FANUC SP9020 Alarm 20: V-Phase Current Offset Causes and Troubleshooting

FANUC SP9020 means that the offset voltage in the spindle amplifier's V-phase current detection circuit is excessively high. On CNC systems that use this alarm mapping, the CNC displays SP9020, while the spindle amplifier displays alarm 20.

FANUC maintenance manuals state that this offset check is performed when power is turned on. The normal repair direction is to replace the spindle amplifier module. If alarm 20 appears immediately after the spindle amplifier control printed-circuit board was replaced, check the connectors between the power unit and the control board before replacing the complete amplifier.

For the surrounding alarm numbers, see the FANUC Serial Spindle Alarm Codes SP9001–SP9033.

What Does FANUC SP9020 Mean?

The spindle amplifier contains current-detection circuits used to monitor spindle motor phase current. At power-on, the amplifier checks the initial offset of this circuitry. Alarm 20 is generated when the offset voltage associated with the V-phase current detection circuit is above the permitted range.

This is an internal current-measurement reference fault. It does not mean that excessive operating current is flowing through the spindle motor's V phase. The word “current” in the alarm name describes the circuit being checked, not a confirmed motor overcurrent event.

For that reason, SP9020 should not automatically be attributed to heavy cutting, rapid acceleration, a locked spindle, motor insulation failure, or a shorted V-phase power lead. Those conditions belong to other alarm procedures unless an additional alarm or test result specifically identifies them.

FANUC defines alarm 20 together with alarm 19:

The detection principle and official repair direction are paired, but the two alarms identify different phase-detection channels. Always record the exact number displayed by the amplifier.

Why SP9020 Normally Appears at Power-On

FANUC B-65285EN/04 and B-65325EN/01 specify that the offset check for alarms 19 and 20 is made when power is turned on. Alarm 20 therefore normally appears while the spindle amplifier is initializing, before ordinary spindle operation begins.

Record the precise timing:

• The alarm appears at every machine power-on.

• The alarm appears intermittently during startup.

• The alarm first appeared immediately after the control board was replaced.

• The alarm began after the amplifier was repaired, transported, removed, or reinstalled.

• The machine shows another spindle or power-supply alarm at the same time.

Timing is especially important when the control printed-circuit board has just been replaced. In that condition, FANUC specifically directs technicians to check the connectors between the power unit and control board. If no internal work preceded the alarm, the manual's primary direction is spindle amplifier replacement.

Repeated power cycling is not a repair. Even if an intermittent alarm clears temporarily, an unreliable connector or abnormal current-detection circuit can still be present.

1. Confirm SP9020 and Amplifier Alarm 20

Before removing hardware, confirm the alarm at both display locations where possible:

• The CNC shows SP9020, SSPA:20 EXCESS OFFSET CURRENT V, or the equivalent serial-spindle alarm used by the installed control series.

• The spindle amplifier status display shows 20.

• The amplifier does not show 12, 19, 22, or another current-related number.

Photograph or record the CNC alarm screen and amplifier display before switching off the machine. Also collect:

• The complete CNC series and model

• The full spindle amplifier, SPM, SVPM, or βiSVSP part number, including every suffix

• The control printed-circuit board number when it can be identified safely

• The spindle software series and edition when available

• The time and operating condition in which the alarm appears

• Details of any recent amplifier or control-board removal, repair, or replacement

The CNC-side code is not identical on every FANUC generation. Some systems use SP9020, while other CNC and spindle combinations may map amplifier alarm 20 to alarm 750, an SP12xx number, or a general serial-spindle alarm. The amplifier display 20, complete equipment identification, and the applicable FANUC manual are the correct diagnostic references.

2. Isolate Power Before Inspecting the Amplifier

Never remove, insert, or press a spindle amplifier control board into position while the unit is energized.

Before opening the electrical cabinet or touching the amplifier:

• Place the machine in the required safe state.

• Switch off and isolate the machine power.

• Wait for the DC link to discharge for the period specified in the applicable maintenance manual.

• Confirm that the charge indicator is off and verify the required electrical condition.

• Follow the machine builder's lockout and electrical-safety procedure.

Spindle amplifiers can retain hazardous voltage after incoming power is removed. Internal inspection and replacement must be performed by personnel qualified to service the specific FANUC amplifier and machine.

3. Check the Control-Board Connectors When Applicable

For αi spindle amplifier modules and βi SVPM units covered by the cited manuals, FANUC states that alarm 20 requires amplifier replacement. The manuals add one specific exception: if the alarm appears immediately after the control printed-circuit board was replaced, check the plugging of the connectors between the power unit and control board.

