Configuring SINAMICS G120 as SIMOTION Servo Backup Drive

This reference covers the hardware and software architecture required to hand off a continuously rotating servo motor from a failed SIMOTION motion controller to a SINAMICS G120 inverter. The topology applies to actuators (valve actuators, conveyor drives, rotary tables) that cannot tolerate a stop, and where loss of the primary controller must be detected, the motor electrically isolated from the failed servo, and re-energized through a vector-controlled drive with flying restart within a bounded time window.

1. Problem Definition and System Constraints

The application requirement is a servo-class motor that must keep rotating when the upstream SIMOTION D / P controller is stopped, powered off, or has a firmware/watchdog fault. The primary drive (typically a SINAMICS S120 or S210) is operated in closed-loop position/speed mode over PROFIBUS or PROFINET IRT. The question is whether a SINAMICS G120 can pick up the load under the following conditions:

• SIMOTION CPU is in STOP or no longer alive on the network.
• PROFINET I/O data has failed (communication timeout, watchdog expiration).
• The motor is still mechanically turning at the moment of handover.
• Position reference is lost; the backup drive only has to maintain rotation, not absolute position.

Two physical constraints dominate the design:

1. Motor topology: the G120 family only supports induction (asynchronous) motors in vector control. Synchronous servo motors (1FK, 1FT, 1FS, SIMOTION S-1FK7) are not supported on the standard CU240B-2 / CU240E-2 / CU250S-2 control units in field-oriented closed-loop mode without an encoder. See the SINAMICS G120 Operating Instructions for the supported motor list.
2. Encoder feedback: a synchronous rotor can only be controlled if the absolute rotor position is known at start-up. Without an encoder, the G120 cannot run a permanent-magnet motor, and the rotor will not align correctly to a 3-phase voltage application.

2. Drive Selection: G120 Variants for Handover

Decision rule: If the existing servo motor is a permanent-magnet synchronous servo (1FK, 1FT family), the G120 cannot run it sensorlessly. Two practical resolutions exist:

• Replace the synchronous servo with an induction servo (e.g., 1PH8 / 1LA9 asynchronous servomotor) so that the G120 can take over with a CU250S-2 in sensorless vector control (SLVC).
• Add a CU250S-2 with the existing encoder wired in. The CU250S-2 closes the speed loop on the encoder, allowing torque-controlled handover while the rotor is still turning.

For a continuously rotating plant, the asynchronous + encoder path is the only viable topology on a G120 because the synchronous sensorless option is not available. Confirm selection against the SINAMICS G120 CU250S-2 Function Manual.

3. Electrical Topology: Contactor-Based Motor Transfer

Two drives cannot be electrically paralleled to one motor. The motor stator must be switched from the SIMOTION-driven primary inverter to the G120 backup. The simplest industrial implementation is a 3-phase contactor pair (or motorized circuit breaker) with mechanical and electrical interlocking.

3.1 Contactor Selection

Drop-out + de-energize + energize budget = approx. 40–60 ms, which is short enough to ride through on most rotating masses provided the contactors are mechanically interlocked and the motor terminals are not shorted to two inverters simultaneously.

3.2 Topology Diagram

3.3 Control of the Contactors

The contactor coils must be driven by a deterministic logic that releases K1 (S120 contactor) before K2 (G120 contactor) is energized. Two options exist:

1. Hardware watchdog (recommended): A 24 V signal sourced from a SIMOTION output is wired through a normally-closed watchdog relay. When SIMOTION is alive, the relay is held energized, K1 stays picked up. When the watchdog times out (typically 250–500 ms of missing signal), the relay drops, K1 releases, and an auxiliary contact from K1 (mirror contact per IEC 60947-4-1) enables K2. This path is independent of PROFIBUS/PROFINET state.
2. PROFINET-controlled: SIMOTION commands K1 close; an external logic (e.g., LOGO! 8, ET200S relay) commands K2 when it detects a PROFIBUS/PROFINET failure on the backup drive's network. This is fragile during network power loss.

4. PROFIBUS / PROFINET Behavior During CPU Stop

When SIMOTION goes to STOP or is powered off, the cyclic I/O data exchanged with the G120 on PROFIBUS/PROFINET does not freeze at the last value. The G120 reacts based on the configuration of the communication fault response (p2041, p0851, p0864) and the watchdog time configured in the project.

Substitution values (output values that the IO controller writes when its own CPU is in STOP) are only relevant on the SIMOTION side. They do not help the G120 because the G120's CPU keeps running and detecting a communication timeout triggers F1910 "PROFIdrive PZD telegram failure" or F08501 on PROFINET. After this fault, the drive coasts to zero by default unless p2109 / p2100 are configured to suppress the fault response.

