FANUC SP9050 Alarm 50: Excessive Speed Calculation in Spindle Synchronization

FANUC SP9050 means that an internal speed-command calculation exceeded its allowable range during spindle synchronous control. The CNC may display SP9050 or SSPA:50 SYNCRONOUS VALUE IS OVER SPEED, while the spindle amplifier displays 50.

The basic 0i-D alarm description tells technicians to check whether the specified spindle speed multiplied by the active spindle-to-motor gear ratio exceeds the maximum motor speed. FANUC's detailed αi- and βi-series amplifier manuals add a second branch: if the gear-ratio data is correct, check the spindle-synchronization position gain. For the applicable systems, FANUC instructs technicians to increase the active position-gain value—not decrease it—when the position-gain calculation limit is the cause.

For nearby serial spindle alarms, see the FANUC Spindle Alarm Codes SP9034–SP9092.

What Does FANUC SP9050 SSPA:50 Mean?

SP9050 is generated inside spindle synchronous control. It is not a general alarm for every spindle overspeed condition, and it does not necessarily mean that the measured spindle has physically accelerated beyond its mechanical limit.

FANUC uses two complementary descriptions:

These descriptions are not contradictory. A high speed command or incorrect gear ratio can make the motor-speed conversion invalid. A position gain that is too low for the required synchronous speed can also make the internal position-error calculation exceed its representable range.

The word “SYNCRONOUS” is misspelled in the original FANUC display message. Keep that spelling when identifying the exact screen text; use the standard spelling “synchronous” in the explanation.

How FANUC Spindle Synchronous Control Works

Spindle synchronous control coordinates two spindles so they rotate at the same commanded speed. Phase synchronous control can additionally align their angular positions. FANUC documents this function for machines such as twin-spindle lathes, where a workpiece is transferred between facing spindles or held by both spindles during acceleration and deceleration.

The control must account for each spindle's:

• Commanded spindle-end speed

• Motor-to-spindle transmission ratio

• Selected mechanical gear or clutch state

• Position feedback and synchronous position error

• Position gain and velocity-loop settings

• Rotation direction and synchronization sequence

The CNC therefore performs calculations beyond simply sending the same rpm number to two motors. If one spindle uses a 1:1 belt ratio and the other uses a 2.5:1 gear ratio, their motors require different speeds even though both spindle noses rotate at the same spindle speed.

For example:

Commanded synchronous spindle speed = 3,000 min⁻¹
Active motor/spindle ratio = 2.5
Calculated motor speed = 3,000 × 2.5 = 7,500 min⁻¹

If the applicable motor is limited to 6,000 min⁻¹, the command and selected gear are not valid together. Reduce the commanded speed or use an approved gear range; do not raise the motor-speed limit beyond the motor or spindle specification.

Why Position Gain Can Also Cause Alarm 50

Parameters 4065–4068 set the spindle position gain used in servo mode and spindle synchronous control for the selected gear range. FANUC's parameter manual shows that, at constant synchronous speed, the expected number of position-error pulses is inversely proportional to the position gain:

Position error ∝ synchronous spindle speed ÷ position gain

As position gain becomes smaller, more stored position error is required to represent the same synchronous speed. If the required internal value exceeds the control's allowable calculation range, alarm 50 can result. This explains why FANUC's official alarm-50 procedure says to increase the position gain when the gear-ratio data is correct.

This instruction must be applied carefully. Excessive gain can cause overshoot, hunting, vibration, high current, or unstable phase matching. FANUC's startup procedure requires checking for overshoot and hunting before reaching the maximum synchronization speed. The active position-gain value must also be coordinated between the first and second spindles.

Parameters 4056–4059: Spindle-to-Motor Gear Ratio

For the applicable 16i, 30i, and 0i spindle parameter groups, parameters 4056–4059 contain the motor-to-spindle ratio for four gear states. The active parameter is selected by the corresponding CTH1 and CTH2 PMC inputs.

With the standard increment system, the stored number represents the motor/spindle ratio multiplied by 100. A 2.5:1 ratio is entered as 250. If parameter 4006#1 (GRUNIT) selects the 1/1000 increment system, the same 2.5:1 ratio is entered as 2500.

The actual gear or clutch state must agree with the CTH signals. An individual spindle has its own spindle-parameter data and corresponding A- or B-channel gear-selection inputs. Check both synchronized spindles; do not inspect only the first spindle or assume the two mechanical ratios are identical.

Parameter numbering differs on some earlier controls. FANUC's older α-series manual lists 6556–6559 for certain FS0 systems and 3056–3059 for Series 15i. Apply the number range shown in the English manual for the installed CNC and spindle software.

Parameters 4065–4068: Synchronization Position Gain

On the applicable systems, parameters 4065–4068 select the spindle position gain for the same four CTH gear states.

