Resolving SINAMICS S120 Parallel ALM Infeed Faults via AOP

When two SINAMICS S120 Active Line Modules (ALM) operate in parallel behind a single CU320-2 Control Unit, the Advanced Operator Panel (AOP30) and STARTER commissioning tool present individual drive objects (DO) that must be monitored separately. Field engineers frequently encounter a fault buffer that reports only the aggregate message "Sub2_Infeeds" or "Sub1_Infeeds" without identifying which physical module triggered the trip. This reference documents the topology, drive-object numbering scheme, AOP navigation sequence, STARTER workflow, and ALM-specific temperature fault codes required to isolate a defective parallel infeed module on a CU320-2 commissioning.

1. SINAMICS S120 Parallel Infeed Architecture

The SINAMICS S120 modular drive system supports parallel connection of line modules (Basic Line Module, Smart Line Module, Active Line Module) and Motor Modules to scale apparent power beyond the rating of a single power block. A single CU320-2 (article numbers 6SL3040-1MA00-0AA0 for the DP variant and 6SL3040-1MA01-0AA0 for the PN variant) supervises all drive objects that are connected via DRIVE-CLiQ to its five ports X100, X200, X201, X202, and X203.

For a parallel infeed consisting of two ALMs (for example, two 6SL3130-7TE25-5AA3 Smart Line Modules or 6SL3130-7TE31-2AA3 ALMs in the booksize format), each module is a discrete SINAMICS drive object of type A_INF (Active Infeed). The CU320-2 hosts the Line Infeed Control (LIC) and the Line Infeed Monitoring (LIM) as additional internal drive objects when the firmware is configured for closed-loop line infeed operation. The DRIVE-CLiQ connections from the CU320-2 to each ALM are established at ports X200 and X201 (and optionally X203 for the second module when not used for a TM31 Terminal Module).

2. Drive Object Numbering for Parallel ALMs

The CU320-2 assigns drive object numbers sequentially during commissioning. The default allocation for a drive system with two parallel ALMs and a single Motor Module is summarized below.

The aggregate DO #1 (the Line Infeed Control) is internally modeled by SINAMICS as a "Master" object that polls every physical infeed DO and combines their status bits. When one of the parallel ALMs reports a fault, the LIC propagates the fault up to the AOP as Sub{n}_Infeeds where n is the subcomponent index of the failing module. The AOP then shows only the aggregated fault code, which is why the source problem states that "DO number not visible" on the operator panel.

The DO number for the actual failing module is stored in the fault buffer of the CU320-2 and can be retrieved either from the AOP fault detail screen or from STARTER. This is the single most important piece of information required to physically identify which ALM triggered the trip.

3. Interpreting the "Sub2_Infeeds" Group Fault

The message "Sub2_Infeeds Fault" is not a SINAMICS error code in itself; it is a component assignment that the fault buffer uses to indicate the failing sub-component within a multi-component drive object. The complete fault record on the CU320-2 always includes:

• The numeric fault code (for example F30004, F30005, F06250).
• The fault value (parameter value captured at the moment of the trip).
• The fault time stamp (in operating hours, parameter r2135).
• The drive object number where the fault originated.
• The sub-component index when the failing object is a parallel group.

The Sub2_Infeeds string is composed of two parts: Sub (sub-component) and 2 (the index of the failing ALM within the parallel infeed). On a two-module parallel infeed, Sub1_Infeeds always refers to ALM #1 (the master by default) and Sub2_Infeeds always refers to ALM #2. Both sub-component strings are mapped through the topology and the parameter p0121 (power unit component number).

4. AOP30 Fault Buffer Navigation

The AOP30 (Advanced Operator Panel, 6SL3055-0AA00-4NA0) is the most common commissioning interface for SINAMICS S120 cabinets. The fault buffer of each drive object is independently accessible through the menu tree described below.

1. Press the MENU key to enter the main menu.
2. Select Diagnostics → Faults / Alarms.
3. Press the soft key DO selection (alternatively labelled Drive selection) and use the cursor keys to choose the suspected ALM drive object. The DO list typically shows:
   — CU320-2 (DO 0)
   — Infeed_1 (DO 1, the LIC aggregate)
   — Infeed_2 (DO 2, ALM #1)
   — Infeed_3 (DO 3, ALM #2)
   — Motor_1 (DO 4, etc.)
4. Press OK to load the fault buffer of the selected DO. The AOP now displays the historic fault list scoped to that object only.
5. For each fault, press the Detail soft key (or press F5 on newer firmware) to expand the fault record and reveal the drive object number, fault value, and time stamp.
6. Repeat for the second ALM DO to determine which module actually registered the temperature trip.

If the AOP30 firmware is older than V2.6, the detail soft key may be hidden. The workaround is to scroll down within the fault line with the down arrow key; the DO number appears as the second line of the entry.

