Clearing SINAMICS S120 F07901 Motor Overspeed on Servo Drives

1. Problem Overview

The SINAMICS S120 drive (catalog number 6SL3120-1TE21-8AD0, a single-axis SINAMICS S120 Active Line Module / Motor Module in Booksize format with 18 A rated output) trips fault F07901 (Drive: Motor overspeed, V05.10) as soon as the operator requests motion through the STARTER / Startdrive commissioning control panel. The drive is configured as control mode Servo [11] with sub-mode Encoderless Speed Control [20] and is wired to a Copeland Scroll compressor motor (Copeland ZP51K5E-TFD-830, a 5 HP / 3.7 kW rated hermetic scroll compressor induction motor commonly paired with variable-speed drives).

The fault occurs even when:

• OFF1 (p840[0]) is wired to a constant 1
• OFF3 sources 1 and 2 (p848[0], p849[0]) are wired to a constant 1
• No external enable is missing from r0046

Status word r0046 reports OFF1 enable missing = YES and OFF3 enable missing = YES, while internal status bits r46.16 (OFF1 enable internal) and r46.18 (OFF3 enable internal) are clear. The drive therefore refuses to release the ramp-function generator (RFG) and the speed setpoint is applied directly to the closed-loop controller, producing an immediate F07901 overspeed trip the first time a setpoint is issued from the control panel.

2. Affected Components, Firmware and Tooling

Although F07901 and its associated warning A07901 are documented across the entire SINAMICS V5.x firmware range, the behavior of the ramp-function generator in Servo control mode is a long-standing design decision inherited from the SINAMICS S120 function diagrams. The RFG in Servo is hidden behind a feature bit that is only checked when the extended setpoint channel is selected during drive configuration.

3. Root Cause Analysis

Servo control mode (p1300 = 11) in the SINAMICS S120 ships with a minimal setpoint channel. By design, the minimal channel connects the main setpoint directly to the speed controller without an intermediate ramp-function generator. This is intentional: high-dynamic Servo applications such as feed axes, packaging spindles and cam-driven axes expect the drive to follow the controller's velocity command with the shortest possible delay and do not benefit from a slow RFG that would distort the contouring quality.

The minimal setpoint channel therefore disables:

• Ramp-function generator (ramp-up / ramp-down / rounding)
• Jog setpoint sources
• Motor potentiometer (MOP)
• Fixed setpoint table
• Skip speeds

When the operator commands motion through the STARTER control panel without an explicit enable signal, the AND gate that conditions OFF1 and OFF3 remains FALSE because the RFG has not yet signaled ramping active = 1. Internally, bit r46.0 (OFF1 enable missing) and r46.2 (OFF3 enable missing) are asserted, even though the user-supplied bits on p840, p848 and p849 are TRUE.

3.1 Why the Speed "Overshoots" Without a Ramp

With p840[0] = 1 and p848[0] = p849[0] = 1, OFF1 and OFF3 appear to be present. However, in the minimal setpoint channel the speed setpoint is applied as a step. The current controller in Servo mode reaches the requested speed in milliseconds. The encoderless speed observer (sub-mode 20) cannot estimate speed until the motor develops back-EMF, so during the first ~50–150 ms the actual speed is unknown to the regulator. The drive compares the unknown actual speed against the speed limit p1082 (or p2000-based limits), and if the difference exceeds the F07901 threshold it trips immediately.

The F07901 threshold is computed by:

F07901 trigger speed = max(p1082, p2000 × p0290) × overspeed_factor

where the overspeed factor is derived from p0290 (load response) and the configured speed limits in the function diagram 8010. A step setpoint that briefly exceeds p1082 (because the motor is unloaded, lightly damped or coasting in the wrong direction at start) trips F07901 within one current-controller cycle.

3.2 Status Word Verification

The combination r46.0 = 1 and r46.16 = 0 indicates that the user side of the OFF1 logic is satisfied, but the internal gating inside the setpoint channel is not. That internal gating is precisely the RFG-enable signal in function diagram 3030 / 3050.

