SP9031 alarm is generated by the spindle amplifier module when it detects a difference between the commanded spindle speed and the actual motor performance. Alarm code corresponds to AL-31 on the spindle drive’s seven-segment display. This fault condition triggers when the motor speed remains below the SST (Speed Setting Time) level continuously, activating what FANUC designates as a “Motor Restraint Alarm”.
Alarm Designation: SP9031 SSPA:31
Alarm Message: (S)Motor Lock / Motor Lock or Velocity Signal Loss
Amplifier Display Code: 31
System Impact: Complete spindle shutdown and machine stoppage.
The fundamental mechanism behind this alarm involves the serial communication between the CNC control unit and the spindle amplifier module. Fanuc Controller continuously monitors feedback signals from the speed sensor or encoder mounted on the spindle. When the amplifier detects that actual motor rotation does not match the commanded speed profile, it immediately triggers the SP9031 alarm to prevent potential mechanical damage or safety hazards.
Root Causes of FANUC Alarm SP9031
Based on field experience and technical documentation analysis, the SP9031 alarm can be categorised into two primary scenarios, which are following .
Scenario 1: Motor Rotates at Very Low Speed
When the spindle motor attempts to rotate but only achieves minimal speed before triggering the alarm, the following causes are most probable:
Parameter Configuration Errors: Incorrect parameter settings related to the spindle sensor configuration represent the most common software-related cause. The sensor type, pulse-per-revolution (PPR) count, and related parameters must precisely match the physical hardware specifications. Parameters such as No. 4001 (sensor type selection), and sensor-related settings documented in the FANUC AC Spindle Motor Parameter Manual.
Motor Phase Sequence Incorrectly Wired: The three-phase power connection sequence to the spindle motor must follow the correct order (U-V-W). An incorrect phase sequence will cause the motor to attempt rotation in the wrong direction or produce insufficient torque, resulting in the motor lock condition. This is particularly common after motor replacement or electrical maintenance work.
Feedback Cable Signal Reversal: Encoder or sensor feedback cable carries critical A-phase and B-phase quadrature signals that indicate rotational direction and speed. If these signals are reversed during cable installation, the control system receives inverted feedback, creating a conflict between commanded and detected motion.
Speed Sensor Physical Damage or Obstruction: Beta zi Sensor mounted on the spindle motor shaft can become physically damaged, obstructed by chips, contaminated or displaced from its mounting position. External encoders connected via timing belts are particularly susceptible to belt damage, coupling failure, or pulley misalignment.
Feedback Cable Degradation: Sensor cable running from the motor to the amplifier module experiences mechanical stress, electromagnetic interference, and environmental exposure. Internal conductor breaks, insulation failure, or connector pin damage can interrupt the critical feedback signals.
Scenario 2: Motor Does Not Rotate At All
When the spindle motor remains completely stationary despite receiving rotation commands, these causes should be investigated:
Mechanical Spindle Lock Engaged: Many CNC machines employ a mechanical spindle orientation lock or brake mechanism for tool changes. If the unlock sequence is incorrect or the mechanical lock fails to disengage, the motor cannot rotate despite receiving electrical commands.
Power Supply Cable Failure: The high-current power cables connecting the spindle amplifier to the motor terminals can develop connection issues, loose terminals, or conductor breaks. Inadequate terminal tightening torque during installation often leads to overheating and connection failure. In below image we can see that power cable was burnt at motor terminal so spindle motor could not get the sufficient power to rotate that’s why it triggered alarm SP 9031.
Spindle Motor Overload or Mechanical Binding: Excessive cutting loads, material jammed in the spindle, broken tools, or mechanical bearing seizure can physically prevent motor rotation. The control system detects the inability to achieve commanded speed and triggers the alarm.
Video of Spindle bearing failure and there is abnormal sound from Spindle motor
Sensor or Feedback Cable Complete Failure: A totally failed speed sensor or speed sensor gap disturbed or speed sensor damaged or scratch on spindle speed sensor during spindle mounting /unmounting condition or completely severed feedback cable, or disconnected connector provides no speed information to the Fanuc controller, resulting in immediate alarm activation.
Spindle Power Module (SPM) Failure: The solid-state power electronics within the spindle amplifier module can fail due to thermal stress, overvoltage transients, or component aging. A failed SPM cannot deliver the required current to drive the motor.
