Resetting p0251 Fan Hour Counter on Sinumerik 840D sl XLM Drive

1. Problem Description

DMG Mori DMC650V vertical machining centers — and other mid-size DMG Mori 5-axis platforms that share the same control architecture (DMU 50, DMU 65 monoBLOCK, DMC 80 monoBLOCK, etc.) — are equipped with a Siemens Sinumerik 840D sl numerical control driving a SINAMICS S120 modular drive line-up. The Active Line Module (ALM) or Smart Line Module (SLM) — referred to in DMG Mori machine documentation as the XLM (eXtended Line Module) — uses one or more internal radial fans to cool the line-stage IGBT modules, DC-link chokes, and capacitor bank.

SINAMICS firmware tracks cumulative fan ON-time inside drive parameter r0251. When the counter crosses the alarm threshold stored in p0252[0], the drive issues a maintenance alarm. On a Sinumerik 840D sl HMI the alarm is mapped into the alarm log and typically displays in the form:

Decoding the message format used by the DMG Mori HMI skin:

• DP003 — the third PROFIBUS/PROFINET drive group (the infeed axis group on a DMC650V). DP001 normally holds the spindle, DP002 holds the linear and rotary feed axes, and DP003 holds the line module.
• Slave003 — the PROFIBUS slave number assigned to the XLM in the SDB/PN configuration. On DMG Mori commissioning sheets the XLM is normally slave 3 or 4 depending on whether a Smart Line Module (SLM), Active Line Module (ALM), or Active Line Module + Active Line Filter (ALF) combination is fitted.
• XLM_00 — drive object name. The _00 suffix is the DO index inside the CU320-2 control unit. The XLM is always DO index 0 on a CU320-2; the Motor Modules are indices 1, 2, 3, …
• (2) — alarm number, here F0502 (Power unit: Fan operating time reached) or A0502 (warning) depending on firmware version and the value of p0252/p0254 configuration. Some HMI versions display the warning first (A0502) and escalate to a fault (F0502) when the value exceeds p0252 by an additional margin.

After replacing the worn fan, the standard maintenance task is to write p0251[0] = 0 to clear the counter. The technician logs in with the machine's service / manufacturer password (Sinumerik access level 2 or 3), opens the Setup menu, navigates to Drive System → Drives → Infeed Parameter, locates p0251 — and is blocked with the message:

Subsequent attempts to access p0003 (BOP / operator panel access level) to escalate to the Expert level also fail, because p0003 itself is not visible at the manufacturer's default access level for that drive object, or it is grayed out on the HMI tree. The remainder of this article explains why the regulator blocks the write, and walks through the complete access and reset sequence required on a DMG Mori DMC650V with Sinumerik 840D sl and SINAMICS S120 firmware ≥ 4.x.

2. Hardware Topology and Drive Objects

A DMC650V is built around a SINUMERIK NCU 710.3B (or 720.3B on later builds) with a CF card holding the Sinumerik 840D sl HMI and PLC image. The drive bus is PROFIBUS DP (classic DMC650V builds) or PROFINET (newer builds with CU320-2 PN). A typical topology from left to right inside the electrical cabinet looks like:

1. NX10 / NX15 terminal module (sensor module hub for the line module) — this is the point at which the line module's DRIVE-CLiQ port is physically brought to a standard RJ45.
2. XLM (Active or Smart Line Module) — DRIVE-CLiQ port 0, addressed as DO 0 on the CU320-2.
3. Motor Module 1 (double-axis, spindle) — typically a 6SL3120-series booksize unit, 36 A or 45 A rated.
4. Motor Module 2 (double-axis, X/Y feed) — 18 A or 24 A booksize unit.
5. Motor Module 3 (double-axis, Z / 4th axis) — 18 A or 24 A booksize unit.
6. Motor Module 4 (rotary/swivel axis, B or C) — 9 A or 12 A booksize unit.

The XLM is the only drive object where p0251 exists in the standard parameter scope; Motor Modules use a different fan monitoring path (or no fan monitoring at all on booksize units under 18 A). For this reason the reset task must be performed against the XLM's drive object, not against any of the Motor Modules.

Drive objects are addressed in SINAMICS using the two-letter prefix plus a drive object number. Parameter access from the HMI is gated by the parameter number itself (p0003 / p0009 / p0010) and by the regulator state machine. The "regulator" referenced in the NO WRITE ACCESS TO ACTIVE REGULATOR message is the closed-loop controller inside the XLM, and the XLM is treated as active whenever the DC link is powered and the line module has not been told to suppress its pulses.

