Configuring Speed Reference Scaling for SINAMICS G120 with Drive ES APL Block FB1721 in PCS 7

In SIMATIC PCS 7 v8.1 SP1, speed setpoint discrepancies when controlling a SINAMICS G120 drive via Drive ES PCS 7 APL block APL_GS (FB1721) occur due to a scaling mismatch between the controller block parameters and the drive's reference frequency parameter p2000. In the PROFIdrive profile, 100% nominal speed is transmitted as hexadecimal 16#4000 (16384 decimal). To ensure accurate speed reference control, parameter RefSpeed on FB1721 must match p2000 in the inverter, the CFC scale structure SP_OpScale must reflect physical RPM units, and the drive must be in valid Automatic (SP_Ext) or Manual mode.

PROFIdrive Speed Reference Architecture and Telegram Scaling

In SIMATIC PCS 7 systems, integrating variable speed drives such as the SINAMICS G120 (6SL3210-1PE21-8UL0) over PROFIBUS DP or PROFINET IO utilizes the standardized PROFIdrive profile (e.g., Standard Telegram 1, 20, or Siemens Telegram 352).

Cyclic communication exchanges two primary process words:
* STW1 / ZSW1: Control Word 1 and Status Word 1.
* NSOLL_A / NIST_A: 16-bit signed integer representing Speed Setpoint and Speed Actual Value.

In PROFIdrive standardization:
* The integer value 16#4000 (hexadecimal), or 16384 (decimal), represents 100.0% of the drive's reference speed.
* In the SINAMICS G120 inverter parameters, reference speed is defined by parameter p2000 (typically set to motor rated speed, e.g., 1500.0 RPM or 50.0 Hz).
* If the PLC driver block and inverter assume different reference speeds, the physical motor speed will diverge significantly from operator setpoints.

Drive ES PCS 7 APL Block FB1721 (APL_GS) Parameter Structure

The Drive ES PCS 7 APL library provides function block APL_GS (FB1721), combining APL look-and-feel with native PROFIdrive communication:

Parameter NameData TypeI/O CategoryEngineering Function and Scaling RuleSP_ExtREAL (STRUCT)InputExternal automated setpoint supplied by upstream process controllers in physical units.SP_IntREALInputInternal manual setpoint entered by operators via the WinCC faceplate.RefSpeedREALInputReference speed in physical units. Must match parameter p2000 in the G120 drive.SP_OpScaleSTRUCTInputOperator scale structure defining high/low display limits on faceplate bargraphs and trends.SP_HiLim / LoLimREALInputOperational setpoint clamping limits in physical units.Out / ActSpeedREAL (STRUCT)OutputActual speed feedback converted from NIST_A back into physical engineering units.

Table 1: Key parameters of Drive ES PCS 7 APL block FB1721.

Aligning Reference Speed (p2000) with Block Scaling in CFC

To ensure that a 1000 RPM setpoint command produces exactly 1000 RPM at the motor shaft:

Verify Inverter Reference Parameters:
* Open the drive parameterization tool (STARTER or Startdrive).
* Check parameter p2000 (Reference Speed, e.g., 1500.0 RPM).
* Check parameter p1082 (Maximum Speed, e.g., 1500.0 RPM).
Configure FB1721 in CFC:
* In SIMATIC Manager, open the CFC chart containing the APL_GS block.
* Set parameter RefSpeed = 1500.0.
* Expand SP_OpScale: set High = 1500.0 and Low = 0.0.
* Set SP_HiLim = 1500.0 and SP_LoLim = 0.0.
Mathematical Normalization in Block Execution:
* When an operator inputs SP = 750.0 RPM, the block calculates:
$$\text{Telegram Value} = \frac{750.0}{1500.0} \times 16384 = 8192 \quad (16#2000)$$
* The inverter receives 16#2000 (50%) and runs at $0.5 imes 1500 = 750 ext{ RPM}$. Mismatches disappear completely.

Troubleshooting Mode Selection and Setpoint Clamping

If setpoint adjustments produce zero motor speed response despite correct scaling:
* Operating Mode Interlocks: Verify whether the block is in Auto or Manual mode. If ModLiOp = 1 (linking interface priority), the block enforces automatic mode and ignores manual setpoints.
* Setpoint Limits (SP_HiLim / SP_LoLim): If SP_HiLim defaults to 0.0, the block clamps all positive setpoints to zero.
* Enable Signals (EnFwd / EnRev): Verify that forward rotation is enabled (EnFwd = TRUE). If directional enables are missing, setpoint generation is suppressed.

Verification

To confirm proper speed reference operation:

In CFC test mode, verify that RefSpeed matches p2000.
In WinCC Runtime, open the G120 faceplate.
Switch the drive to Manual mode and enter setpoint SP = 750.0 RPM.
Start the drive: confirm that motor actual speed feedback (ActSpeed) accelerates steadily and stabilizes at 750.0 RPM.
Check STARTER online monitor: verify that parameter r2050[1] (received speed setpoint) reads 50.0% (16#2000) and output frequency reflects 25.0 Hz.

Limitations and Operational Precautions

Never modify parameter p2000 in the G120 inverter without simultaneously updating RefSpeed on the CFC block; altering reference speed in the drive without PLC alignment will distort all process control loops.
Drive ES PCS 7 APL requires appropriate engineering and runtime licenses (6SW1700-5JD00-1AA0). Always validate licensing packages with an authorized Siemens distributor.
Ensure maximum acceleration/deceleration ramp times in the drive (p1120, p1121) coordinate with upstream ramp limiters on FB1721 to prevent mechanical drive train jerking.

Engineer FAQ

What is the difference between standard FbDrive and Drive ES APL_GS?

FbDrive (FB179) is a basic driver block from the standard library that only handles raw telegrams. APL_GS (FB1721) from Drive ES APL includes full APL operating modes, interlocks, feature bits, and high-resolution faceplates.

Can FB1721 control non-Siemens drives running PROFIdrive?

Yes. Any third-party variable frequency drive supporting PROFIdrive Standard Telegram 1 or 20 can be controlled using APL_GS, provided telegram structures align.