Timing Belt Failure (External Encoder Systems): For spindles equipped with external shaft-mounted encoders, timing belt connecting the spindle shaft to the encoder pulley can break, become excessively loose, or jump teeth. This disconnects the mechanical coupling between actual spindle rotation and encoder feedback.
Comprehensive Step-by-Step Diagnostic Procedure
Systematic troubleshooting follows a logical sequence from simple verification steps to complex component testing & diagnostic methodology:
Step 1: Initial System Assessment and Alarm Documentation
Before physical intervention, document the complete alarm condition. Record the exact alarm number, message text, machine operating mode when the alarm occurred (MDI, Auto, Manual), commanded spindle speed, and any preceding events such as heavy cutting loads or power interruptions. Check the diagnostic display for additional fault codes that may provide supplementary information about the failure mode.
Access the CNC diagnostic screens to view spindle load meter readings (typically shown as a percentage at the bottom of the position screen). If the load meter shows extremely high values (above 100-150%) even without cutting load, this strongly suggests mechanical binding, sensor issues, or motor problems.
Step 2: Visual and Physical Inspection
Safety First: Ensure the machine is placed in emergency stop condition, main power is disconnected, and appropriate lockout/tagout procedures are followed before beginning physical inspection.
Conduct a thorough visual inspection of the spindle motor assembly. Look for obvious signs of damage, coolant contamination, oil leaks, or physical impacts to the motor housing. Check that all cooling fans are operational and air passages are not blocked.
Mechanical Freedom Test: With power isolated and the spindle unlocked, attempt to manually rotate the spindle by hand. The spindle should rotate freely with consistent resistance. Unusual grinding, binding, or rough spots indicate mechanical problems such as bearing failure or internal damage that must be resolved before electrical troubleshooting continues
Step 3: Power and Connection Verification
Verify that all power supply connections are secure and properly terminated. Check the three-phase power cables from the spindle amplifier to the motor, ensuring terminal block screws are tightened to specified torque values. Loose power connections cause arcing, overheating, and intermittent faults.
Inspect the motor feedback cable from the amplifier connector (typically labeled JYA2 depending on amplifier model) to the motor sensor connector. Look for physical damage, cuts, excessive bending, pinch points, or areas where coolant may have penetrated the cable jacket. Verify connector pins are not bent, corroded, or pushed back in the connector housing.
Step 4: Spindle Drive Belt and Mechanical Coupling Inspection
For spindles utilizing belt drive transmission, inspect the drive belt connecting the spindle motor to the spindle shaft. Check for proper tension—the belt should not be excessively tight or loose. A loose belt will slip under load, preventing the motor from driving the spindle effectively. A broken or damaged belt requires immediate replacement.
For machines equipped with external encoders, inspect the encoder mounting and the timing belt connecting the spindle shaft to the encoder shaft. Verify the encoder coupling is intact and properly secured. A broken coupling disconnects the feedback signal from actual spindle rotation
Step 5: Spindle Sensor and Encoder Testing
The speed feedback sensor is critical for closed-loop spindle control. Access the sensor according to your specific motor model—some sensors are located behind a red protective cap at the rear of the motor, requiring fan removal for access.
Visual Sensor Inspection: Check that the sensor is properly secured and has not moved from its mounting position. Verify the sensor gap or alignment relative to the rotating encoder disc or magnetic ring. Physical obstruction by metal chips, coolant residue, or debris will prevent proper signal generation.
Electrical Signal Testing: Using diagnostic displays on the CNC or spindle amplifier diagnostic board, check for the presence of speed feedback signals. Slowly rotate the spindle by hand and observe whether speed pulses are displayed. Absence of pulses indicates sensor failure, cable break, or connection problems.
Step 6: Parameter Verification and Configuration Check
Access the CNC parameter settings and systematically verify all spindle-related parameters. Key parameters requiring verification include:
Parameter 4001: Spindle motor and sensor type selection. This parameter defines whether the system uses an internal motor-mounted sensor, external encoder, or other feedback device.
Sensor PPR Parameters: The pulses-per-revolution value must exactly match the physical encoder specification. Common values include 1024, 2048, or 4096 PPR. A mismatch causes incorrect speed calculation and alarm conditions.
