Resolving SINAMICS S120 F07900 Motor Locked on 1FT7 Servo Axes

Problem Overview

SINAMICS S120 drives configured with a CU320-2 PN control unit, a Smart Line Module, and a Double Motor Module driving two 1FT7042-5AF70-1BG2 servo motors (DRIVE-CLiQ, integrated encoder) report fault F07900 — "Drive: Motor locked / speed controller at its limit" immediately after the standstill (stationary) measurement completes, both from the STARTER control panel and from the S7-300 CPU via EPOS basic positioner. The fault appears the first time a non-zero speed setpoint (for example, 100 rpm) is commanded, regardless of which SERVO drive object (SERVO_02 or SERVO_03) is active.

This behavior is reproducible after a full project download, after a factory reset + automatic device configuration, and after switching the active drive object in the STARTER control panel. A short, sluggish shaft rotation is typically observed before the drive pulses are inhibited and F07900 is latched.

Affected Components and Topology

The fault typically surfaces in the following configuration, which is the reference scenario for the procedure below:

The 1FT7042-5AF70-1BG2 is a SIMOTICS S servo motor with a built-in DRIVE-CLiQ encoder. The motor has only two physical connections: an orange power cable with a pre-fitted connector on the motor side, and a green DRIVE-CLiQ signal cable. The power cable reaches the Motor Module as a free-end lead that the user must connect to the U2/V2/W2 (or U/V/W depending on module family) terminal block of the corresponding axis.

Root Cause: Power Phase Sequence vs. Encoder Direction

F07900 is raised by the SINAMICS closed-loop control when it detects a contradiction between the speed setpoint issued by the higher-level controller, the speed controller output (torque demand), and the measured encoder velocity. The drive concludes the motor must be mechanically blocked and trips to protect the system.

For an unloaded 1FT7 motor that has just passed the standstill measurement, the three most probable causes in this topology are:

1. Power phase sequence reversed on the Motor Module terminal block. The magnetic field in the stator rotates in the opposite direction to the direction expected by the encoder-based field-orientation model. The speed controller saturates trying to close the gap and the "motor locked" detection trips. This is the most common cause when a non-prefabricated (free-end) cable is used or when the cable was ordered separately from the motor and crossed in the field.
2. Encoder physically paired with the wrong motor. On a Double Motor Module, axis X1 and axis X2 share a common housing. The DRIVE-CLiQ topology wizard validates that the encoder type matches, but it does not verify that encoder X1 is mechanically mounted on the same shaft as power X1. If the green DRIVE-CLiQ leads were inadvertently swapped at the Motor Module end, the field-orientation model rotates correctly but on the wrong physical shaft, so the controlled shaft reports near-zero speed while the controller commands a positive speed.
3. Torque limit / current limit too low relative to friction + load. The standstill measurement typically sets p1400/p1520/p1521 based on motor data. If the load-side friction or any external torque is significant, the speed controller may saturate against the torque limit and the "motor locked" time threshold expires.

The DRIVE-CLiQ encoder interface communicates the absolute position, commutation angle, and pole-pair information to the control unit over a digital link. Because the commutation is computed in the drive from the encoder feedback, swapping power phases U ↔ V (or V ↔ W, or U ↔ W) on the Motor Module terminal is a real, physical phase sequence change and is not auto-corrected by DRIVE-CLiQ. DRIVE-CLiQ only resolves the logical direction (positive/negative setpoint) via parameter p0410[p].bit / p1821 (direction of rotation reversal), not the electrical phase wiring.

Relevant SINAMICS Parameters

Before touching wiring, capture the current state of the parameters below. They are visible in STARTER under Drive → Commissioning → Motor/Encoder and Drive → Messages/monitoring → Speed messages.

Solution A — Verify and Correct Power Phase Sequence

This is the field-proven fix for the reference case and is the first action to take whenever F07900 is raised on a DRIVE-CLiQ motor right after the standstill measurement on an unloaded shaft.

