9091 occurs.
Browse FANUC servo, spindle, and power drives, visit the FANUC Technical Support Center, or contact REACO CNC for assistance.
Reference Source: FANUC technical documentation. The correct parameter values, detector checks, and repair procedure depend on the installed CNC, spindle amplifier, software, BiS motor, feedback system, and machine-builder design.