Parameter 4080: Regenerative power limit setting affects how the drive handles braking and deceleration.
Parameter 4082: Acceleration/deceleration time constant. Insufficient time constants can cause the motor to exceed torque limits during rapid speed changes, triggering overload protection
Phase Sequence Parameters: Certain parameters define the expected phase sequence relationship between the motor rotation direction and encoder direction. Verify these match the actual mechanical configuration.
Step 7: Systematic Component Isolation Testing
If previous steps have not identified the fault, systematic component substitution helps isolate the defective element:
Feedback Cable Replacement: Replace the motor feedback cable with a known-good spare. Cable failures can be intermittent and difficult to detect with standard electrical testing. Ensure proper cable routing away from high-current motor power cables to minimize electromagnetic interference. We have faced this alarm was triggered when one wire of feedback cable(shown in below image) was out from the connector. So it is necessary to check feedback cable also.
Sensor/Encoder Replacement: If feedback cable replacement does not resolve the alarm, replace the speed sensor or encoder assembly. When installing a new encoder, verify the PPR specification matches the original unit exactly. After mechanical installation, adjust sensor alignment and air gap according to FANUC service procedures.
Spindle Amplifier Module Testing: Advanced troubleshooting may require spindle amplifier testing or substitution. This requires qualified personnel with appropriate test equipment and knowledge of high-voltage safety procedures
Step 8: Post-Repair Testing and Verification
After corrective actions are completed, follow a safe restart procedure:
- Ensure all guards, covers, and safety interlocks are properly installed and functional.
- Power on the CNC control and verify no alarms are present at startup.
- Command the spindle to rotate at a low speed (typically 100-300 RPM) using MDI mode with an M03 or M04 command.
- Gradually increase spindle speed in increments, monitoring the load meter, motor sound, and vibration at each speed level.
- Verify proper spindle orientation (M19) function if applicable to your machine configuration.
- Conduct test cuts at various speeds and loads to confirm stable operation under real working conditions.
Critical Safety Precautions for Diagnosis and Repair
Working on spindle systems involves significant electrical, mechanical, and rotational hazards. Every technician must strictly observe these safety protocols:
Electrical Safety
Complete Power Isolation: Before any physical work on the motor, cables, or amplifier components, disconnect all electrical power at the main circuit breaker. Use proper lockout/tagout devices with personal locks to prevent accidental energization. Verify zero voltage at all terminals using a calibrated voltage meter before proceeding.
High Voltage Awareness: Spindle amplifiers contain high-voltage DC link capacitors that can retain lethal voltage for several minutes after power shutdown. Wait a minimum of five minutes after power disconnection and verify zero voltage before touching any terminals or internal components[. The discharge time may be longer for larger amplifier models.
Proper Grounding: Ensure the spindle motor and amplifier cabinet are properly grounded to building earth ground. Verify continuity of ground connections periodically as part of preventive maintenance.
Connector Handling: When disconnecting feedback cables or connectors, do not pull on the cable itself—grasp the connector body firmly. Avoid touching connector pins with bare hands, as skin oils can cause corrosion. If coolant contamination is present in connectors, clean thoroughly with electrical contact cleaner and dry completely before reconnection.
Mechanical Safety
Rotating Machinery Hazards: Never approach rotating spindles or attempt to touch moving parts. Ensure all machine guards and interlocks are functional before operating. Keep loose clothing, hair, jewelry, and tools away from rotating components
Manual Spindle Rotation: When manually rotating the spindle for testing, ensure all cutting tools are removed, the work area is clear, and no one else is in the vicinity of the machine. Apply rotation force gradually and be alert for unexpected movement.
Mechanical Lock Verification: Before assuming a motor fault exists, positively verify the mechanical spindle lock or brake is fully disengaged. Attempting to electrically drive a mechanically locked spindle can damage the motor, amplifier, or mechanical components.
Tool Holder Removal: Remove all tool holders and cutting tools before spindle testing to eliminate additional rotating mass and reduce risk of tool ejection.
Diagnostic Safety
Emergency Stop Accessibility: Ensure emergency stop buttons are immediately accessible and functional before conducting any spindle rotation tests. Test the emergency stop function before beginning troubleshooting work.
