Siemens SINAMICS S120: Switching CU320-2 DP to Sensorless Mode

Overview

Switching a Siemens SINAMICS S120 drive with a CU320-2 DP Control Unit from encoder-based (sensored) to sensorless vector control is a useful capability for fault diagnostics, temporary troubleshooting, redundant-mode verification, and applications where the speed encoder is unavailable. The change is governed by parameter p1300 on the drive object, and the operating mode can be switched on-the-fly using Drive Data Sets (DDS) without rewiring the control cabinet.

The CU320-2 DP is the standard Control Unit of the SINAMICS S120 modular drive system. It supports simultaneous operation of up to six servo / six vector drive objects, encoder evaluation through Sensor Modules (SMC10, SMC20, SME20, SME25, SME120, SME125), and the full SINAMICS parameter tree for open-loop and closed-loop control. Its firmware stores both sensored and sensorless control structures, so selecting one or the other is a parameter-driven decision rather than a hardware-locked one, provided the motor model has been identified.

Sensorless vector control in SINAMICS uses a model-based speed and flux estimator that relies on motor nameplate data, the stator resistance / leakage inductance from the motor data identification routine, and the saturation characteristic of the induction machine. Because no physical speed feedback exists in this mode, the regulator bandwidth is intentionally lower than in sensored mode and the low-speed / zero-speed holding torque is limited. The mode is therefore well suited for commissioning trials, fault recovery, and continuous processes that operate in the medium-to-high speed range.

Hardware Context: CU320-2 DP and Sensor Modules

The CU320-2 DP is the DP (PROFIBUS) variant of the second-generation Control Unit. The other variant, CU320-2 PN, adds PROFINET. The Drive-CLiQ topology on a CU320-2 typically looks like this:

• CU320-2 DP — central controller; slots for option modules (CBE20, TM31, etc.).
• Sensor Module Cabinet (SMC) — mounted inside the cabinet near the motor cables; reads the encoder signals (e.g., SMC20 for incremental sin/cos 1 Vpp encoders, SMC10 for DRIVE-CLiQ encoders).
• Sensor Module External (SME) — mounted on the motor when the encoder is DRIVE-CLiQ capable (SME20, SME25, SME120, SME125).
• Line Module (Active Line Module, Smart Line Module, Basic Line Module).
• Motor Module (single or double axis) — actual power stage that drives the motor.

The encoder is read by the Sensor Module and the position/speed value is forwarded to the Control Unit via Drive-CLiQ. To go sensorless, the encoder wiring may stay in place, but the Sensor Module must either be deactivated in the topology or its evaluation must be disabled in the encoder parameter set. Otherwise the drive will drop out with an encoder fault on the next ramp-down or pulse enable.

Prerequisites

Before changing p1300, the following conditions must be met:

1. Commissioning PC with STARTER (legacy, V5.x) or StartDrive (TIA Portal V15 and later) installed and an Ethernet / PROFIBUS connection to the drive.
2. Online access to the drive object to be reconfigured (STARTER online mode / TIA online).
3. CU320-2 DP firmware that supports sensorless vector control. Sensorless vector control (p1300 = 20) is supported by the standard SINAMICS S120 firmware since the V2.x generation and remains present in current firmware versions; check the firmware release notes if running a non-standard build.
4. Motor data identification completed at least once (p1910 / p1960 routines), or stator resistance / leakage reactance entered manually in the motor data set (p0350, p0354, p0356, p0358, p0360).
5. Drive is in a safe state: pulse inhibit on the drive object, no active traversing command.
6. When physically removing the encoder: the corresponding Sensor Module port must be reconfigured to deactivated in the topology; otherwise the drive will signal F07900 (sensor module not ready) or F31117 (encoder signal error).

P1300 Control Mode Parameter — Reference

Parameter p1300 on a SINAMICS drive object selects the open-loop/closed-loop control mode. The settings relevant to this discussion, taken from the SINAMICS S120/S150 List Manual, are:

The setting cited in the source — p1300 = 20 for "Speed or velocity control (sensorless)" — is the standard fallback for fault-tolerant operation. Modes 21 and 22 add torque-controlled behavior but require the same motor model and impose similar bandwidth limits.

Drive Data Sets (DDS) for Runtime Switching

When the application requires occasional switching between sensored and sensorless modes — for example, a continuous casting strand that normally runs sensored but must drop to sensorless when the encoder trips — Drive Data Sets are the correct mechanism. The SINAMICS parameter model has:

• DDS0 — default Drive Data Set, parameter set indexed by p0180.
• CDS — Command Data Set (control word / setpoint source), indexed by p0170.
• EDS / MDS — Encoder Data Set and Motor Data Set, indexed by p0140 / p0130.