The B-65280EN/05 standard setting shown for this αi-series table is 1000, corresponding to 10 s⁻¹, but the correct commissioned value depends on the exact spindle system and machine. Do not replace verified machine-builder data with the manual's standard value without confirming the complete application.

For spindle synchronous control, FANUC states that the same position-gain value must be specified for the first and second spindles. Because each spindle's CTH signals can select a different member of the 4065–4068 group, compare the active value on both spindle channels rather than comparing only the same parameter address.

1. Confirm the Exact Alarm and Capture the State

Before changing the program or parameters, record:

• The complete CNC message and 50 on the spindle-amplifier display

• CNC model, spindle-amplifier models, and spindle software series/edition

• Which physical units are the first and second synchronized spindles

• Whether speed synchronization only or phase synchronization is active

• The synchronous spindle-speed command at the moment of the alarm

• Actual spindle and motor speeds shown for both spindle channels

• Actual mechanical gear range for each spindle

• Applicable CTH1A/CTH2A and CTH1B/CTH2B states

• Active values from 4056–4059 and 4065–4068 for both spindle parameter groups

• Each motor's exact model, suffix, and maximum permitted speed

• Any recent motor, gearbox, PMC, amplifier, or parameter work

Alarm timing is useful. An alarm that appears immediately when a high synchronous speed is entered points first toward speed conversion or gear-ratio data. An alarm that occurs even at a moderate speed after parameter restoration may point toward a ratio-scaling or position-gain problem.

2. Calculate the Motor Speed for Both Spindles

For each spindle, use the actual selected gear ratio:

Calculated motor speed
= |commanded spindle-end speed| × |motor rotations per spindle rotation|

Compare the result with the verified maximum speed for that exact motor and spindle arrangement. Perform this check for both synchronized spindles because their mechanical ratios and motor limits can differ.

Review the program, macro variables, PMC speed command, spindle override, constant-surface-speed state, synchronization cycle, and gear-selection M code. The executed speed may not be the value shown in the currently highlighted program block.

If the calculation is excessive, correct the command or approved gear selection. Do not repeat an invalid high-speed test merely to see whether the alarm recurs.

3. Verify Motor-Specific Parameters

Compare the spindle data against:

• The exact motor nameplate and every suffix

• The installed spindle amplifier and spindle software

• A verified machine backup or machine-builder commissioning sheet

• The applicable English FANUC Spindle Motor Parameter Manual B-65280EN

• The specified maximum motor speed and motor-model data

On applicable 16i/30i parameter tables, parameter 4020 represents maximum motor speed. Other generations may display another number or derive values through a different initialization procedure. Do not increase the value above the motor specification to clear SP9050.

Motor-parameter initialization is a later recovery step, not a universal first action. It can overwrite machine-specific gear, detector, orientation, synchronization, and tuning data. Initialize only when parameter loss or incompatibility is confirmed and the exact motor code, English procedure, and original machine values are available.

4. Verify Gear Ratios and GRUNIT Scaling

For both spindle channels:

1. Identify the physically engaged gear or clutch.

2. Confirm the corresponding CTH1/CTH2 state on the PMC or spindle monitor screen.

3. Determine which of 4056–4059 is active.

4. Calculate the real ratio from motor revolutions per spindle revolution.

5. Confirm the stored data uses the scaling selected by 4006#1 (GRUNIT).

6. Compare the parameter with a known-good backup and the machine drawing.

Common errors include a reversed ratio, a misplaced zero, an incorrect 1/100 versus 1/1000 unit, a default 1:1 setting on a geared spindle, or CTH signals that do not match the engaged range.

Do not reduce the ratio parameter simply because it lowers the calculated motor speed. These parameters describe the real transmission. FANUC warns that incorrect gear-ratio data can cause unexpected spindle operation, including failure to stop normally during orientation.

5. Check the Active Synchronization Position Gain

Only after the speed and gear-ratio checks are correct should parameters 4065–4068 be evaluated.

• Identify the active position-gain parameter for each spindle from its CTH state.

• Confirm that the active gain data agrees between the two synchronized spindles.

• Compare the settings with the original machine backup and commissioning data.

• Check whether a very low or zero value was introduced by editing or parameter restoration.

• Review the expected position error for the commanded synchronous speed.

For applicable Series 16i systems, B-65280EN/05 lists diagnosis 414 for first-spindle position error, 415 for second-spindle position error, and 416 for the absolute synchronous error. Applicable Series 30i documentation lists different diagnostic addresses, including 418 and 425. Use the exact manual for the CNC rather than assuming one diagnostic number is universal.

If the gear-ratio data is correct and the position-gain calculation limit is confirmed, FANUC's alarm-50 procedure says to increase the synchronization position gain. Make controlled changes according to the machine-builder or FANUC adjustment procedure, with a complete backup and qualified personnel.

