Resolving Siemens 802D Alarm 25030 After Simodrive 611 Power Module Replacement

Alarm 25030 on a Sinumerik 802D-controlled turning centre, surfacing immediately after a Simodrive 611 LT-module (line/transistor power section) swap, is one of the most frequently misdiagnosed spindle faults in field service. The drive is mechanically intact, the motor rotates briefly under command, and the controller inhibits the axis within a fraction of a second. Operators typically assume the new power section is defective, or that the firmware is incompatible, when the actual root cause is a mismatch between the commanded velocity and one of the speed-monitoring thresholds, a corrupted or partial parameter set, an inverted phase, or — as confirmed in the field case documented here — a multi-stage gearbox that was not re-engaged correctly after the module change. This reference walks through the diagnostic ladder, the specific machine data to check, the SimoCom U validation procedure, and the mechanical checks that must always be performed before returning the spindle to production.

Affected Hardware and Configuration

The reported case involved a turning CNC controlled by a Sinumerik 802D with a Simodrive 611 U (universal) spindle drive. The failed and replacement modules were:

The trailing designator in the 6SN1123 MLFB identifies the power-section variant (current rating, internal configuration, and resolver/external-encoder interface). A 0LA1 and a 0LA3 are both 611U LT modules but they are not bit-for-bit equivalent: the inverter stage, the brake-chopper sizing, and the assigned drive parameters (notably P1100–P1103 on the 611U control board) differ between the two suffixes. Swapping a 0LA1 for a 0LA3 is mechanically compatible but it is not a transparent, drop-in replacement — the spindle must be re-commissioned so the new power-section code is written into the drive's parameter set and the speed/position controller is re-tuned to the new IGBT stage. Skipping this re-commissioning is the single most common reason alarm 25030 appears on first spindle command.

For the full 611U configuration list, refer to the SIMODRIVE 611 universal / digital configuration manual and the Sinumerik 802D base software / diagnostics guide in the Siemens Industry Online Support portal.

Alarm 25030: Definition and Triggers

Alarm 25030 belongs to the Sinumerik 802D axis-monitoring class. According to the 802D Diagnostics Guide, alarm 25030 is the "actual velocity has exceeded the monitoring threshold" alarm. The 802D continuously compares the position-controller-derived actual velocity against the threshold defined in machine data MD 36200 AX_VELO_LIMIT. When the actual velocity exceeds this limit, the controller clears the axis enable, ramps down the drive, and latches alarm 25030.

The standard remedies listed in the Diagnostics Guide are:

1. Check the speed setpoint cable (bus / PROFIBUS cable from the NCK to the drive).
2. Check the actual values and the position-control direction.
3. Reverse the position-control direction by setting MD 32110 $MA_ENC_FEEDBACK_POL = < -1 / 0 / 1 >.
4. Increase the monitoring limit value in MD 36200 $MA_AX_VELO_LIMIT.

On Simodrive 611U, alarm 25030 is also raised when the 611U's own speed monitoring trips and the fault word is propagated to the 802D. The relevant 611U parameters to inspect are:

If the 802D also reports alarm 25020 (position-control monitoring) or 25050 (contour monitoring), the root cause is the same physical event — an uncontrolled velocity excursion — and the diagnostic path in this document applies unchanged.

Pre-Replacement Preparation

Before removing the suspect LT module, capture the following so you can verify the new module behaves the same way after re-commissioning:

1. Back up the 802D NC machine data. On the 802D, navigate to Commissioning → NC data → MD and save the MD list to a CF card or to a backed-up commissioning file (MMC). The original machine-builder commissioning file is mandatory for a like-for-like module replacement.
2. Back up the 611U drive parameters. Connect a PC running SimoCom U (Siemens ordering designation SimoCom U Tool, ordering number 6SN1117-2CA00-0AA0) to the 611U's serial X471 service port. Read the parameter set, save it as a .dpt file, and copy it to a known-good laptop.
3. Label the power cabling. Mark U, V, W and the protective earth at both ends of the motor cable and at the DC-link terminals. A swapped U-V-W pair inverts the rotating field and is the second-most common cause of 25030 after a parameter fault.
4. Verify the new module suffix. The new 6SN1123-1AA00-0LA3 must be the suffix listed in the machine-builder BoM. Do not substitute a similar suffix (e.g., 0LA2 or 0LA4) without re-validating the entire spindle parameter set.
5. Verify input voltage and DC bus. Confirm the 600 V DC link is discharged before unplugging. The 611U has a 5-minute discharge time after the line contactor opens; measure at the DC bus test points before contact.