To switch the control mode on the fly:

1. Create a second Drive Data Set: set p0180 = 2 to enable DDS indexing on the drive object.
2. Copy the parameter set from DDS0 to DDS1 using the STARTER / StartDrive "Copy DDS" function (right-click drive object → Drive Data Sets → Copy).
3. On DDS1, set p1300[1] = 20 (sensorless), and on DDS0 keep p1300[0] = 21 or your sensored value (e.g., p1300 = 1 for V/f closed-loop with encoder, or a higher mode number for servo control).
4. Wire the DDS selection to a digital input or a PROFIdrive control word bit. In SINAMICS, the active DDS is controlled by bit 0 and bit 1 of the drive's p0820 / p0821 BICO chain. Connect a free DI on the CU320-2 DP (e.g., p0820 = r0722.x) to switch DDS at runtime.
5. Save to ROM (p0977 = 1) so the DDS assignment survives a power cycle.

When the DDS changes, SINAMICS rebuilds the active control structure on the next pulse enable. If the encoder evaluation is still active, the drive can be configured (p1300 = 20 with encoder present) to use the encoder for diagnostics while running the sensorless model in parallel — a useful state for verifying the model quality against the actual speed.

Commissioning with STARTER (Legacy Tool)

STARTER is the original Siemens commissioning tool for SINAMICS and remains supported for S120. Procedure to switch the drive to sensorless:

1. Open STARTER, go online with the CU320-2 DP target.
2. Navigate to the drive object (DO) for the motor you want to switch.
3. In the navigation tree, open Configuration → Open-loop/closed-loop control.
4. Set p1300 = 20. STARTER will trigger a configuration re-load and prompt for the change to be applied.
5. If the motor has not yet been identified, run the motor data identification under Commissioning → Motor identification. The two routines that matter are:
   • p1900 = 2 — standstill identification (measures stator resistance and leakage inductance without rotation).
   • p1910 = 1 — rotating identification (full motor model, requires safe direction of rotation).
6. Save the project and load to the drive (Target system → Load to target device).
7. Save to ROM (p0977 = 1) so the change survives a power cycle.
8. Verify with the control panel: in STARTER, right-click the drive object → Control panel → enable → ramp to a low speed (e.g., 200 rpm) → check the actual speed (r0021) tracks the setpoint (r0020).

Refer to the STARTER commissioning manual for CU320-2 DP on the Siemens Industry Online Support portal for the matching project setup steps.

Commissioning with StartDrive (TIA Portal)

StartDrive is the TIA Portal integration for SINAMICS drives. It exposes the same parameter tree as STARTER but inside the TIA project.

1. Add a SINAMICS S120 device with CU320-2 DP to the TIA project.
2. Go online with the device.
3. Open the drive object's Parameter view and search for p1300.
4. Set the value to 20, write to the device (Online → Write to device).
5. Run motor identification from the Commissioning panel of the drive object (StartDrive exposes p1900 and p1910 as buttons in the commissioning wizard).
6. Save the project to the PLC and load to the drive.

For StartDrive-specific instructions, consult the StartDrive function manual on the Siemens Industry Online Support site.

Encoder Removal Procedure

To physically remove the encoder from a running cabinet — for example, while a Sensor Module has failed and a replacement is not on site — the recommended sequence is:

1. Bring the drive to a controlled stop (OFF1) and ensure pulse inhibit.
2. Open STARTER / StartDrive online, navigate to Configuration → DRIVE-CLiQ topology.
3. Mark the relevant Sensor Module port as deactivated (right-click the port → Deactivate).
4. Execute Configuration re-load ("Download" the topology from the PG/PC to the drive). The drive will accept the new topology and clear the encoder assignment.
5. Set p1300 = 20 as described above.
6. Power-cycle the drive. After the next ramp-up, the drive will boot into sensorless mode without a Sensor Module in the topology.
7. Run the motor identification if the motor data has been deleted from the project (p1910 = 1, rotating).