Prerequisites

• Drive is safely de-energized, DC-link discharged (≥ 5 min wait after line contactor OFF, verify with a suitable DC-link meter at the Smart Line Module test points).
• All safety circuits (STO via terminals EP at the Motor Module) are in the safe state.
• Documentation of which orange power cable belongs to which physical motor and which axis socket on the Double Motor Module.

Step-by-step

1. Power down the Line Module, lock the main disconnect, and confirm zero voltage at the Motor Module input. Discharge the DC link per the S120 Equipment Manual instructions.
2. Trace each power lead from the motor connector back to the Motor Module terminal block. The 1FT7042 power cable has three phase cores plus protective earth. Color coding on Siemens prefabricated cables: U = black, V = black with white stripe, W = black (the printed sleeve near each lug identifies the phase). Confirm the assignment against the schematic in the S120 List Manual (search for "Motor connection").
3. Connect U → U2, V → V2, W → W2 on the Motor Module terminals. The terminal layout for the 6SL3120-2TE13-0AAx class Double Motor Module is: axis X1: U2 V2 W2, axis X2: U2 V2 W2 on the lower terminal block, with PE (ground) on the dedicated grounding bar.
4. Re-torque the terminal screws to the value specified on the module label (typically 2.5 Nm for the 6SL3120 double motor module family).
5. Verify DRIVE-CLiQ topology: in STARTER online, expand the device tree and confirm that the encoder of motor X1 is on DRIVE-CLiQ port X1 of the Double Motor Module and the encoder of motor X2 is on port X2. If the topology wizard in HW Config / TIA Portal was used, the project topology must match the physical topology — if it does not, the wrong p0141 / p0142 encoder assignment is in effect.
6. Power up the Line Module, wait for DC-link ready, and bring the drive back online in STARTER.
7. Run a directional test using the STARTER control panel: command a small positive speed (e.g. 30 rpm) on SERVO_02 with the motor uncoupled. Verify the shaft physically rotates in the direction defined as "positive" by your mechanical convention. If it rotates the wrong way, swap two phases and leave the encoder wiring as-is — do not change p1821. If it rotates correctly, no further wiring change is needed; proceed to the optimization measurement.

Solution B — Confirm Encoder-to-Motor Pairing

When phase sequence is correct but F07900 still trips on a particular drive object, the most likely remaining cause is that the green DRIVE-CLiQ cable of motor X1 has been plugged into port X2 of the Double Motor Module (or vice versa). The drive is then closing the loop on the wrong physical shaft.

1. With both motors mechanically accessible and uncoupled, command a slow speed (10 rpm) on SERVO_02 from the STARTER control panel.
2. Visually confirm that the physical motor attached to power X1 (not power X2) is the one rotating. If the motor on X2 rotates, the green DRIVE-CLiQ cables are swapped at the Motor Module.
3. Power down, swap the two DRIVE-CLiQ connectors between X1 and X2 on the Motor Module, and repeat the test.
4. Once the rotation matches the active drive object, re-run the standstill measurement on both axes to update the commutation offset in p0431 and the torque/current limit in p1520/p1521.

Solution C — Relax the Motor-Locked Detection (Diagnostic Only)

Use this only to confirm the protection path is the source of the trip, never as a permanent workaround.

1. In STARTER, open Drive → Messages/monitoring → Speed messages.
2. Set p2175 = 0.00 rpm to disable the motor-locked speed threshold.
3. Set p2198.0 = 0 if you also want to disable the monitoring flag entirely.
4. Download to the drive and run the same 100 rpm test from the control panel.
5. If F07900 still trips, the fault is in the torque/current loop, not the speed monitoring — return to Solutions A and B.
6. Revert p2175 to the recommended 6.00 rpm (or the calculated value for your application) before commissioning the machine.

