SINAMICS G120 CU250s-2 PN Torque Limit Behavior in Winder Applications

The SINAMICS G120 modular drive family with the CU250s-2 PN Control Unit is a frequent choice for winding and unwinding stations in converting lines, slitter-rewinders, and foil or paper lines. Engineers commissioning these drives routinely encounter a tight cluster of symptoms around torque control:

1. The drive enters torque-controlled operation but the speed plateaus at a fixed RPM that is below the configured maximum, instead of climbing to n_max when the winder is unloaded.
2. At low web speeds everything looks stable; as the line accelerates the Starter/TIA Portal commissioning tool flags "Torque limit reached" and the actual speed locks to a ceiling value.
3. Attempts to move the upper and lower torque limit from the PLC through PROFINET PQW words produce no change on the drive, even though the analog input address shows the value being written.

All three symptoms share a single underlying cause: the torque ceiling is below the torque required for acceleration. This reference explains the root cause, the exact parameter map on the CU250s-2 PN, the PROFINET/PZD wiring required to move torque limits from an S7-1200/1500, and the commissioning procedure that fixes the issue without changing application software.

1. Root Cause: Torque Demand vs. Acceleration Torque

A winder running in torque mode must still accelerate. The motor must produce the following torque at all times:

M_motor = M_tension + M_friction + M_inertia + M_acceleration

Where:

• M_tension = F_web × D/2 (web force × roll radius)
• M_friction = Coulomb + viscous losses (bearings, gear, drag from adjacent rollers)
• M_inertia = J_roll × dω/dt (effective inertia of roll + core + web)
• M_acceleration = (J_motor + J_coupling) × dω/dt

The drive enforces:

|M_motor| ≤ max(|p1520|, |p1521|)

If the absolute value of M_motor required for the current operating point exceeds the configured torque limit, the speed controller saturates and the drive cannot deliver the torque required for further acceleration. The result is a fixed, sub-maximum speed plateau. This is not a closed-loop torque control failure; the controller is doing exactly what it was told. The ceiling is the problem.

The "Torque limit reached" indicator that appears at higher RPMs is the standard r1407.12 status bit, generated by the monitoring function in the vector control. It always lights when the torque setpoint is clamped to the active limit, regardless of whether the limit was sourced from p1520/p1521, scaled through p1522/p1523, or written from the PLC.

2. SINAMICS G120 Torque Limit Parameter Reference

All torque limits on the CU250s-2 PN are referenced to rated motor torque (p2003) and are scaled in 0.01 % units when written through PROFINET (16384 = 100 %). The relevant parameter map is summarized below.

When p1522 or p1523 is connected to a connector source other than the fixed 1, the active torque limit becomes p1520 × p1522 (upper) or p1521 × p1523 (lower). This is the entry point for PLC-driven torque limit override.

3. PZD and Telegram Configuration for Torque Limits

The CU250s-2 PN supports the standard Siemens telegrams plus a free configurator (telegram 999). Three telegrams are useful for torque-limited winder control:

Telegrams 350 and 352 are the path of least resistance for winders because the PLC writes a torque limit value directly into the connector input of the torque limit scaler. The drive handles the multiply with p1520/p1521 and the clamping internally.

3.1 Enabling Telegram 350 in STARTER

1. Open the drive project in STARTER (or TIA Portal with the G120 Startdrive plug-in).
2. Navigate to Communication → PROFINET → Telegram Configuration.
3. Set p0922 = 350 (or p2079 = 1 to extend telegram 1 with additional words).
4. Compile and download. The I/O addresses in the S7 hardware configuration update automatically.