Where the repair history makes this check applicable, inspect for:

• A faceplate or control board that is not fully inserted

• Uneven seating or loose retaining hardware

• A partially engaged or misaligned board-to-power-unit connector

• Bent, contaminated, corroded, or visibly damaged contacts

• Damage caused during previous removal, transportation, repair, or installation

• A replacement control board whose complete part number or revision has not been confirmed

If the board is not installed correctly, remove and reinstall it only according to the procedure for the exact amplifier model. Inspect connector alignment before reinsertion and seat the assembly fully without forcing it.

The technician's recommendation to remove and reinsert the front control board is therefore technically reasonable when poor seating or recent board work is relevant. It should not be presented as a universal first step for every SP9020 case. The FANUC αi and βi SVPM procedures tie the connector check specifically to an alarm that appears immediately after control-board replacement.

Do not exchange boards between different amplifiers merely to test the alarm unless the exact part number, revision, software support, and approved service procedure have been confirmed.

4. Replace the Spindle Amplifier if Alarm 20 Remains

If alarm 20 is confirmed and the applicable control-board connectors are installed correctly, FANUC instructs technicians to replace the spindle amplifier, SPM, or SVPM unit.

Before selecting a replacement, verify:

• The complete FANUC amplifier part number and every suffix

• The amplifier family and hardware specification

• Rated input, output capacity, and supported spindle motor configuration

• The original control-board and connector arrangement

• Compatibility with the CNC, spindle motor, spindle software, and machine configuration

Do not select a replacement only by housing size, power rating, connector appearance, or a shortened part number. FANUC model suffixes and hardware revisions can affect electrical and software compatibility.

A qualified repair facility may perform board-level testing of the control board, power section, connectors, and V-phase current-detection circuitry where model-specific service documentation permits. However, the standard FANUC field action for a confirmed alarm 20 remains replacement of the spindle amplifier when the connector condition is correct.

If a tested replacement or repair assessment is required, browse FANUC servo, spindle, and power drives or send REACO CNC the full amplifier model and suffix for a compatibility check.

SP9020 Is Not a V-Phase Motor Overcurrent Alarm

Because the alarm name contains “current,” it is easy to treat SP9020 as a motor or power-circuit overcurrent. That interpretation is incorrect.

Motor power-cable and insulation tests may be appropriate when alarm 12, another related alarm, or an independent measurement points to a power-circuit fault. FANUC does not list them as the standard troubleshooting route for alarm 20.

Similarly, changing cutting conditions or reducing spindle load will not correct an internal V-phase current-detection offset that fails during power-on initialization.

Difference Between SP9019 and SP9020

SP9019 and SP9020 are companion current-detection offset alarms, but they must not be merged into one diagnosis:

If the amplifier display changes from 20 to 19, record the new number and diagnose the U-phase offset alarm separately. Do not continue calling it SP9020 simply because both alarms share the same general repair direction.

FANUC SP9020 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm the CNC indication and alarm 20 on the spindle amplifier display.

2. Record the complete CNC, amplifier, control-board, and spindle software identification.

3. Confirm that the alarm occurs during power-on initialization.

4. Determine whether the amplifier or its control board was recently replaced, repaired, removed, transported, or reinstalled.

5. Isolate machine power and wait for DC-link discharge according to the applicable safety procedure.

6. If the alarm began immediately after control-board replacement, inspect the connectors between the power unit and control board.

7. Correct any improper connector plugging or board seating, then retest at power-on.

8. If alarm 20 remains with the board and connectors installed correctly, replace the spindle amplifier with an exact compatible unit.

9. After replacement, confirm that SP9020 and amplifier alarm 20 no longer appear and complete the machine builder's spindle test.

10. If a different alarm appears, diagnose the newly displayed number separately.

Do not begin by changing spindle parameters, editing the PMC ladder, replacing the spindle motor, or adjusting cutting conditions. FANUC does not identify those actions as the standard remedy for alarm 20.

Technical References

• FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Alarm Codes 19 and 20

• FANUC AC Servo Motor βi Series, AC Spindle Motor βi Series, Servo Amplifier βi Series Maintenance Manual, B-65325EN/01, Alarm Codes 19 and 20

• FANUC AC Servo Motor βi-B/βi Series, AC Spindle Motor βi Series, Servo Amplifier βi-B Series Maintenance Manual, B-65425EN/02, SP9020 alarm mapping

Always use the manual edition that applies to the exact CNC, spindle amplifier family, control board, spindle software, and machine configuration.

Need Help With FANUC SP9020 Alarm?

REACO CNC provides independent FANUC spindle amplifier testing, repair, compatibility checking, and replacement support. Send us the CNC alarm screen, amplifier display, complete CNC model, exact spindle amplifier model and suffix, control-board number when available, and details of any recent amplifier work.

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 connector inspection and replacement procedure depends on the exact CNC, spindle amplifier series, control board, software, and machine configuration.