Step-by-Step Diagnostic Procedure

Execute the following steps in order. Do not skip the lower steps when a higher step appears to clear the alarm — in the field case documented here, every step (a)–(e) was performed and the alarm persisted, which correctly steered the engineer toward the mechanical root cause at step (f).

1. Confirm the 802D alarm log. Read the alarm history under Diagnosis → Alarm log. Multiple 25030 entries with no 25020 or 25021 in between confirm a clean velocity-excursion event rather than a position-controller instability.
2. Cross-check MD 32000 and MD 36200. The maximum axis velocity (MD 32000) and the monitoring threshold (MD 36200) must be consistent. As a rule of thumb: MD 36200 = 1.05 × MD 32000 to 1.10 × MD 32000. If MD 32000 has been changed (for example, by a parameter reset), MD 36200 must follow.
3. Verify the encoder polarity. Issue a small M03 S100 command and observe the actual velocity feedback. If the actual velocity reads negative while the setpoint is positive, the encoder polarity is inverted. Set MD 32110 $MA_ENC_FEEDBACK_POL to the opposite sign, or swap the resolver / encoder leads per the 802D commissioning manual.
4. Check the 611U drive enables in SimoCom U. Connect SimoCom U and verify that all enable indicators in the lower-left status block are GREEN. Any orange or red LED — particularly Pulse enable and Controller enable — will cause the drive to ignore or immediately revoke the command.
5. Compare 611U parameters against the original commissioning file. Load the original .dpt file into SimoCom U and run an Online → Parameter list → Compare. The only difference should be the power-section code (P1100–P1103) and the resulting current-loop gains. If any other parameter has drifted, restore it to the original value.
6. Re-commission the spindle. If the power-section code has changed, run the SimoCom U commissioning wizard and re-load the motor code, the current-loop gain, the speed-loop gain, and the field-weakening threshold. A power-section swap that changes the IGBT module forces a fresh Calculate controller data pass.
7. Verify the gearbox mechanical stage. Command M03 S100 with a small programmed gear stage. Observe the actual velocity feedback. If the actual velocity reads approximately 10× the setpoint, the gearbox has been left in the wrong stage (typically the high-speed stage while the controller still commands the low-speed ratio). Engage the correct gear stage and re-test.

Machine Data Verification: MD 32000, MD 36200, MD 32110

The three machine data words that govern the velocity envelope on the 802D are:

The relationship is simple but rigid:

MD 36200 = (1.05 to 1.10) × MD 32000

If MD 32000 = 3000 rpm (a typical spindle maximum for a turning centre with a 3-stage gearbox in the high-speed range), then MD 36200 must be set between 3150 rpm and 3300 rpm. Setting MD 36200 = 5000 rpm to "stop the alarm" is unsafe — a real runaway that exceeds the mechanical rating of the spindle bearings will no longer be caught by the controller.

MD 32110 is the most dangerous of the three to leave in the wrong state. A polarity error of the spindle encoder means that on the first M03 command the controller believes the spindle is decelerating (because the actual velocity reads opposite to the setpoint) and increases the output. The drive then accelerates the spindle past MD 36200 in a fraction of a second, the monitoring trips, and alarm 25030 is raised. If you see the actual velocity feedback swing the wrong way at the moment of command, do not raise MD 36200 — invert MD 32110 instead.

SimooCom U Commissioning Tool Validation

SimoCom U is the PC-based commissioning tool for Simodrive 611U drives. The tool talks to the drive over a serial RS-232 cable on connector X471, or over PROFIBUS if the optional CBP module is fitted. The tool gives full read/write access to the drive's parameter set and a live diagnostic screen that mirrors the drive's enable word, the actual velocity, the current, and the DC-link voltage.

Validation sequence after the new LT module is bolted in:

1. Connect SimoCom U, go online, and read the parameter list.
2. Open Commissioning → Drive configuration and confirm the displayed power-section code matches the 6SN1123-1AA00-0LA3 suffix on the new module's nameplate.
3. Open Commissioning → Motor configuration and confirm the motor code matches the nameplate of the 1PH / 1PH2 spindle motor.
4. Open Diagnostic → Status word and check every status LED. All six enable indicators (Pulse enable, Controller enable, Speed controller enable, Setpoint enable, Drive enable, On) must read GREEN before the drive will accept a command from the 802D.
5. Open Commissioning → Controller data and click Calculate. The wizard uses the new power-section code and the motor code to compute the current-loop, speed-loop, and field-weakening parameters. Accept the calculated values and write them to the drive.
6. Save the parameter set to a .dpt file on the laptop.