3.2 Custom telegram 999

If the application requires additional words (e.g. additive torque trim, friction compensation override), set p0922 = 999 and configure:

p2051[0] = r0052 (ZSW1)        ; Status word 1
p2051[1] = r0021 (NIST)        ; Speed actual
p2051[2] = r0080 (M_actual)    ; Torque utilization
p2061[0] = r0050 (STW1)        ; Control word 1
p2061[1] = p1155 (N_soll)      ; Speed setpoint
p2061[2] = p1522 source        ; Upper torque limit scaler
p2061[3] = p1523 source        ; Lower torque limit scaler
p2071[0..15] = 0               ; Diagnostics for send words

These assignments let the PLC write torque limits into the right input, which is the common reason a PQW write "appears to do nothing" – the word is wired to the wrong connector or to a status word on the output side.

4. PROFINET Slot Mapping: PQW → Drive Parameter

CU250s-2 PN occupies a single PROFINET slot with subslots per telegram word. In TIA Portal / STEP 7 the I/O address layout for a default project with telegram 1 is:

The address in the original configuration (PQW356 / PQW358) maps to a slot offset of 354 with two 16-bit words of padding in front, which is consistent with telegram 350 and a slot reserved for the safety module. With telegram 1 in that slot, PQW356 would write the third word — but on a default project there is no third word for telegram 1, so the value is dropped into the void. Switching to telegram 350 or 999 wires the third word to p1522.

4.1 PLC example: S7-1500 / TIA Portal STL

// Assume drive I/O start address = 256 (re-read in hardware config)
// PQW256 = STW1, PQW258 = NSOLL, PQW260 = torque upper, PQW262 = torque lower
// PIW256 = ZSW1, PIW258 = NIST, PIW260 = MIST, PIW262 = fault/warn

// Convert torque percent (REAL 0..100) to 16-bit (0..16384)
"DB_Winder".Torque_Upper_Pct := 110.0; // 110 % during accel, 95 % at steady state
"DB_Winder".Torque_Upper_Raw :=
    REAL_TO_INT("DB_Winder".Torque_Upper_Pct * 16384.0 / 100.0);

// Write to drive (assumes area pointer via PUT/GET or MOVE to PQW)
MOVE_BLK(VARIANT:="DB_Winder".Torque_Upper_Raw, OUT:=PQW260);

// Read actual torque
"DB_Winder".Torque_Actual_Raw := PIW260;
"DB_Winder".Torque_Actual_Pct := INT_TO_REAL(PIW260) * 100.0 / 16384.0;

5. Winder-Specific Torque Components

A correct torque limit for a center winder must leave headroom for the four components described in section 1. A simplified static calculation at radius R (in metres), web width b (m), web tension F (N/m) and effective friction M_f (Nm):

M_tension_max = (F_max × b) × (R_core + h_max)   // h_max = max web thickness on roll
M_inertia_max  = (1/2) × (ρ π b h_max (D_core + h_max)^2) × (dω/dt)
M_accel_max    = J_drive_train × (dω/dt)

The minimum required torque limit at radius R and target line accel a (m/s²) is:

p1520_required  ≥ 100% × (M_tension + M_friction + M_inertia_max + M_accel_max) / M_rated

with M_rated read from r0333 (rated motor torque). For a 22 kW, 1460 rpm motor with rated torque 144 Nm and a winder operating with full roll of 1.5 m diameter, F = 50 N/m at b = 1 m, the tension torque at full roll reaches 37.5 Nm. With M_{inertia_max} of 12 Nm at dω/dt = 0.5 rad/s², the required limit sits near 50 Nm / 144 Nm = 35 %. A default p1520 = 200 % is more than enough; the binding constraint is rarely the steady-state tension but the transient during ramp-up at low roll diameter, where M_tension is small but M_inertia is large because the angular acceleration is high.

For inertia and friction compensation, the SIMATIC "Converting Toolbox" distributed by Siemens regional partners contains the Winder Control Block library for S7-300/400/1200/1500. It includes friction and inertia models pre-tuned for typical web materials. The block is not on the public Siemens support portal and is supplied only through local Siemens field application engineers.

