FANUC SP9057 Alarm 57: Excessive Deceleration Power and Troubleshooting

FANUC SP9057 means that the regenerative resistor circuit associated with an applicable αiPSR power supply has detected excessive deceleration energy, abnormal resistor temperature, an open resistor circuit, or an abnormal resistance condition. The CNC reports SP9057—often with the abbreviated message CONV. EX. DECELERATION POW.—the spindle amplifier indication is 57, and the associated Power Supply (PS) may display H.

FANUC's documented troubleshooting direction is to reduce acceleration/deceleration duty, check the cooling environment, inspect the regenerative resistor cooling fan, verify the resistor wiring and specification, and replace the resistor if its measured resistance is abnormal. A power supply replacement is not the first conclusion from SP9057 alone.

For the surrounding serial spindle alarm numbers, see the FANUC Spindle Alarm Codes SP9034–SP9092.

What Does FANUC SP9057 Mean?

During deceleration, a spindle or servo motor can return electrical energy to the DC link. In an αiPSR regenerative-resistor-type power supply system, that energy is dissipated as heat by the specified regenerative discharge resistor. SP9057 is generated when the system determines that this regenerative path is operating abnormally or beyond its allowable thermal or electrical condition.

FANUC documentation identifies three closely related detection conditions:

• An overload was detected in the regenerative resistance.

• Thermostat operation or a short-time overload was detected.

• The regenerative resistor was disconnected, or an abnormal resistance was detected.

The alarm therefore does not prove that the resistor itself has failed. It can also result from excessive regenerative duty, inadequate cooling, a stopped resistor fan, incorrect resistor selection, loose or open wiring, or an abnormal thermostat circuit. The timing of the alarm is an important diagnostic clue.

SP9057 Is a Power-Supply-Side Regenerative Alarm

Although SP9057 appears on the CNC and is reported through the spindle alarm system, the condition is associated with the converter/common power supply and its regenerative discharge circuit. Do not begin by treating it as a spindle position sensor, motor feedback, orientation, or CNC program-memory alarm.

The distinction between αiPSR and αiPS also matters. The αiPSR configuration uses a regenerative discharge resistor to absorb returned energy. Other FANUC power-supply configurations can handle regeneration differently and use different alarm indications and procedures. Before testing or ordering parts, record the complete power supply model and confirm that the installed unit and resistor arrangement match the applicable FANUC manual.

Why Deceleration Creates Regenerative Power

When a rotating spindle or feed axis slows down, its motor acts temporarily as a generator. The returned energy must be handled by the drive system. The amount of regenerative energy increases with factors such as rotating inertia, speed change, deceleration rate, and how often the cycle repeats.

A single aggressive stop can create a high short-term regenerative load. Repeated starts and stops can raise the resistor's average temperature even when each individual deceleration appears normal. This is why SP9057 may occur only after several cycles, during a particular production program, or after cycle time has been shortened.

The correct response is not simply to make every deceleration slower. First determine whether the alarm is caused by actual operating duty, cooling, wiring, or a defective or incorrectly specified resistor. Any program or parameter change must remain within the machine builder's limits and preserve machining and safety requirements.

1. Confirm All Three Alarm Displays

Before switching off the machine, record the displays and operating state:

• Confirm SP9057 or CONV. EX. DECELERATION POW. on the CNC alarm screen.

• Confirm 57 on the spindle amplifier display.

• Check whether the Power Supply displays H.

• Record whether a servo alarm appeared at the same time.

• Note the exact cycle step, commanded speed, load, and whether the alarm occurred during acceleration, deceleration, steady running, or power-up.

Also record:

• CNC series and model

• Complete Power Supply model and every suffix

• Complete spindle and servo amplifier models

• Regenerative resistor or regenerative discharge unit part number

• Whether the resistor has forced-air cooling

• Any recent program, parameter, motor, drive, resistor, fan, or cabinet-cooling change

Do not rely only on the English wording shown by one CNC software version. The numerical combination—SP9057, amplifier 57, and Power Supply H where applicable—is the more reliable identification.

2. Determine When the Alarm Occurs

The occurrence pattern helps separate a duty problem from an electrical fault.

This table is a diagnostic guide, not a substitute for the model-specific circuit diagram. Multiple causes can exist at the same time—for example, a partially blocked fan can make a previously acceptable duty cycle trigger the alarm.

3. Reduce Excessive Acceleration/Deceleration Duty

FANUC's maintenance procedure for αiPSR Power Supply alarm H specifically directs technicians to reduce how frequently acceleration and deceleration occur when regenerative power is excessive. Review the program and machine cycle for:

• Repeated spindle start/stop commands

• Frequent spindle reversals

• Large speed changes with little time between cycles

• Rapid deceleration of a high-inertia spindle, chuck, fixture, or workpiece

• Simultaneous deceleration of multiple spindle or servo motors sharing the same Power Supply

• A recently shortened cycle time

• A new workholding configuration with greater rotating inertia

If the machine builder permits it, a controlled test may use a lower commanded speed, fewer repeated reversals, a longer interval between cycles, or a longer approved acceleration/deceleration time. The objective is to reduce regenerative duty and observe whether the alarm pattern changes.