Incremental Testing: When testing spindle rotation after repairs, always start with the lowest possible speed and increase gradually. Monitor for unusual vibration, noise, or heating. Be prepared to immediately activate emergency stop if abnormal conditions develop.
Personnel Clearance: Ensure all personnel are clear of the machine’s operating envelope before initiating spindle rotation. Establish clear communication protocols when multiple technicians are working together.
Diagnostic Tool Safety: When using oscilloscopes or multimeters for electrical measurements, use proper test lead insulation and probe guards. Connect measurement equipment before applying power and disconnect after power is removed.
Component Handling Safety
Motor Lifting: Spindle motors are heavy precision components. Use appropriate lifting equipment (hoists, cranes) with properly rated slings when removing or installing motors. Never attempt to manually lift motors that exceed safe manual handling limits.
Static Discharge Protection: Electronic components such as sensors, encoder boards, and amplifier modules are sensitive to electrostatic discharge. Use proper ESD wrist straps and work surfaces when handling these components.
Environmental Contamination: Protect electronic components from coolant, cutting oil, and metal chip contamination during disassembly and reassembly. Cover open motor housings and connectors with clean protective material when work is interrupted.
Documentation and Communication Safety
Alarm Documentation: Thoroughly document all alarm conditions, symptoms, and troubleshooting steps performed. This information is invaluable for recurrent problems and supports continuous improvement.
Parameter Change Documentation: Record all parameter values before making changes. Document the original value, new value, and reason for the change. Incorrect parameter settings can cause unpredictable machine behavior and safety hazards.
Management Communication: Inform production management and machine operators of any safety concerns discovered during diagnosis. Do not return a machine to service if any safety-related defects remain unresolved.
Preventive Measures to Avoid Future SP9031 Alarms
Proactive maintenance significantly reduces the occurrence of spindle alarms and extends equipment life:
Regular Feedback Cable Inspection: Implement quarterly inspection schedules for all motor feedback cables. Look for abrasion, flex fatigue at cable entry points, and connector wear. Replace cables showing any signs of degradation before failure occurs.
Proper Cable Routing and Support:
- Ensure feedback cables are routed separately from motor power cables to minimize electromagnetic interference.
- Provide adequate support and strain relief to prevent mechanical stress.
- Environmental Protection: Implement effective coolant management and chip evacuation systems to prevent contamination of electrical components.
- Consider splash guards or protective bellows for particularly harsh environments.
- Parameter Backup: Maintain current backups of all CNC parameters. After any parameter changes, create a new backup with clear documentation of what was changed and why
- Spindle Load Monitoring: Train operators to monitor spindle load meter readings during operation. Excessive loads indicate inappropriate cutting parameters or tool problems that can lead to motor damage over time.
- Scheduled Encoder Maintenance: For external encoder systems, implement preventive replacement of timing belts and couplings at manufacturer-recommended intervals before failure occurs.
- Thermal Management: Ensure spindle motor cooling systems (fans, liquid cooling) are properly maintained. Monitor motor operating temperature and investigate any temperature increases.
Conclusion
FANUC alarm SP9031 (S)Motor Lock represents a critical spindle system fault requiring systematic diagnostic approach and strict adherence to safety protocols. As detailed in this comprehensive blog, the alarm stems from either complete motor stoppage or inability to achieve commanded speed, caused by mechanical issues, feedback system failures, parameter errors, or component faults. Successful resolution demands methodical troubleshooting beginning with basic visual inspection and progressing through electrical testing, parameter verification, and component isolation. Every diagnostic step must be performed with paramount attention to electrical and mechanical safety, including proper lockout/tagout procedures, high-voltage precautions, and controlled testing protocols. The key to minimizing downtime from SP9031 alarms lies in preventive maintenance—regular inspection of cables and sensors, proper environmental protection, load monitoring, and systematic documentation. By implementing the procedures outlined in this guide, maintenance technicians can efficiently resolve current alarm conditions and establish practices that prevent future occurrences.
Remember that spindle systems represent significant investments in both capital and production capability. When alarm conditions exceed your facility’s technical capabilities, do not hesitate to engage qualified FANUC service engineers who possess specialized training, diagnostic equipment, and access to technical resources necessary for complex troubleshooting scenarios. Your commitment to safety, systematic diagnosis, and professional standards ensures reliable CNC operation and protects both personnel and equipment assets.
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