3. Root Cause: Why the Regulator Blocks the Write

SINAMICS S120 firmware implements a write-protection layer on critical parameters. The protection has three independent gates, all of which must be satisfied for a parameter write to take effect:

The NO WRITE ACCESS TO ACTIVE REGULATOR message means Gate 3 has failed. The HMI access level (Gate 1) and the commissioning filter (Gate 2) are usually fine — the technician has logged in with the manufacturer password, p0010 is 0 because no commissioning routine is active, and p0003 was either left at the default value of 1 (Standard) or set to 3 (Expert) by the operator. The problem is that the XLM is still running: the line contactor is closed, the DC link is charged, the XLM is gating IGBTs, and the closed-loop current regulator is in the follow-up or closed-loop state.

SINAMICS refuses to let the user reset a service-life counter while the regulator is closed-loop because the counter reset is a non-volatile write to the EEPROM, and an EEPROM write during active regulation can collide with the regulator's own background storage tasks. The firmware's safety policy is: no parameter writes that touch the EEPROM while the regulator is closed-loop, regardless of the access level.

The fix is to drive the regulator into a benign state by inhibiting the XLM pulses. On a DMG Mori HMI, the only operator-facing way to inhibit the line module pulses is to press the physical EMERGENCY STOP button (or to set the SINUMERIK safety signal DBX0.5 / DBX2.5 via the PLC, which the EMERGENCY STOP already does). EMERGENCY STOP triggers the SINUMERIK stop sequence, the PLC drops the OFF2 / OFF3 lines on the PROFIDrive telegram, the XLM pulses are cancelled within ~20–50 ms, and p0251 becomes writable.

4. SINAMICS Parameter Reference

The full parameter set that controls fan service-life monitoring on a SINAMICS S120 line module is listed below. All values are taken from the standard SINAMICS S120/S150 List Manual, parameter sheet for the Active Line Module and Smart Line Module control diagrams.

The full parameter list is published in the official SINAMICS S120/S150 List Manual (function block diagram section 4.2, "Power Unit"). The parameter list is the canonical reference for every index [0…n] of p0251, p0252, and the alarm masks A0502 / F0502.

5. Prerequisites and Safety Lockout

Before opening the electrical cabinet or driving the HMI into the infeed parameter screen, complete the following checklist. None of these steps are optional on a DMG Mori DMC650V.

1. Machine state. Spindle stopped, all axes parked at safe positions, no tool in the spindle, no part clamped. Coolant pump off.
2. Access level. Log in on the Sinumerik HMI with the service or manufacturer password. On a DMC650V the default OEM passwords are listed in the DMG Mori service binder; they are not the public Siemens default. The login prompt is reached via the user-management softkey in the HMI top bar.
3. Drive state. Confirm the drives are at standstill. Check r0002 (drive status word) on the HMI: must be 0 (Commissioning / ready, no run enable) before the E-stop step.
4. EMERGENCY STOP accessibility. The red E-stop button must remain reachable throughout the procedure. Do not lock it in the released position; the standard practice is to press and hold it (some operators use a soft-touch latching collar — DMG Mori service notes do not require this).
5. HMI screenshot tool. Have a way to capture the Infeed Parameter screen for the maintenance log. The standard Sinumerik screenshot softkey is MENU SELECT → Diagnostics → Screen → Save.
6. Power cycle plan. Have already inspected the new fan mechanically (spins freely, correct part number, no damaged blades). Replacement of the fan before the counter reset is mandatory; resetting p0251 with a worn fan still in place defeats the maintenance purpose.

6. Step-by-Step Reset Procedure

The procedure below uses only the standard Sinumerik HMI softkeys — no PC, no Starter / Startdrive, no PLC source change. It is the procedure recommended by Siemens for in-process maintenance on a 840D sl with SINAMICS S120.