Do not make unrecorded changes to spindle, servo, or power-supply parameters. Back up the current settings, identify the exact parameter function in the correct English manual, and follow the machine builder's procedure. An arbitrary time-constant change can affect cycle time, synchronization, orientation, stopping distance, or machine safety.

If SP9057 occurs during a standard machine cycle that previously ran reliably, investigate cooling and hardware before permanently slowing the process. A new alarm under an unchanged duty cycle often indicates that cooling performance, a connection, or the resistor assembly has deteriorated.

4. Check the Regenerative Resistor Installation and Wiring

The FANUC αiPSR procedure includes checking the wiring when the regenerative resistance is not detected. With power safely isolated and the DC link fully discharged according to the applicable manual, inspect the complete circuit between the Power Supply and the regenerative discharge unit.

In the illustrated 200-V input example, K41 is associated with the regenerative discharge resistance and K42 with the thermostat circuit. These designations and connector arrangements vary by Power Supply and regenerative unit, so use the diagram for the exact installed hardware.

Check for:

• A regenerative resistor that is missing or not fully connected

• Loose, backed-out, overheated, oxidized, or damaged terminals

• Broken conductors, damaged insulation, or heat-hardened cable

• Incorrect connector keying or a connector installed in the wrong receptacle

• Incorrect K41/K42 routing for the installed model

• An open thermostat circuit

• Evidence that a non-original resistor or cable was installed

• Incorrect grounding or mounting of the regenerative discharge unit

Do not bypass the thermostat or short the alarm circuit to keep the machine running. The thermostat is part of the protection system. Bypassing it can hide an actual overtemperature condition and create a fire or burn hazard.

5. Verify the Regenerative Resistor Specification

FANUC lists insufficient regenerative resistance as a cause of alarm H and directs technicians to review the resistor specification. “Insufficient” refers to the regenerative capability of the installed arrangement, not merely whether a resistor is physically present.

Confirm all of the following against the applicable FANUC and machine-builder documentation:

• Complete regenerative discharge unit part number

• Rated resistance

• Continuous and short-time regenerative capacity

• Natural-cooled or forced-cooled configuration

• Required fan voltage and airflow, where applicable

• Power Supply compatibility

• Cable and connector specification

• Installation clearance and mounting orientation

Do not select a replacement by appearance, connector shape, or a generic resistance value. A resistor with the wrong resistance or power rating can cause repeat alarms or damage the regenerative circuit. Never substitute a lower-resistance unit in an attempt to absorb more energy unless the exact combination is explicitly approved for that Power Supply.

6. Inspect Cooling, Cabinet Temperature, and Airflow

The SP9057 alarm list directs technicians to check the cooling condition and surrounding temperature. Inspect:

• Cabinet ambient temperature

• Cabinet air conditioner or heat exchanger operation

• Intake and exhaust filters

• Blocked vents or recirculating hot air

• Dust, oil mist, or debris on the regenerative unit

• Clearance around the resistor and its cooling path

• Heat from nearby components

• Fan direction and airflow

If the alarm appears only after the cabinet warms up, do not assume that a reset has repaired it. Allowing the resistor to cool may clear the thermostat temporarily, but the underlying duty or cooling problem remains.

The regenerative resistor can remain extremely hot after the machine stops. Follow the specified cooling time and do not touch the resistor, radiator, terminals, or adjacent metalwork until the equipment is safe.

7. Check the Regenerative Resistor Cooling Fan

Some regenerative discharge units use forced cooling. FANUC identifies a stopped regenerative resistor cooling fan as a cause of αiPSR alarm H. Confirm that the installed configuration is supposed to have a fan, then check:

• Whether the fan rotates when commanded

• Correct fan supply voltage

• Connector seating and cable condition

• Fan blade obstruction

• Contamination or bearing seizure

• Correct airflow direction

• Whether the fan starts intermittently or stops when warm

If the fan is defective, replace it with the exact specified fan or replace the complete regenerative discharge unit if the fan is not separately serviceable for that model. Do not fit an unverified general-purpose fan based only on physical size.

This fan should not be confused with the Power Supply internal cooling fan. A stopped Power Supply internal fan is associated with a different alarm condition, such as SP9059 on the systems covered by the cited alarm list.

8. Measure the Resistor and Thermostat Circuit Safely

Only qualified personnel should perform resistance or continuity tests. Isolate machine power, observe the specified DC-link discharge time, verify absence of dangerous voltage, and disconnect the circuit as required by the applicable manual before measuring it. Measuring through the connected power electronics can give a misleading result and may damage the meter or equipment.