5. STARTER and Startdrive Workflow

The most efficient diagnostic path is to use the STARTER commissioning tool (now superseded by Startdrive but functionally equivalent for this task). The procedure to inspect both parallel ALMs simultaneously is:

1. Open the offline project, go online with the drive unit, and double-click the drive unit icon in the project navigator.
2. Right-click Drive unit → Faults/Alarms → Fault buffer. This opens the unit-level fault list that aggregates every fault on every DO. The Drive object column shows the DO number alongside the fault code, making the failing module immediately obvious.
3. Switch to the per-DO view by clicking the ALM node in the navigator (for example Infeed_2). The Diagnostics → Faults/Alarms tab now lists only the faults that originated in that specific ALM.
4. Open the Control panel for each ALM to view r0002 (operating display) and confirm the active state of each module. The status word r0899 shows power unit status (commissioning, ready, run, fault).
5. For a temperature trip, scroll to Diagnostics → Measured values and read r0035 (actual coolant temperature where the sensor is mounted), r0037 (actual power unit temperature), and r2135 (fault time).

The same workflow applies in TIA Portal V15 and later with the SINAMICS HSP installed. The advantage of the engineering tool over the AOP is that all drive objects can be inspected in a single screen without navigating through menus.

6. ALM Temperature Fault Codes and Diagnostics

The SINAMICS S120 List Manual (SINAMICS S120/S150 List Manual, 109739154) documents the fault numbers below that are most commonly triggered by cooling failures on parallel ALMs.

All codes are read on the CU320-2 under Diagnostics → Fault buffer; the Object number column is the key to physical module identification.

7. Cooling System Verification Per ALM

Once the faulty ALM is identified via DO number, the cooling system must be verified module by module. The standard liquid-cooled SINAMICS S120 booksize ALM uses an integrated heat sink with optional internal fan. Follow this verification sequence:

1. Confirm the heat sink and intake of each ALM are clean and unobstructed. Air-cooled ALMs use one or two 6SL3162-1AF00-0AA0 fans rated at 24 V DC.
2. Measure the supply voltage at the fan connector (X12 on the ALM). Less than 21 V indicates the internal 24 V rail is sagging under load.
3. For liquid-cooled ALMs, verify the coolant flow rate at the inlet manifold. The minimum flow for the 6SL3130-7TE25-5AA3 is 8 L/min with a 70/30 water-glycol mixture and a maximum inlet temperature of 45 °C.
4. Inspect the inlet temperature sensor. The sensor connects to terminal X21 of the ALM and is read as r0035[0] on the corresponding DO. A reading of −200 °C indicates an open sensor; a reading clamped at the maximum indicates a short.
5. Compare the heat sink temperature of both ALMs via r0037[2] (ALM #1) and r0037[3] (ALM #2). A delta greater than 8 °C between two parallel modules of the same rating is a strong indicator of cooling asymmetry.
6. Run the drive in idle and observe r0035 over 30 minutes. The temperature should stabilise below 60 °C in a 25 °C ambient. A monotonic rise indicates reduced coolant flow or blocked fins.

8. Parameter Reference for Parallel ALM Operation

The following parameters are mandatory review items when commissioning two parallel ALMs on a CU320-2. They are taken from the SINAMICS S120/S150 List Manual and the SINAMICS S120 Line Infeed Function Manual.

Load sharing between the two ALMs is supervised by the LIC. When one module begins to take more than 50 % of the demanded current, r0077 (current setpoint) and r0078 (actual current) diverge. The firmware includes a current-balancing routine that adjusts the gating of the IGBTs to equalise the load; however, this routine does not compensate for external cooling failure, which is why the temperature trip on one module can occur even when the system current is well below rated.

9. Recommended Commissioning and Diagnostic Procedure

The following step-by-step sequence is recommended after any temperature trip on a parallel infeed to physically identify and resolve the issue:

1. Connect to the CU320-2 with STARTER (or Startdrive) and go online.
2. Open the unit-level fault buffer. Record the fault code, fault value, and drive object number for every entry.
3. Drill down into each ALM DO and copy the local fault buffer for archiving.
4. Acknowledge the fault only after copying. Premature acknowledgement can clear time-stamps.
5. Compare r0037[2] and r0037[3]. The module with the higher temperature at the moment of trip is the suspect.
6. Verify the DRIVE-CLiQ cable integrity on the suspect module. A faulty DRIVE-CLiQ cable can cause the LIC to lose the temperature signal and react conservatively. Recommended cable: 6SL3060-4Axx0-0AA0 series.
7. Power down, lock out / tag out (LOTO), and inspect the cooling path of the suspect ALM (fan, heat sink, coolant).
8. After repair, power up the cabinet, run the topology check, and reset the fault using p3981 = 1 (acknowledge all faults) on the CU320-2.
9. Run the cabinet at 25 % rated load for 30 minutes and confirm both ALM heat sink temperatures track within 5 °C.

10. Troubleshooting Matrix

11. Related Diagnostic Aids and Documentation

For deeper analysis, the following official Siemens documents are recommended:

• SINAMICS S120/S150 List Manual (LH1) — complete parameter and fault code reference.
• SINAMICS S120 Line Infeed Function Manual — explains parallel connection commissioning including p0120, master/slave configuration, and PROFIdrive telegrams for infeeds.
• SINAMICS 828D List Manual, Section Infeed closed-loop control — complementary closed-loop control description for A_INF_828 variants.
• SINAMICS S120 Commissioning Manual — STARTER topology configuration and Automatic Configuration procedure.
• AOP30 Operating Instructions — menu tree, fault buffer, and DO navigation on the panel.