1. Press the EMERGENCY STOP button. All drives drop to "pulses cancelled" within ~20–50 ms. r0899.11 (OFF2) and r0899.12 (OFF3) go to 0 on every drive object, including the XLM. The HMI alarm bar will turn red; this is expected.
2. Confirm access level. On the top bar of the Sinumerik HMI, tap the access-level icon. It must show "Manufacturer" or "Service" (level 2 or 3). If it shows "User" or "Keyswitch", re-enter the password.
3. Open the drive menu. Press MENU SELECT → SET UP → DRIVE SYSTEM. The full path on a DMC650V HMI is:
   MENU SELECT → SET UP → DRIVE SYSTEM → Drives → INFEED PARAMETER
4. Select the XLM drive object. The Drives list shows one entry per drive object. The XLM is the entry named XLM_00 or INF_00 (DMG Mori uses XLM, Siemens default uses INF — both refer to DO index 0). Tap the XLM row; the right pane shows the parameter list scoped to that DO.
5. Check p0003. Scroll to p0003 in the parameter list. If it is hidden, the access level is not high enough — go back to step 2. If it is shown and the value is 1, change it to 3 (Expert). The numeric softkey opens a small keypad. Press Input to confirm.
6. Locate p0251[0]. With p0003 = 3, p0251[0] is now visible. The column "Value" shows the current hour count (e.g. 31 240 h). The column "Unit" shows h.
7. Write 0 to p0251[0]. Tap the value field, enter 0, press Input. The HMI asks for a confirmation pop-up ("Save parameter in drive?"). Press Yes. The drive firmware performs the EEPROM write. This takes 1–3 seconds. If the write succeeds, the value stays at 0.
8. Verify the write. The HMI shows the new value 0 in p0251[0] and 0 in r0251.0 (the read-only mirror). The alarm A0502 / F0502 may still appear in the alarm list until the drive is power-cycled.
9. Power-cycle the control. Turn the main disconnect to OFF. Wait at least 20 seconds (SINAMICS CU320-2 discharge time). Turn the disconnect back ON. The NCU boots, the HMI re-initialises, the XLM re-charges the DC link, the alarm A0502 / F0502 is no longer raised, and the alarm log shows the entry with a green "cleared" timestamp.
10. Release the EMERGENCY STOP button. Twist to release. Acknowledge any residual alarms with the Cancel key. Confirm the drives re-enable when the run-inhibit chain is reset.

The complete menu trail is reproduced below as a single-line command path that mirrors the HMI softkey tree:

On builds that ship the older Sinumerik Operate 4.5 HMI, the softkey label INFEED PARAMETER may be replaced with INF_00 or INF (XLM). The location in the tree is identical.

7. Verification and Commissioning

After the power cycle, perform the following four checks before returning the machine to production:

1. Alarm buffer clear. Open Diagnostics → Alarm → History. Confirm the F0502 / A0502 entry has status "cleared" and that no new alarm is generated on a 5-minute idle run.
2. Counter read-back. Re-enter the Infeed Parameter screen. Confirm p0251[0] = 0 and r0251.0 = 0. If either is non-zero, the EEPROM write did not persist; repeat step 7 of the procedure above.
3. Fan tachometer. Read r0197. With the new fan installed, the value should be in the range 2500–3500 rpm for a booksize line module (6SL3130) or 1500–2800 rpm for a chassis line module (6SL3330). A reading of 0 rpm means the fan connector was not seated, or the DRIVE-CLiQ link is down.
4. Heatsink temperature. Read r0037 on the XLM. Under no-load idle, the value should stabilise below 45 °C within 10 minutes. If it climbs above 55 °C, the fan is wired wrong (rotating backwards) or the airflow path is blocked.

For a full functional check, run a spindle warm-up program (DMG Mori standard: M03 S500, dwell 5 min) and re-read r0037. The temperature should stay below 60 °C at rated line current. Document the values in the machine's service log per ISO 9001 / IATF 16949 audit requirements.

8. Alarm F0502 / A0502 Diagnostics

The two alarms raised by the fan service-life monitoring differ in severity and in the way the drive responds.

The DMC650V typically surfaces A0502 first and only escalates to F0502 if the operator ignores the warning for several weeks. Both alarms are documented in the SINAMICS S120 List Manual, alarm buffer section, fault code 0502. The full alarm text and remediation path is also in the SINAMICS S120 Function Manual, Drive Functions, chapter 10.4 (Power-unit protection functions).

If the HMI displays the message with the prefix DP003.Slave003:XLM_00, the alarm is being routed through the SINUMERIK alarm server and the drive bus slave number has been inserted by the HMI. The drive-internal alarm number is still 0502 — the (2) in the HMI text is a Sinumerik alarm sequence number, not the drive-internal fault number.