Compare the measured value with the exact specification for the installed resistor. Do not use a universal “normal” resistance value; FANUC regenerative units differ by model and capacity.

Check:

• Resistor continuity

• Measured resistance against the specified tolerance

• Thermostat contact state at a safe, known temperature

• Intermittent opens caused by vibration or heat

• Cable continuity from the resistor assembly to the Power Supply

• Signs of arcing, cracked insulation, or overheated terminals

If the resistance is abnormal, replace the specified regenerative resistor or discharge unit. If the thermostat or internal wiring is integral to the assembly, follow the replacement procedure for that assembly rather than bypassing the failed protective device.

9. Evaluate the Power Supply Only After External Causes Are Eliminated

Replacing the common Power Supply is not FANUC's first listed remedy for SP9057/57. Consider Power Supply diagnosis only after the following have been confirmed:

• Operating duty is within the machine and drive specification.

• The correct regenerative resistor is installed.

• Resistor and thermostat wiring is intact.

• Resistance values meet the exact specification.

• Cooling fan and cabinet cooling are normal.

• Connectors and terminals are secure.

• The alarm persists under an approved diagnostic test.

At that point, use the model-specific FANUC maintenance procedure to determine whether the regenerative detection or switching circuit inside the Power Supply is faulty. Do not condemn the spindle amplifier merely because it relays the alarm to the CNC.

If a verified replacement is required, browse FANUC servo, spindle, and power drives or provide REACO CNC with the complete Power Supply, spindle amplifier, and regenerative resistor part numbers for compatibility checking.

Difference Between SP9057, SP9058, and SP9059

These alarms are all associated with the converter/common Power Supply, but they identify different monitored conditions.

Do not replace a regenerative resistor for SP9058 or SP9059 unless separate evidence identifies a resistor problem. Likewise, do not diagnose SP9057 solely as a Power Supply internal fan alarm. Always follow the exact displayed number.

Difference Between SP9057 Alarm 57 and Power Supply Alarm 8

Both conditions involve regenerative energy in an αiPSR system, but FANUC documents them separately:

Because CNC generations and drive combinations differ, record every active display rather than assuming that all regenerative alarms have the same remedy.

What Not to Do With SP9057

• Do not repeatedly reset the alarm and continue production without finding the heat source.

• Do not bypass the thermostat or short its input.

• Do not touch the regenerative resistor immediately after operation.

• Do not measure resistance until power is isolated and the DC link is safely discharged.

• Do not install a resistor selected only by ohms, physical size, or connector appearance.

• Do not increase acceleration or deceleration parameters without confirming the resulting regenerative duty.

• Do not assume that “replace the cooling fan” means any fan in the cabinet; identify the fan associated with the installed regenerative unit.

• Do not replace the spindle amplifier first when the Power Supply and regenerative circuit are reporting the abnormality.

FANUC SP9057 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm SP9057, amplifier indication 57, and Power Supply indication H where applicable.

2. Record the exact machine action, speed change, load, cycle frequency, and temperature when the alarm occurs.

3. Identify the complete Power Supply and regenerative discharge unit models.

4. Check whether acceleration/deceleration duty or rotating inertia recently increased.

5. Inspect cabinet temperature, airflow, filters, and the regenerative resistor cooling arrangement.

6. Isolate power, wait for the specified discharge time, and inspect resistor and thermostat wiring.

7. Verify that the installed resistor specification matches the Power Supply and machine design.

8. Test the specified cooling fan and replace it or the assembly if defective.

9. Measure the isolated resistor and thermostat circuit against the exact model specification.

10. Replace the resistor only when its condition or specification is confirmed abnormal.

11. Evaluate the Power Supply only after duty, cooling, wiring, fan, and resistor checks pass.

12. Retest with the machine builder's approved procedure and diagnose any new alarm separately.

Technical References

• FANUC Series 0i-MODEL D / Series 0i Mate-MODEL D Maintenance Manual, B-64305EN/03, SP9057 / Amplifier Indication 57

• FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Power Supply Alarm Code H (αiPSR)

• FANUC Servo Amplifier αi Series Descriptions, B-65282EN/05, αiPSR regenerative discharge resistor, thermostat, fan, and cable connections

Always use the English manual edition that applies to the exact CNC, Power Supply, regenerative discharge unit, servo/spindle amplifiers, and machine-builder configuration.

Need Help With FANUC SP9057 Alarm 57?

REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC power supplies, spindle amplifiers, servo amplifiers, cooling components, and regenerative assemblies. Send the CNC alarm screen, amplifier display, Power Supply display, complete part numbers and suffixes, resistor model, operating condition, and details of when the alarm occurs.

Visit the FANUC Technical Support Center for additional FANUC alarm guidance, or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The correct resistor specification, wiring, cooling arrangement, test method, and replacement procedure depend on the exact drive hardware and machine configuration.