Sensorless Control Performance and Limitations

Sensorless vector control on SINAMICS has known limits. Engineers running the conversion should expect:

• Speed accuracy at low speed: below approximately 5% of rated speed, the model-based speed estimate loses precision. A typical datasheet figure is ±0.5% of rated speed above 5% n_nom, deteriorating to ±5% or worse below 5% n_nom.
• Zero-speed holding torque: limited. For continuous-load applications that must hold position at zero speed, sensorless mode is not suitable. The drive can hold torque down to roughly 1-2 Hz electrical, but with reduced accuracy.
• Torque linearity: approximately ±5% of rated torque in the medium speed range, worsening at very low speeds.
• Bandwidth: speed controller bandwidth in sensorless mode is typically 30-50% of the bandwidth achieved in sensored mode because the speed estimate is filtered to suppress noise.
• Behavior with load steps: the sensorless model tracks load transients, but overshoot and settling time increase relative to sensored mode.

For a more general background on the trade-offs between sensored and sensorless control, see Texas Instruments' educational material "Sensored vs. sensorless control" — although that resource is framed around BLDC motors, the underlying control-theory trade-offs (bandwidth vs. zero-speed performance) are analogous to induction-motor sensorless vector control.

Fault Code Handling When Switching to Sensorless

Common fault codes encountered when transitioning a sensored drive to sensorless mode:

Refer to the SINAMICS S120/S150 List Manual on Siemens Industry Online Support for the complete fault / alarm dictionary.

Verification and Acceptance Test

After switching to sensorless mode and saving the parameter set, run the following verification sequence:

1. Read back p1300 from the drive and confirm the value matches what was written (r0010 / parameter read in STARTER or StartDrive).
2. Check motor data: p0350 (stator resistance), p0354 (rotor resistance), p0356 (stator leakage reactance), p0358 (rotor leakage reactance). If any of these are zero, the motor data has not been identified and sensorless mode will not function.
3. Run a low-speed ramp via the control panel from 0 to 200 rpm in 2 seconds. Monitor r0021 (actual speed), r0027 (actual current), and r0046.1 (status: encoder evaluation active). The drive should ramp smoothly without r0046.1 going active.
4. Run a load step: apply a step load (e.g., 50% of rated torque) and observe the speed dip and recovery time. The dip should be within 3% of setpoint speed and recovery should complete in < 500 ms for a typical 30 kW induction motor.
5. Reverse direction: command a slow reversal via the control panel and check that r0021 changes sign cleanly without oscillation.
6. DDS switching test (if DDS was used): toggle the DDS selection bit and verify that the drive swaps control structures cleanly on the next OFF1 → ON cycle.
7. Save to ROM (p0977 = 1) and power-cycle the drive to confirm that the configuration is non-volatile.

Field-Proven Caveats and Application Notes

For continuous processes that previously ran on encoder feedback, switching to sensorless as a temporary fallback is a common field practice, but the long-term implications must be understood:

• Continuous casting and process control: source commentary notes that sensorless fallback on continuous casting machines was found to be less reliable than running the process with the encoder as it was. Sensorless mode is appropriate for a controlled ramp-down or short diagnostic run, not as a permanent operating state for high-quality process control.
• Motor heating: sensorless vector control at very low speeds can produce higher stator currents for a given torque, increasing motor heating. Check the thermal model parameters (p0610, p0611) before running in a thermally constrained application.
• Model accuracy over time: the stator resistance in the motor model drifts with temperature. For long-duration sensorless runs, the actual speed may drift by 1-2% of setpoint as the motor warms. Use a temperature sensor (KTY84 / PT1000) on the motor and feed it to p0601 for thermal model compensation if precision is required.
• Starting behavior: sensorless mode requires the drive to "find" the flux before releasing the ramp-function generator. This adds a start delay of typically 50-200 ms compared to sensored mode. For applications that require sub-100 ms response, sensorless is not appropriate.
• Active Line Module interactions: if the cabinet has an Active Line Module with regenerative feedback, the sensorless mode does not interact with the line module, but the ramp-down profile should be reviewed to ensure DC link voltage stays within the limits.

For a deeper treatment of the model-based sensorless control strategy at the academic level, the ScienceDirect paper "An internal power angle control strategy for high-speed sensorless brushless DC motor drives" is a useful reference for the underlying theory, although it covers BLDC rather than induction-motor sensorless vector control.

Troubleshooting Matrix

Summary

Switching a SINAMICS S120 with CU320-2 DP from sensored to sensorless operation is a parameter change (p1300 = 20) plus a topology edit (deactivate the Sensor Module) plus a motor identification routine (p1900 = 2 or p1910 = 1). The change can be done at commissioning time (p0010 = 1) or at runtime through Drive Data Sets. The result is a drive that operates with a model-based speed estimate, with reduced low-speed performance and reduced torque linearity compared to the encoder-fitted configuration. This makes sensorless mode a useful fallback for fault diagnosis and short-term operation, not a permanent replacement for encoder feedback on a process-critical axis.