FANUC SP9031 Alarm 31: Motor Lock or Detector Signal Loss

FANUC SP9031 means that the spindle motor failed to rotate as commanded and either stopped or remained at a very low speed. The CNC displays SP9031 / SSPA:31 MOTOR LOCK OR DISCONNECT DETECTOR, while the spindle amplifier displays 31.

This alarm does not by itself prove that the motor sensor is defective. The same speed-response failure can be produced by excessive mechanical load, a spindle that remains locked, incorrect motor phase sequence, incorrect speed-detector parameters, crossed phase A/B feedback signals, a damaged feedback cable, an open motor power circuit, an incorrect switching sequence or a faulty spindle amplifier.

For the surrounding serial spindle alarms, see the FANUC Serial Spindle Alarm Codes SP9001–SP9033.

What Does FANUC SP9031 Mean?

FANUC Series 0i-D/0i Mate-D Maintenance Manual B-64305EN/03 describes SP9031 as a condition in which the motor cannot rotate at the specified speed. The speed remains at or below the applicable SST detection level continuously while a rotation command is present. The alarm list directs technicians to check the load condition and the motor sensor cable.

FANUC Servo Amplifier αi Series Maintenance Manual B-65285EN/04 gives the practical symptom in simpler terms: the motor failed to rotate as specified and has either stopped or is rotating at a very low speed. It then separates troubleshooting into those two observable conditions.

The phrase “disconnect detector” in the CNC alarm message should therefore be interpreted carefully. A missing or incorrect speed-feedback signal can make the amplifier detect little or no rotation, but a mechanically locked spindle or an interrupted motor power circuit can create the same speed-response result. Confirm how the motor behaves before replacing the sensor, motor or amplifier.

Main Causes of FANUC Alarm 31

The verified diagnostic directions are excessive load or mechanical restraint; a lock, brake or clamp that remains engaged; incorrect motor or detector parameters; incorrect motor phase sequence or phase A/B feedback; a faulty feedback cable or sensor; an open motor power circuit; an incorrect switching sequence; and spindle-amplifier failure. Switching checks apply only to machines equipped with that function, while amplifier replacement follows the applicable external checks.

Do not combine all these items into one parts-replacement list. The correct order depends on whether the motor turns slowly, does not turn at all, or alarms only under cutting or switching conditions.

1. Confirm the Alarm and When It Occurs

Before changing wiring or parameters, record:

• The complete CNC message, spindle number and 31 indication on the amplifier

• The complete CNC, amplifier and motor models with suffixes, plus the installed sensor type

• Whether the motor is stationary, rotates slowly, jerks, or begins normally and then stalls

• Whether the alarm occurs at start, during cutting, orientation or switching

• Any recent motor, amplifier, cable, contactor, parameter or ladder work

Observe the load meter and the machine sequence at the moment the alarm occurs. If another spindle, servo or power-supply alarm appears at the same time, record it and diagnose the complete alarm combination.

Use the exact machine electrical drawing and applicable FANUC manual. JYA2 is identified as the motor sensor connection in the 0i-D SP9031 alarm table and is used for several αi motor-sensor configurations, but connector designations can differ with the amplifier, sensor and machine architecture.

2. Check the Load and Mechanical Condition

B-64305EN/03 lists load status as the first corrective direction for SP9031. If the alarm appears during cutting or when the spindle attempts to accelerate, first determine whether the motor is being prevented from reaching the commanded speed.

Check the load-meter reading, cutting condition and whether the spindle, belt, gearbox, coupling or driven mechanism rotates freely under the machine builder's procedure. A controlled no-load test can help separate process load from a mechanical or electrical condition.

If the alarm followed a heavy cut or a spindle stall, correct the machining or mechanical cause before testing the electrical system. Repeatedly resetting the alarm while the spindle remains restrained can expose the motor and amplifier to further stress.

However, a light-load alarm does not automatically prove an electrical fault. A brake, clamp, gearbox or switching mechanism can remain engaged even when no cutting load is present.

3. Check for an External Spindle Lock

When the motor does not rotate at all, B-65285EN/04 first directs the technician to check whether the sequence is locking the spindle.

Confirm that the spindle brake or clamp releases before the rotation command, that orientation or tool-change logic has not left the spindle locked, and that the PMC sequence receives the required hydraulic, pneumatic and interlock completion signals.