Do not apply the technician note that recommends reducing 4065–4068 as a generic SP9050 fix. Lowering gain increases the theoretical position-error count required at a given speed and moves the calculation in the wrong direction for the specific alarm-50 “position gain setting limit” described by FANUC.

6. Check Gear Selection and Synchronization Sequence

The parameter can be correct while the wrong parameter is active. Use the ladder diagram and PMC diagnostics to verify:

• Both spindles report their actual gear states correctly.

• Gear changes are complete before synchronous control is enabled.

• CTH inputs remain stable throughout acceleration and synchronization.

• The correct speed-synchronization command is supplied.

• Both spindle rotation commands and directions match the intended sequence.

• The control waits for the synchronization-completion signal before phase-dependent machine motion.

Do not force CTH, SPSYC, SPPHS, or completion signals merely to bypass the alarm. Forced signals can create a mismatch between the CNC's internal state and the actual spindle mechanics.

7. Retest at a Verified Safe Speed

After correcting the confirmed cause, begin below the calculated motor-speed limit. Monitor both spindle and motor speeds, active gear states, position errors, synchronous error, load, vibration, and completion signals.

Increase the synchronous speed gradually only under the applicable commissioning procedure. FANUC instructs technicians to verify that overshoot or hunting does not occur before the maximum synchronization speed is reached. Stop if either spindle becomes unstable, reports abnormal load, or generates another alarm.

If a different alarm appears, diagnose the new number separately. Alarm C8, C9, a position-coder alarm, or an overload alarm describes a different detected condition and should not automatically be treated as SP9050.

What Not to Do for SP9050

• Do not raise the motor maximum-speed setting beyond the motor specification.

• Do not falsify 4056–4059 to reduce the calculated speed.

• Do not decrease 4065–4068 as a generic alarm-50 remedy; FANUC specifies increasing the gain when the ratio is correct and the gain limit is confirmed.

• Do not increase position gain without checking both spindle channels for equal active gain and monitoring stability.

• Do not initialize spindle parameters without a full backup and exact motor data.

• Do not bypass gear-selection or synchronization interlocks.

• Do not replace the feedback cable, position coder, spindle motor, or spindle amplifier solely because SP9050 is displayed.

SP9050 is an internal calculation-range alarm. Hardware investigation becomes appropriate only when another alarm or measured evidence identifies a separate hardware problem.

Difference Between SP9049, SP9050, C8, and C9

SP9049 is the preceding differential-speed alarm and is covered separately.

FANUC SP9050 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm SP9050/SSPA:50 on the CNC and 50 on the spindle amplifier.

2. Record both spindle channels, commanded and actual speeds, active gear states, CTH signals, motor models, and alarm timing.

3. Calculate motor speed for each spindle from the synchronous spindle command and actual motor/spindle ratio.

4. Reduce an excessive command or select an approved operating gear; never exceed the verified motor limit.

5. Confirm motor-specific data and maximum-speed settings against the correct English FANUC manual and machine backup.

6. Verify parameters 4056–4059, GRUNIT scaling, physical gear state, and CTH selection for both spindles.

7. If the gear-ratio data is correct, identify the active 4065–4068 position-gain value on each spindle.

8. Confirm equal active gain values and review the position-error diagnostics.

9. Increase the synchronization position gain only under the applicable FANUC or machine-builder adjustment procedure while monitoring overshoot and hunting.

10. Retest synchronization from a calculated safe speed and verify speed and phase completion signals.

Technical References

• FANUC Series 0i-MODEL D / 0i Mate-MODEL D Maintenance Manual, B-64305EN/03, SP9050 / SSPA:50 alarm description

• FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Alarm Code 50 troubleshooting

• FANUC AC Servo Motor βi Series, AC Spindle Motor βi Series, Servo Amplifier βi Series Maintenance Manual, B-65325EN/02, SP9050 troubleshooting

• FANUC AC Spindle Motor αi/βi Series Parameter Manual, B-65280EN/05, Spindle Synchronous Control, gear-ratio parameters, position-gain parameters, and diagnostics

All references above are English-language FANUC manuals. Always use the edition that applies to the exact CNC, spindle amplifier, spindle software, motor, transmission, and machine-builder configuration.

Need Help With FANUC SP9050 Alarm 50?

REACO CNC provides independent troubleshooting, testing, repair, compatibility checking, and replacement support for FANUC spindle systems. Send us the CNC alarm screen, amplifier display, complete CNC and amplifier models, spindle software information, both motor nameplates, commanded speed, actual gear states, CTH signals, parameters 4056–4059 and 4065–4068 for both spindles, and details of when the alarm occurs.

If testing identifies a drive-hardware requirement, browse FANUC servo, spindle, and power drives. For diagnostic assistance, visit the FANUC Technical Support Center or contact REACO CNC.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation. The applicable speed limit, parameter number, gain, gear ratio, diagnostic address, and commissioning procedure depend on the exact CNC, spindle software, motor, gearbox, and machine.