9. Related Fan Counters and Monitoring

The XLM is not the only drive object with a fan service-life counter. On DMC650V-class machines, two more counters are present and should be reset at the same fan service event:

The NX module (sensor module hub) drives the cabinet ventilation fan and has its own DRIVE-CLiQ presence. Its p0251 counter runs faster than the line module's because the cabinet fan runs whenever the cabinet door switch is closed, not only when the drives are enabled. The threshold is typically lower (e.g. 20 000 h). The same access / pulse-inhibit logic applies.

For a complete fan-health picture, a service engineer should also read r0197 on each DO, log the values, and compare to a baseline taken at commissioning. A drop of 30 % in r0197 indicates a failing fan even if the hour counter has not yet tripped the alarm.

10. Fan Replacement Best Practices

Replacing the fan and resetting the counter are two halves of the same maintenance task. Skipping either half leaves the machine in an unsafe state. The field-proven sequence is:

1. Order the OEM fan. DMG Mori part numbers for the XLM internal fan are listed in the spare-parts catalogue; the bare Siemens part (e.g. 6SL3162-1AD00-0AA0 for a booksize line module fan) is interchangeable but voids the OEM warranty if the machine is still under contract. Check the cabinet sticker for the line module part number before ordering.
2. Lock out and tag out. Open the main disconnect, apply LOTO, wait 5 minutes for the DC link to discharge (the XLM has internal discharge resistors; the time constant is on the order of 1–2 minutes for a 16 kW ALM, longer for larger modules).
3. Verify zero energy. Measure the DC link with a category-IV meter at the XLM DC test points. Voltage must be below 50 V DC before any work on the fan assembly.
4. Replace the fan assembly. Remove the fan cover, swap the fan, re-seat the connector, refit the cover. Do not run the line module with the cover removed; the airflow profile is part of the cooling design.
5. Power up, verify tachometer. Close the disconnect, wait for NCU boot, navigate to the XLM screen, confirm r0197 is non-zero. If r0197 = 0, the connector is reversed or the DRIVE-CLiQ link to the NX is down.
6. Reset the counter. Execute the procedure in section 6 above.
7. Log the event. Stamp the maintenance log with date, technician ID, fan part number, serial, and post-replacement r0197 / r0037 / p0251 readings.

11. Alternative Reset Methods

If the HMI procedure is not available — for example, the HMI panel has failed, the operator password is unknown, or the E-stop wiring is broken — there are three alternative paths. Each has stricter access and audit requirements.

The Starter / Startdrive path requires the drive expert password. On a DMG Mori machine this is the OEM-modified password, not the public Siemens default. Contact DMG Mori Service with the machine serial number to obtain the binding. The sequence is: connect → go online with XLM_00 → p0003 = 3 → p0251[0] = 0 → RAM-to-ROM save → power cycle. The "RAM-to-ROM" step is the Starter equivalent of the HMI "Save parameter in drive?" confirmation.

The PLC path is documented in the Sinumerik 840D sl Programming Manual as a side effect of the standard drive-parameter read/write FB (PI service _N_NVR_RD / _N_NVR_WR). It is not recommended for field use because a typo in the parameter number can corrupt the drive image; the HMI path remains the audited, supported, and DMG Mori-recommended method.

12. Troubleshooting Matrix

The following matrix covers the most common field failures on the DMC650V fan service-life procedure and the corrective action for each.

For deeper diagnostics, cross-reference the drive-side alarm buffer (r0945 / r0947) with the Sinumerik alarm server log. The two logs should agree; if the Sinumerik log shows the alarm as cleared but the drive log still shows it active, the EEPROM write succeeded but the drive's RAM copy has not been refreshed — power cycle is the only reliable cure.

13. Field Reference: Quick Command Path

The shortest possible field checklist, suitable for taping to the inside of the cabinet door on a DMC650V:

1. Login: Manufacturer password (level 2/3).
2. E-stop: PRESS and HOLD.
3. HMI: MENU SELECT → SET UP → DRIVE SYSTEM → Drives → INFEED PARAMETER.
4. DO: XLM_00.
5. p0003 = 3 (Expert).
6. p0251[0] = 0.
7. Confirm: HMI "saved" message.
8. Read-back: r0251.0 = 0.
9. Power cycle: Disconnect OFF → 20 s wait → Disconnect ON.
10. Verify: F0502 / A0502 cleared, r0197 > 0, r0037 < 45 °C after 10 min idle.
11. Release E-stop. Acknowledge any residual alarms. Document in service log.

Total in-process time for a trained service engineer: 8–12 minutes. Fan mechanical replacement adds 25–40 minutes including LOTO and heatsink cool-down.