Do not bypass an interlock or force a contactor to test the spindle. Monitor the relevant commands and completion signals according to the machine builder's diagnostic procedure. If a lock is physically engaged, correct the mechanical or sequence problem before energizing the motor again.

4. If the Motor Rotates Only at Very Low Speed

If the motor moves but remains at very low speed, FANUC's detailed alarm 31 procedure focuses on four areas: parameters, motor phase sequence, phase A/B feedback and the feedback cable or sensor.

Check the motor and detector parameter set. Compare the installed motor and sensor with the original machine parameter backup and the applicable FANUC AC Spindle Motor αi/βi Series Parameter Manual B-65280EN. A parameter set for a different motor, detector or amplifier configuration can produce incorrect current and speed response.

Do not initialize spindle parameters as a casual first test. Back up the current data, record the exact motor/amplifier/detector specifications, obtain the correct motor table and compare the values with the original machine-builder data. Use the initialization procedure for the exact CNC and spindle software only when authorized.

Check the motor power phase sequence. Confirm that the motor power leads are connected in the required order at the amplifier, switching circuit and motor. This is especially important after motor, amplifier, contactor or cable replacement. Do not reverse phases without first comparing the actual wiring with the FANUC connection manual and machine drawing.

Check phase A/B feedback. On configurations using phase A/B motor-sensor signals, confirm that the A and B channels and their complementary signals are connected to the correct terminals. Crossed phases can make the detected direction or speed relationship inconsistent with the power command.

Check whether speed feedback is present. With the machine safely isolated from unintended motion and according to the approved procedure, manually rotate the motor or spindle and observe the motor-speed value on the CNC diagnosis screen or spindle check board. If no speed is displayed, inspect and test the feedback cable first. Replace the cable if it is open, damaged or incorrectly terminated. If a known-good cable and correct connection still produce no signal, test the motor sensor and replace the sensor or motor only when the fault is confirmed.

5. Inspect the Motor Feedback Cable and JYA2 Path

The 0i-D alarm list specifically directs technicians to replace the motor sensor cable at JYA2 for applicable configurations. Before replacement, inspect the complete feedback path:

• Confirm the interface, connector orientation, pin assignment and locking hardware

• Inspect for loose contacts, soldering errors, bent pins, contamination and coolant ingress

• Test for broken conductors or intermittent continuity, especially at flex points

• Verify the cable specification, shield termination and separation from motor power wiring

JYA2 can carry phase A/B and one-rotation signals for several αi M, MZ and BZ sensor arrangements, but it is not safe to assume that every FANUC spindle motor uses the same pins or cable. Use the connection diagram for the exact equipment.

Replacing the feedback cable is justified when the signal is absent, the cable fails continuity or insulation checks, the connector is damaged, or movement of the cable changes the symptom. It should not be used to avoid checking an obvious mechanical lock or open motor power circuit.

6. Parameters 4104 and 4106: Use Only the Correct Motor Data

Beijing FANUC guidance calls for checking parameters 4104 and 4106 during commissioning of applicable electric spindles and direct-drive motor configurations. These numbers must not be described as universal encoder-selection parameters.

In B-65280EN, parameter 4104 is associated with current-loop proportional gain, while 4106 is associated with current-loop integral gain or a related motor-category-specific current-loop value. Their correct settings depend on the exact motor group and parameter table.

Use these parameters only when the alarm began during commissioning, motor replacement or parameter restoration; the installed motor documentation identifies the values; the exact motor table is available; and the original settings have been backed up.

Do not tune 4104 or 4106 by trial and error to suppress SP9031. Incorrect current-loop gains can create unstable operation, abnormal current or further alarms. For a machine that previously ran correctly and has not lost its parameters, first check the new mechanical, wiring, feedback or switching condition that appeared before the fault.

7. If the Motor Does Not Rotate at All

After confirming that the spindle is not mechanically or logically locked, check the motor power circuit.

Verify that the motor power lead, terminals and connectors agree with the machine drawing; no required phase is interrupted through a contactor; the commanded contactor closes and returns the correct auxiliary feedback; and any recent replacement preserved the original phase order.