5. Flying Restart Parameterization (p1200)

Flying restart allows the G120 to connect to a motor that is already rotating without first waiting for the rotor to stop. The relevant parameters are on the CU250S-2 control unit. Refer to the SINAMICS G120 CU250S-2 List Manual for full descriptions.

5.1 Flying Restart Search Algorithm

1. At ON command, the inverter injects a search current (defined by p1202 × r0331 rated motor current).
2. It sweeps the output frequency from p1203 × (1.3 × current speed) down to zero in two stages to identify rotor flux angle and speed.
3. Once the search current signature matches the rotor saliency, the drive locks onto the rotor speed and accelerates to setpoint.
4. Typical search time is 200–800 ms; for fast handover, pre-charge the DC link and keep the drive in "ready" state so the algorithm begins immediately on K2 close.

For a continuously rotating motor, a 1-second search window is acceptable. For sub-500 ms handover, consider pre-rotating the G120 speed observer from a separate speed signal (e.g., a back-up tachometer) using a CU250S-2 with SSI encoder input on the backup path.

6. Encoder and Feedback on the Backup Path

If the primary drive uses a synchronous servo with a high-resolution absolute encoder (EnDat 2.2 / DRIVE-CLiQ), the same encoder cannot be shared with the G120 because the S120 owns the DRIVE-CLiQ link. A separate incremental encoder on the motor shaft must be wired to the CU250S-2, with the following pinout discipline:

• HTL encoder (24 V) on the CU250S-2 sub-D connector: tracks A, /A, B, /B, R, /R, 24 V, GND.
• TTL encoder (RS422) if higher resolution is required: tracks A, /A, B, /B, R, /R, 5 V, GND.
• Encoder must be physically present at the motor so the G120 can close its speed loop during handover.

Set p0400 = 1 (encoder type) and configure p0408, p0410, p0411 to the encoder's pulses-per-revolution (PPR). If the motor is an induction servo, sensorless mode is acceptable and the encoder is not strictly required; however, the CU250S-2 with an encoder provides better low-speed torque and a faster flying restart.

7. Double-Redundancy Architecture Reference

For a system where both the motor and the controller are duplicated, the academic literature provides a formal topology that matches this application. The IEEE article on double-redundancy electric actuator controllers describes a motor + controller dual-channel architecture where the second drive is engaged on detection of a primary fault. The handover logic in the academic paper uses a digital signal processor to coordinate a contactor-based switchover within 50 ms; the G120 + watchdog approach above is the commercial realization of the same architecture.

8. Commissioning Procedure

1. Pre-commissioning: Run the S120 with the motor in normal mode. Verify all cabling and encoder feedback.
2. Power up the G120 independently on its own line, configure it for sensorless vector control (or encoder-based) with p1200 = 2, motor data from p0340 = 1, 3 (motor identification at standstill, then rotating).
3. Run the G120 stand-alone on the motor (no contactor engaged), confirm it can start, run, and stop the motor at various speeds. Capture the ramp times and fault behavior.
4. Test flying restart: Spin the motor externally (or use S120 to spin it, then command G120 ON while motor is moving), verify the G120 catches the rotor without tripping F07900 "Motor stalled" or F30002 "DC link overvoltage".
5. Wire contactors and watchdog. Verify mirror contact logic and mechanical interlock.
6. Simulate SIMOTION failure: Pull the SIMOTION power, observe that within the configured watchdog time (≤ 500 ms) K1 opens, K2 closes, the G120 receives ON, executes flying restart, and the motor continues.
7. Capture handover transient on an oscilloscope: monitor motor current, contactor state, G120 ZSW1 bit 6 (acknowledgement of enable) and ZSW1 bit 3 (fault present).

9. Verification Checklist

10. Limitations and Edge Cases

• Synchronous servo motors are not natively supported on the G120 in sensorless mode. If the existing motor is 1FK / 1FT, you must either replace it with an induction servo or add a CU250S-2 with encoder feedback.
• Position reference is lost during handover. The backup drive runs the motor as an open-loop speed/encoder-vector drive; absolute position is not preserved. Downstream process must tolerate position loss.
• Re-engagement to SIMOTION is non-trivial: when SIMOTION returns, the G120 must be turned off, the motor switched back to S120, and SIMOTION must perform a new referencing. Schedule this in a planned downtime window.
• Braking: the G120 in sensorless mode cannot brake as precisely as the S120 in closed-loop torque control. Avoid hard-stop requirements on the backup path.
• Thermal: the backup drive's power module is normally cold and may have higher starting resistance; the first few start attempts may show different thermal behavior.

11. Recommended Part Numbers

12. Frequently Asked Questions