The screenshot referred to in the source field report showed a partially-discharged enable word on SimoCom U — a common red herring. After a power-section swap, the drive may also be in a latched-fault state from the old module's final fault. Clear all latched faults (SimoCom U: Diagnostics → Clear fault) before commanding the spindle.

Wiring and Phase Verification

A Simodrive 611U LT module drives a 3-phase AC motor from the U2, V2, W2 terminals on the front of the module. The cable between the module and the motor is a 4-core shielded cable (U, V, W, PE). If two of the three phase conductors are swapped (for example, U and V reversed), the rotating field in the motor inverts and the motor runs the wrong way. The 802D sees the inverted direction through the encoder and reacts by trying to correct the polarity — which, because the encoder is now reading a negative rotation, is interpreted as runaway.

Verification procedure:

1. De-energise the cabinet, wait five minutes, verify zero DC-link voltage, then disconnect the motor cable at the LT module terminals.
2. Using a continuity tester, ring out U, V, W, and PE from the module side to the motor side. Each conductor must trace to the same letter at the motor junction box.
3. Verify the shield is bonded to PE at the cabinet end and isolated at the motor end (or bonded on both ends through a high-frequency low-impedance bond — follow the machine-builder drawing).
4. Re-connect, torque to the spec on the terminal label (typically 4 Nm for the 6SN1123-1AA00-0LAx LT module), and re-verify with a second continuity test.

The original field report explicitly verified that the U, V, W phases were not swapped, which correctly moved the diagnostic focus away from the wiring and toward the gearbox.

Mechanical Verification: Gear Box Position

The single most common reason a 25030 alarm persists after a clean electrical re-commissioning of the spindle is a multi-stage gearbox that has been left in the wrong stage. On a 3-stage turning-centre spindle, the gearbox is typically shifted by an external gear-change actuator (a hydraulic or electric shift cylinder plus a position switch for each stage). The 802D commands the spindle through the M40–M45 range and selects the gear stage that the controller thinks is currently engaged.

The failure mode looks like this:

1. Mechanic removes the LT module for replacement and disconnects the spindle encoder cable and the motor cable at the LT module end.
2. While the cables are loose, the gearbox is bumped (or was previously left in mid-shift), so the gear-change actuator is sitting in the high stage position while the controller still has the low stage selected.
3. The new LT module is installed, the spindle is re-commissioned in low stage, and the controller commands M03 S100 — expecting a spindle speed of 100 rpm at the chuck.
4. The gearbox is actually in high stage (1:10 ratio), so 100 rpm commanded at the motor becomes 1000 rpm at the chuck. The actual velocity exceeds MD 36200. Alarm 25030 trips.

Diagnostic check: with the spindle stopped and the spindle override forced to 0 %, command M03 S100 and watch the actual velocity in the 802D's Axis service screen. If the actual velocity reads approximately the gearbox ratio × the commanded velocity, the gearbox is in the wrong stage. Shift the gearbox to the correct stage, re-home the spindle, and re-test.

Final Resolution Path

Summarising the diagnostic ladder for a 802D / Simodrive 611U spindle that raises alarm 25030 after a 6SN1123-1AA00-0LAx power-section swap:

Field-Validated Lessons

• A power-section swap is not firmware work. The 611U control board firmware does not change when the LT module changes. Re-flashing the firmware will not clear alarm 25030 and may introduce additional fault codes (notably F-0070 or F-0071 on the 611U side) if the firmware version drifts away from the boot loader version.
• Original parameter files are irreplaceable. If the machine builder's commissioning file is lost, contact the OEM or a Siemens service partner with the 802D serial number and the 611U control board's order number (6SN1118-xDAxx-xAAx/ABx). The parameter set can be regenerated from the motor nameplate, but the gearbox ratios, the encoder pole count, and the OEM-specific protection limits will not be recovered without the original file.
• Mechanical disturbances count. A gearbox that was bumped during the module swap is statistically the most common cause of a 25030 alarm that survives a clean electrical re-commissioning. Always verify the gear-stage position switch and the actuator state before declaring the spindle healthy.
• Do not raise MD 36200 to clear the alarm. The monitoring limit exists to protect the mechanical envelope. Raising it past 1.10 × MD 32000 hides the real fault and risks a runaway event on the next rapid move.
• Allow the DC link to discharge. Five minutes minimum, then verify with a meter. Hot-swapping the LT module with a charged DC link will trip the line-side input fuse and may damage the new module's IGBT stack on insertion.

Frequently Asked Questions