If the machine uses spindle switching, output switching or winding switching, B-65285EN/04 requires confirmation that the applicable magnetic contactor is on. Beijing FANUC guidance also calls for checking the dual-winding motor parameters and ladder sequence when the alarm appears during winding changeover.

These switching checks apply only when the machine has that function. Confirm that the previous contactor has released before the selected contactor closes, that the selected motor or winding is connected, and that the sequence matches the original machine-builder logic. Do not bridge contactors or defeat electrical interlocks.

Motor-cable and motor-insulation tests may be performed when the applicable FANUC or machine-builder procedure calls for them, but the general αi alarm 31 flow does not identify insulation failure as the first universal cause. Use properly rated test equipment and isolate the amplifier before testing; never apply a megohmmeter through connected amplifier electronics.

8. When Should the Spindle Amplifier Be Replaced?

FANUC lists the spindle amplifier as the final direction when the motor does not rotate at all. Before replacement, verify that the spindle is free, the lock sequence is correct, motor/detector parameters match the hardware, phase and feedback wiring are correct, the power and sensor cables have been tested, applicable switching contactors operate correctly, and the complete replacement model and suffix are compatible.

Do not replace the amplifier only because the CNC message contains the words “motor lock.” A replacement amplifier will not correct a mechanical restraint, incorrect phase order, missing sensor signal or open contactor.

If replacement is confirmed, browse FANUC servo, spindle and power drives or send REACO CNC the complete model and suffix for compatibility checking. After installation, restore only the verified parameter data and perform the machine builder's controlled no-load and load tests.

Difference Between SP9031, SP9029 and SP9073

SP9031 may involve a missing speed signal, but it is broader than SP9073. Conversely, SP9029 records sustained excessive load and should not be treated as identical to a motor that cannot rotate at all. Always diagnose the exact CNC and amplifier numbers displayed.

FANUC SP9031 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm SP9031 on the CNC and 31 on the spindle amplifier; record the spindle number and all simultaneous alarms.

2. Observe whether the motor is stationary, rotates only at very low speed, or stalls during cutting or acceleration.

3. Check the load meter, cutting condition and spindle mechanical freedom.

4. Confirm that the spindle brake, clamp and external locking sequence release correctly.

5. If the motor rotates slowly, verify the motor and detector parameters against the original backup and exact B-65280 motor table.

6. Check the motor power phase sequence and the phase A/B feedback connection where those signals are used.

7. Manually rotate the motor or spindle under the approved safe procedure and confirm that a motor-speed indication is present.

8. Inspect and test the motor feedback cable and the applicable JYA2 or model-specific interface; replace the cable when its fault is confirmed.

9. If the motor does not rotate, check the motor power lead and any spindle, output or winding-switching contactors and ladder sequence.

10. Use parameters 4104 and 4106 only for applicable commissioning or restoration work with the exact motor data; do not tune them by trial and error.

11. After all applicable external checks are correct, test and repair or replace the spindle amplifier.

12. Perform a controlled no-load retest, followed by the machine builder's load test, and diagnose any new alarm separately.

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 31

• FANUC Series 0i-D/0i Mate-D Maintenance Manual, B-64305EN/03, SP9031 alarm list

• FANUC AC Spindle Motor αi/βi Series, Built-in Spindle Motor Bi Series Parameter Manual, B-65280EN, motor-specific parameters and parameters 4104/4106

• FANUC Servo Amplifier αi Series Descriptions, B-65282EN, motor sensor connections and JYA2 phase A/B signals

• Applicable machine-builder electrical drawings and spindle switching or winding-switching sequence documentation

• Beijing FANUC SP9031 technical guidance, motor lock or detector disconnection troubleshooting

Always use the manual edition and parameter table that apply to the exact CNC, spindle amplifier, motor, sensor, spindle software and machine configuration.

Need Help With FANUC SP9031 Alarm?

REACO CNC provides independent FANUC spindle amplifier, motor and feedback-system testing, repair, compatibility checking and replacement support. Send us the CNC alarm screen, spindle amplifier display, complete CNC/amplifier/motor models with suffixes, motor sensor type, parameter backup, details of when the alarm occurs, and a short video showing whether the spindle is stationary or rotating slowly.

Visit the FANUC Technical Support Center for more FANUC alarm guides, or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The applicable load, locking, parameter, feedback, power-circuit, switching and amplifier procedure depends on the exact spindle system and machine configuration.