6. Commissioning Procedure in STARTER and TIA Portal

1. Mechanical data review. Record core diameter, max roll diameter, max web tension, web width, max line speed, and max line acceleration. Compute the required p1520/p1521 headroom per section 5.
2. Motor data auto-identification. Run p1900 = 2 (motor identification, complete) on a stationary, unloaded motor. Confirm r0334 (cos φ), r0333 (rated torque), r0337 (rated EMF) are sensible.
3. Speed loop optimization. Use p1960 or the STARTER speed-controller auto-tuning tool. Capture p1460 (Kp_n), p1462 (Tn_n) and back them up before commissioning the torque loop.
4. Telegram selection. Set p0922 = 350 for simple torque-limit override, or = 999 for free configuration. Re-download the project so the PROFINET interface restarts on the new telegram.
5. Torque limit static setting. With the PLC disconnected from the drive, set p1520 and p1521 to the calculated limits (e.g. 110 % upper, -50 % lower for a center winder that is only allowed to brake via the drive).
6. PLC handshake. Verify that the drive responds to NSOLL=0 in torque-limited mode. With STW1 bit 10 (control by PLC) high and the torque limit set, the drive should produce zero torque at zero speed setpoint.
7. PLC torque override. Enable the PLC write path to p1522/p1523 via the third and fourth PZD. Confirm with trace that the active limit r1538/r1539 follows the PLC word.
8. Line test. Run the winder through a complete reel build. Capture r0078 (torque setpoint), r0080 (torque utilization), r0079 (speed setpoint), r0021 (speed actual), and the active limits.

7. Trace Diagnostics: Reading Actual Torque vs. Limit

A 4-channel trace with a 1 ms timebase is the fastest way to determine why a winder stalls. Recommended signals:

If r0078 sits flat against r1538 and r0021 stops climbing, the limit is the bottleneck. If r0078 tracks a value below r1538, the controller is not demanding more torque and the issue is in the upstream setpoint (line speed reference, web tension feedback, dancer position).

8. Troubleshooting Matrix

9. Alternative Approaches and Free Blocks

For engineers who need a turnkey winder rather than a hand-tuned torque limit, three Siemens-supported options exist:

1. SIMATIC Converting Toolbox — Winder Control. A library of FBs for S7-300/400/1200/1500 containing open-loop and closed-loop tension control with friction and inertia models. Distributed by Siemens regional partners.
2. Technology object "TO_Winder" in the TIA Portal technology objects library (available from V16 with Startdrive V16 Update 3). Provides structured state machine for empty-core start, ramp-up, taper tension, and stop.
3. DCC (Drive Control Chart) on the CU250s-2 PN. Graphical function block editor resident on the drive; useful for trim functions such as inertia feedforward, diameter calculation, and taper tension profile without consuming PLC scan time.

For the FBs in the converting toolbox that produce SCL compile errors (a typical situation when a project references newer/older block versions), regenerate the source from the library version that matches the STEP 7 / TIA Portal release installed, then update the block types in the SCL compilation cache. The issue is normally a missing dependency in the master library, not a syntax error in the calling FB.

10. Verification Checklist

After applying any change, complete the following on the running line:

• Trace r0078 (torque setpoint) for at least 30 s of steady-state operation at full roll. It must remain below r1538 (upper limit) with at least 5 % margin.
• Toggle the PLC torque-limit PQW from 50 % to 110 %. r1538 must track within one PROFINET cycle (~2 ms).
• Run a full reel build, capture r0021 (speed actual) and r0023 (output voltage). Speed must follow the line reference within the configured tolerance band.
• Decel the line from full speed to zero. DC link voltage must stay below the configured threshold (default 760 V on a 400 V unit) with the configured decel ramp.
• Read the diagnostic buffer with r0945[0..7] / r0947[0..7]. The buffer must remain empty across 100 complete reel builds before the torque limits are signed off.

Refer to the SINAMICS G120 CU250s-2 operating instructions and the SINAMICS G120 function manual for the definitive parameter reference. The PROFINET fieldbus interface description documents the slot model and telegram structure. For firmware ≥ V5.2, review the release notes referenced under G120 firmware V5.2 SPx for changes to p1522/p1523 behaviour and the free block library.