Troubleshoot SINAMICS G180 PROFIBUS Frequency Swings in PCS 7

A G180 frequency excursion toward 50 Hz, then near zero, cannot be diagnosed from the reported symptom alone. In PCS 7, first confirm the G180's PROFIBUS control and setpoint sources, the STW1/PZD values exchanged with the drive, and whether related CFC blocks execute in a consistent wake-up alarm. Siemens' G180 application example for PCS 7 V9.0 SP1 documents the required bus-source parameters and places Event16T and MotSpdCL in the same wake-up alarm; it does not identify the cause of this specific transient.

Symptoms and Applicability

The reported system uses SINAMICS G180, PROFIBUS and PCS 7 V9.0. On a bus start command, the frequency reportedly rises close to 50 Hz, falls near zero and then approaches the setpoint. The author observed normal behavior after “auto tuning” under inverter management software (IMS), but the transient recurred while control was via the bus. The drive CFC program was said to use both OB35 and OB1. The exact G180 order number, firmware, telegram, parameter set, CFC block versions and recorded PZD values were not provided.

Siemens Entry ID 109474410 V3.1 documents a G180 PROFIBUS solution for PCS 7 V9.0 SP1 with APL. This version-matched application is useful for checking the integration and bus parameterization; its example does not prove that the same block arrangement or cause applies to every project.

How PCS 7 Transfers Drive Commands

In the documented example, FbDrive sends two output words to the G180. STW1 carries command bits and PZD 2 OUT carries the scaled frequency setpoint. The drive must be configured to accept the relevant control and setpoint sources from PROFIBUS. For the example, Siemens specifies P-INTERFACE > Op.-source = BUS and P-MACHINE POWER CONV > P-speed default > n SetpSource = BUS 1. The setpoint's 0–100% scaling is mapped through the documented bus setpoint parameter; verify the correct parameter names and scaling for the installed G180 version and telegram.

Siemens' example also notes that if both control and setpoint sources are assigned to PROFIBUS, status word ZSW1 bit 9 signals that the automation system should take over control. Local IMS control, bus command bits and setpoint source selection must therefore be checked as separate items; auto-tuning and control-source ownership are not interchangeable concepts.

Check the CFC Execution Context

The original system places drive logic in both OB1 and OB35. This is a significant project detail to investigate, but the discussion does not show which blocks are in each OB or prove that split execution caused the swing. In the Siemens V9.0 SP1 example, Event16T is connected to MotSpdCL and the manual states these two components must be in the same wake-up alarm. Check the exact block instructions and execution assignment for every component in the project's drive template. Avoid moving blocks between OBs as an unverified fix: that changes update timing and can affect control behavior.

Troubleshooting Procedure

Preserve the existing PCS 7 project and capture a time-correlated trace of the frequency setpoint, PZD 2 OUT, STW1, returned PZD data and ZSW1 during a controlled start.
Confirm the configured PROFIBUS telegram and that PCS 7 input/output addresses match the G180 hardware configuration. The Siemens example calls for matching input and output addresses for its telegram configuration and checking all PZDIn and PZDOut connections after compilation.
Compare the actual G180 control and setpoint source parameters with the version-matched G180 instructions. For the PCS 7 V9.0 SP1 example, check P-INTERFACE > Op.-source and P-MACHINE POWER CONV > P-speed default > n SetpSource are assigned to the intended source (BUS/BUS 1).
Decode STW1 and ZSW1 according to the configured telegram. In particular, verify the ON/enable/setpoint-related bits, bus-control ownership, and status response; do not assume a single command bit proves the drive accepted the intended setpoint.
Compare the PZD 2 OUT value against the configured bus setpoint scaling and desired frequency. Siemens' example uses a 0–100% normalized transfer; the actual frequency represented by 100% depends on the drive parameterization.
Review the CFC execution assignment for the drive channel and associated event/faceplate blocks. Keep components together where the applicable Siemens template requires the same wake-up alarm, and evaluate any project-specific split between OB1 and OB35 before changing it.
Test one change at a time in a controlled, safe condition. Compare the same command under local IMS and bus control while recording the PZD/control-source behavior. The source reports the IMS/auto-tune contrast but provides no logs to identify causation.

