Configuring SINAMICS G120 OPC Setpoint as RPM via Profibus

Overview

The SINAMICS G120 frequency inverter with the CU240S-DP-F Control Unit communicates with higher-level PLCs and SCADA systems through standard telegrams on Profibus DP. Internally, every speed reference in the drive is a frequency expressed in Hz, never in mechanical RPM. The Profibus process data are normalized integers in the NORM data type, where 0x4000 (16384 decimal) corresponds to 100% of the reference frequency defined by parameter P2000. This convention is described in the SINAMICS CU240S Function Manual.

When a SCADA or HMI issues a setpoint in RPM through an OPC DA/UA server, the application must convert the engineering value to a normalized 16-bit or 32-bit integer using P2000 as the 100% reference. The drive then converts the integer back to Hz for the ramp generator and motor model. Because P2000 is internally clamped at 650.00 Hz on the CU240S, the reference frequency must be set to the maximum operating frequency rather than to a contrived value of 3600 Hz that would map 1 RPM directly to a single bit. The PLC must perform the scaling in every direction.

Prerequisites

• SINAMICS G120 inverter with CU240S-DP-F Control Unit (parameter set documented in the CU240S function manual)
• Starter (legacy) or Startdrive (TIA Portal) commissioning tool installed on the engineering workstation
• SIMATIC NET OPC server or third-party OPC DA/UA gateway with Profibus DP connectivity
• Standard telegram 354 configured on the drive, or equivalent PZD telegrams such as 1, 20, 350, 352
• Motor nameplate values: rated voltage, rated current, rated frequency, rated RPM, cos phi, and connection (star/delta)
• Maximum mechanical speed requirement defined by the process (example: 3600 RPM at a 60 Hz base)
• PROFIdrive profile awareness on the OPC client side so NORM-to-engineering scaling is correct

Reference Frequency Scaling Fundamentals

All PZD words exchanged with a SINAMICS drive follow the NORM convention defined in the PROFIdrive profile. The reference variable is the value of P2000, which the drive treats as 100% of the speed setpoint and the speed actual value.

The speed setpoint path inside the drive is:

1. P1070 selects the main setpoint source (default = analog input 755)
2. P1071 scales the main setpoint (default = 1.0)
3. r2050 receives the cyclic Profibus setpoint word; the CU240S auto-interconnects it to P1070 when a standard telegram is selected
4. P2000 defines what 100% means in Hz
5. The ramp generator (P1120 acceleration, P1121 deceleration) shapes the Hz value before it is delivered to the motor model

Bit 10 (decimal 1024, hex 0x0400) of control word 1 is the "control from PLC" bit and must be set before the drive accepts cyclic setpoints. Watch r0054 to confirm the drive is in remote control.

Step-by-Step Parameter Configuration

1. P310 = motor rated frequency (Hz). Example: 60.00 Hz for a 4-pole 50/60 Hz induction motor.
2. P311 = motor rated RPM (mechanical RPM). Example: 3545 RPM for a 60 Hz motor with approximately 5% slip.
3. P1082 = maximum frequency (Hz). Calculated as:

   P1082 = (Max_RPM / Rated_RPM) × P310

   For 3600 RPM maximum on a 3545 RPM / 60 Hz motor:

   P1082 = (3600 / 3545) × 60 = 60.930 Hz

4. P2000 = P1082. The reference frequency equals the maximum frequency so that 100% on Profibus equals maximum RPM.
5. Confirm P2050[0] interconnects the receive word for the speed setpoint (default for telegram 354).
6. Confirm P2050[1] interconnects the send word for the speed actual value.
7. Verify the bits of control word 1 in P3980 and P3981 so that bit 10 (PLC control) routes to r0054 bit 9 correctly.
8. Save the parameter set to the CF card and back it up with STARTER or Startdrive.

OPC Client Scaling Calculations

With P2000 = P1082 = Max_Hz, the PC converts RPM to a 16-bit NORM word as follows:

OPC_Setpoint_WORD = Commanded_RPM × (16384 / Max_RPM)

Round the result to the nearest integer before sending. The drive-side decoding is automatic:

Internal_Hz = (OPC_Setpoint_WORD / 16384) × P2000

For the reverse path (drive actual speed back to RPM in the HMI):

Actual_RPM = Received_WORD × (Max_RPM / 16384)

When using 32-bit double-word telegrams (telegram 350 or 352), replace 16384 with 2147483648 (2^31) in the same formulas.

Worked Example

Motor: 3545 RPM rated, 60 Hz rated, 3600 RPM maximum. P1082 = P2000 = 60.93 Hz.

The granularity at 3600 RPM is 3600 / 16384 = 0.220 RPM per LSB, approximately 0.006% of full scale. Compared to a 1 Hz granularity at a 60 Hz base (60 / 16384 = 0.00366 Hz/LSB), the RPM scaling delivers significantly more resolution when the application requires precision below 100 RPM.