Solution D — Check Torque and Current Limits

If both wiring paths are correct and F07900 still appears at higher speeds, verify the following with the drive online:

• r0027 (actual speed smoothed) versus r0062 (speed setpoint) — if the gap is sustained and the torque setpoint r0079 saturates against r1520, the speed controller is in current limit.
• r0067 (actual current) versus r0068 (current limit) — saturation here confirms torque limit too low.
• Mechanical load: free the shaft completely. Even a sticky bearing or misaligned coupling at standstill can hold n_act below p2175 long enough for F07900 to trip after p2177 (3.0 s) elapses.
• If the standstill measurement ran with load attached, re-run it with the motor uncoupled and re-accept the values. The measurement writes new limits for p1520/p1521.

EPOS Integration with S7-300

Once the F07900 path is resolved, the same S7-300 application code (FB 1 / FB 2 / FB 3 EPOS blocks, or a manual telegram 111 setpoint) will follow the position setpoint without raising the fault. Two notes specific to PROFINET IRT between an S7-300 (315-2 PN/DP) and a CU320-2 PN:

1. The PROFINET send/receive cycle must match the drive-side clock (p2048 / p0922 = 100). The default STARTER telegram for EPOS on a CU320-2 PN with the 1FT7 motor is telegram 111 (positioning, with 32-bit position, velocity override, and control/status words). The 315-2 PN/DP supports this, but the IRT slot must be configured topologically in HW Config.
2. Bit 0 of the EPOS control word (STW1.0 — ON/OFF1) must be set on each axis. The S7 application must also clear STW1.1 (no OFF2) and STW1.2 (no OFF3), and set STW1.3 (enable operation) and STW2.4 (interpolate). If any of these bits are false the drive will not open the speed controller and the next setpoint will trip F07900 because the controller is disabled.

Verification Procedure

After implementing any of the solutions above, perform the full verification before returning the machine to production:

1. Clear the fault buffer in STARTER (Drive → Diagnostics → Fault buffer → Acknowledge all) and run a power-on reset (line contactor OFF, then ON, after the DC-link discharge time).
2. Run the commutation-angle test via the control panel: Commissioning → Motor/Encoder → Commutation. The drive should report SUCCESS with the new angle stored in p0431.
3. Run the rotating measurement on each axis separately, with the motor uncoupled. Accept the new controller gains (p1460 / p1470 / p1472 — speed controller Kp and Tn).
4. From the STARTER control panel, ramp each axis through −1500 → 0 → +1500 → 0 rpm in steps of 250 rpm. Capture r0027, r0062, and r0079 in the trace. Confirm no F07900 / F07901 / F08501 appears in the fault buffer.
5. From the S7-300, issue a relative positioning command of 10 revolutions at 200 rpm on SERVO_02, then SERVO_03, then both simultaneously. Confirm position following error r2561 stays within the configured window (p2546) and that no F011 / F07900 is raised.
6. Re-enable the motor-locked detection (p2175 = 6.00) and confirm the protection is operational by blocking the shaft mechanically for ≥ 4 s during a 100 rpm command. F07900 should re-appear — this confirms the protection path is alive and the production configuration is safe.

Parameter Cross-Reference Table

Fault Code Family Reference

F07900 belongs to a small group of closely related faults that the speed and current monitoring channels can raise. Knowing the siblings helps disambiguate in the field.

Safety Notes for Working on the SINAMICS S120

Documentation References

• SINAMICS S120 Equipment Manual — DC-link discharge time, terminal layouts, wiring
• SINAMICS S120/S150 List Manual — full parameter list including p2175, p2177, p1520, p1821, p0431
• SINAMICS S120 Commissioning Manual — STARTER control panel procedure and standstill/rotating measurement
• SINAMICS S120 Communication Manual (PROFINET) — telegram 111, IRT configuration
• SIMOTICS 1FT7 Configuration Manual — 1FT7042 connector pin-out, prefabricated cable part numbers
• SINAMICS S120 EPOS Function Manual — basic positioner control words, p2546 following error window

FAQ