Interpreting IMS and Auto-Tuning Observations

The reported improvement after auto-tuning is an observation, not evidence that the transient is caused by motor data, a ramp parameter, the PLC scan cycle or the PROFIBUS telegram. Likewise, a transient that occurs only under bus control points to checks of bus control ownership, setpoint source, PZD content and PCS 7 execution context, but does not isolate one. Use G180 commissioning documentation applicable to the exact drive version to determine whether further motor identification or tuning is appropriate; do not repeat commissioning actions on a connected load solely as a diagnostic guess.

Expected Results

For the configured bus mode, the G180 should accept the intended control word and interpret the setpoint according to the configured bus scaling and source. PCS 7 feedback should reflect the drive status and measured frequency through the configured PZD mapping. Verify the entire transition from command to accepted status to actual response; do not treat a final frequency near setpoint as proof that the startup transient is understood.

Verification

During a controlled start, compare the outgoing STW1 and PZD 2 OUT with the drive's returned ZSW1 and frequency feedback. Verify the bus source remains selected, setpoint scaling is correct and the relevant CFC blocks update in the intended wake-up alarm. Repeat only under approved operating conditions and with a defined stop/rollback method. No laboratory testing was performed for this article.

Limitations and Operational Precautions

The official G180 application example retrieved is valid for PCS 7 V9.0 SP1 with APL, while the source only specifies PCS 7 V9.0. No source trace identifies the active telegram, control-word bit sequence, ramp/tuning parameters, drive firmware, motor data or exact timing of the excursion. Siemens documentation supports the parameter and block-coordination checks above but does not establish the ticket's root cause. Because the reported excursion may affect torque and the driven load, do not repeatedly command starts on connected production equipment to reproduce it. Assess mechanical/process risk and validate changes in a controlled condition under site procedures.

Engineer FAQ

Does the frequency swing prove that OB1 and OB35 are incompatible? No. The source reports that they are both used and a forum participant suspected the split, but no trace or project inspection confirms that cause.

Which drive parameters select PROFIBUS control and setpoint? The PCS 7 V9.0 SP1 example specifies P-INTERFACE > Op.-source = BUS and P-MACHINE POWER CONV > P-speed default > n SetpSource = BUS 1; confirm exact paths for the installed G180 firmware/manual.

What does STW1 bit 10 indicate in the cited example? The example's telegram table defines bit 10 as bus-operation selection: one state enables management by PROFIBUS and the other rejects PZD output data. Decode the value using the project's actual telegram and manual.

Why check PZD 2 OUT scaling? The example transfers a normalized bus setpoint, with drive parameters mapping the normalized value to a frequency. A scaling mismatch can make the command differ from the intended frequency.

Does auto-tuning prove a motor parameter caused the issue? No. The reported sequence does not isolate auto-tuning from other changed conditions or provide evidence of the underlying cause.

Technical References

Siemens, Configuration of SINAMICS G180 Frequency Converters on PROFIBUS in PCS 7, Entry ID 109474410, V3.1 (10/2020), valid for PCS 7 V9.0 SP1 with APL; §§4.2–4.3, 5.2, 6.2, 8.2–8.7 cover CFC block coordination, telegram words, bus source parameters and setpoint scaling: https://cache.industry.siemens.com/dl/files/410/109474410/att_1034375/v1/109474410_PCS7_SINAMICS_G180_PB_V90_SP1_Doc_en.pdf
Siemens, Configuration of SINAMICS G180 Frequency Converters on PROFIBUS in PCS 7, Entry ID 109474410, V2.1 (10/2020), earlier version with the same PCS 7 V9.0 SP1 application topic and detailed setpoint mapping: https://cache.industry.siemens.com/dl/files/410/109474410/att_1034371/v1/109474410_PCS7_SINAMICS_G180_PB_V90_SP1_Doc_en.pdf

Troubleshoot SINAMICS G180 PROFIBUS Frequency Swings in PCS 7