Telegram 354 and P2050 Monitor Connectors

Telegram 354 is a 4-word (8-byte) PZD telegram suitable for a single setpoint and a single actual value. The SINAMICS G120 list manual assigns the receive and send words to specific connectors:

The drive does not present values in RPM on the bus. P2050[1] returns the NORM word whose 100% corresponds to P2000. To display RPM on the HMI, the OPC client must multiply the received value by Max_RPM / 16384. The CU240S function manual documents the connector-to-telegram assignment under function diagram 1020 (process data).

OPC Symbol Configuration

One of the most common commissioning faults is declaring OPC symbols as 8-bit bytes instead of 16-bit words. Each PZD occupies one 16-bit word, and the high byte of a wrong symbol will alias the adjacent word, scrambling setpoint and feedback. Always configure OPC items as:

• VT_UI2 or VT_I2 for 16-bit PZD
• VT_UI4 or VT_I4 for 32-bit PZD (when double-word scaling is enabled on telegram 350 or 352)
• Match byte order to the Profibus DP master convention (little-endian on Siemens masters, big-endian on some third-party gateways)
• Use a separate item for each PZD; do not pack two speed setpoints into a 32-bit item unless the drive is configured for that telegram

Alternative Path: Free Blocks and P1071 Scaling

If the OPC server writes directly to a parameter via the acyclic channel (DPV1, STARTER routing, or SIMATIC NET parameter services) instead of going through cyclic Profibus, the standard telegram interconnections are bypassed. In this case the SCADA-side conversion is unnecessary if a free interconnectable block is used to multiply the value by an additional scale factor.

SINAMICS G120 provides free interconnectable blocks under parameter range P2800-P2890. A typical configuration uses the free multiplier block:

1. Connect P1070 (main setpoint source) to r2878 (multiplier output).
2. Set P2871 = 1.0 (default input X).
3. Set P2872 = scaling factor Y. For RPM-to-Hz on a 3545/60 motor: Y = 60 / 3545 = 0.01693.
4. Write the RPM value into the free block input through P2877. The drive computes Hz = RPM × 0.01693 before the ramp function generator.

When the OPC server uses an acyclic parameter channel, writing P1070 directly is also possible. In this case set P1071 (main setpoint scaling) to the same conversion factor. The parameter list manual notes that P1071 is a multiplier applied after P1070 but before the ramp generator. For belt-driven loads where the gear ratio converts motor Hz to load RPM, set P1071 = gear ratio so the OPC user can write load RPM directly.

Verification and Commissioning

1. Open Starter or Startdrive in online mode and connect to the CU240S-DP-F.
2. In the Commissioning → Trace panel, enable the trace on r2050[1] (received setpoint) and P2050[1] (sent actual).
3. From the OPC client, write 16384 to the speed setpoint item. The drive should ramp to P2000 = Max_Hz and reach Max_RPM on the motor shaft.
4. Read the actual speed value back from the drive and confirm: Actual_RPM = P2050[1] × (Max_RPM / 16384).
5. Switch control word bit 10 ON (status word bit should mirror on ZSW1) and confirm ZSW1 reports "Operation enabled" (bit 2 set, decimal 4).
6. Decrement the setpoint in 100 RPM steps and confirm linearity within ±1 RPM across the full range.
7. Trigger an OFF1, OFF2, and OFF3 sequence from the OPC client and verify the drive executes the ramp-down without fault.
8. Repeat the procedure for each of the seven drives in the project to confirm consistent scaling across the fleet.

Troubleshooting Matrix

Field-Proven Caveats

• When the same SCADA writes RPM to multiple drives, replicate the scaling formula in the OPC client macro so a single parameter change (Max_RPM) updates all instances without manual edits to each tag.
• When the SCADA refreshes the symbol database after a firmware upgrade, verify the new CU240S firmware does not reset P2000. On recent firmware versions, P2000 is included in the parameter backup file and survives a factory reset only if the backup is restored manually.
• For very high-inertia loads, decouple the ramp acceleration (P1120) from the reference frequency update. The OPC layer should never write the setpoint faster than the drive can ramp; otherwise the drive rejects the value or faults with F07900.
• If the motor has a separately excited field, P2000 must reflect the field-weakening range, not the base speed. Set P2000 = P1082 even if that exceeds the motor rated frequency, but ensure the CU240S clamping at 650 Hz still covers the requirement.
• Do not declare OPC speed tags as floating-point items. The PROFIdrive NORM is an integer; floating-point declarations trigger silent value truncation by the OPC server.
• When migrating from PROFIBUS to PROFINET, the G120 with CU240S-DP-F must be replaced with a CU240S-PN-F; the same scaling formulas apply because PROFIdrive is transport-independent.

FAQ