4. Solution: Activate the Extended Setpoint Channel

The official remedy is to enable the extended setpoint channel during drive configuration. Once enabled, the SINAMICS inserts the ramp-function generator, MOP, fixed setpoints and jog between the main setpoint and the speed controller, and the internal OFF1/OFF3 gating then evaluates TRUE on r0046.0 and r0046.2.

4.1 Procedure in STARTER / Drive ES

1. Open the project in Drive ES Starter V5.1.1.2 and navigate to the drive object representing the S120 axis (DO1 / SERVO_1).
2. Right-click the drive and select Configuration -> Configure DDS / CDS (or open the original configuration wizard if the drive object is brand-new).
3. In the configuration wizard, on the page Select additional components / functions, locate the check box Extended setpoint channel (German: Erweiterter Sollwertkanal, function module ID per function manual: FM 1).
4. Tick the box, click Next and complete the wizard. STARTER will set the function module activation bits (p0108.x) and download a new parameter set to the drive.
5. After the download, perform a POWER ON or click Copy RAM to ROM and restart the Control Unit so the new function module assignment takes effect.
6. Re-open the configuration and confirm that the RFG appears in the function navigator (it should now be visible under Setpoint channel -> Ramp-function generator).

4.2 Manual Parameter Equivalent (Advanced Users)

If you cannot re-run the wizard, you can manually enable the extended setpoint channel by setting the function module bit directly. This is what the wizard does internally:

p0108.8 = 1          // Function module: Extended setpoint channel
p1001[0] = 0          // Fixed setpoint 1 (rpm)
p1120[0] = 10.0       // Ramp-up time
p1121[0] = 10.0       // Ramp-down time
p1130[0] = 0.0        // Ramp-up initial rounding (s)
p1131[0] = 0.0        // Ramp-up final rounding (s)
p1132[0] = 0.0        // Ramp-down initial rounding (s)
p1133[0] = 0.0        // Ramp-down final rounding (s)
p1134[0] = 0.0        // Ramp-down final rounding off3
p1135[0] = 5.0        // OFF3 ramp-down time

Then perform Copy RAM to ROM and a POWER ON. After the restart, verify r0108.8 = 1 (function module active) and the RFG parameters become visible in the parameter list.

4.3 Verification After Enabling RFG

1. Go online and read r0108.8 - it must report 1.
2. Read r0046 - bits 0 and 2 should now clear as soon as p840, p848, p849 are TRUE.
3. Open the STARTER control panel, enable the drive (checkbox Enable) and issue a small fixed setpoint (e.g. 100 rpm) for 2 seconds.
4. Monitor r0021 (actual speed smoothed) and r0022 (ramp-function generator output). The RFG output must ramp linearly between 0 and the setpoint following p1120 / p1121.
5. Verify in the trace recorder that the speed actual follows the ramp output without exceeding p1082.
6. If F07901 still trips, raise p1120 / p1121 by a factor of 5 to slow the start, then re-test.

5. Alternate Approach: Switch to Vector-Sensorless Control Mode

A second option is to abandon the Servo control mode and reconfigure the drive as Vector with sensorless speed control (p1300 = 20). In Vector control, the extended setpoint channel with RFG is the default and the F07901 behavior under step setpoints is generally more forgiving because the speed observer model is asynchronous-machine-based and tracks back-EMF more accurately at zero and low speed.

Use Vector-sensorless when:

• The motor is an asynchronous induction machine (such as the Copeland ZP scroll series, which is a 3-phase squirrel-cage induction motor).
• The application requires a controlled ramp-up (compressors, pumps, fans).
• Bandwidth requirements are below ~50 Hz closed-loop bandwidth.

Keep Servo control mode only when:

• The motor is a permanent-magnet synchronous motor (PMSM).
• The application requires > 100 Hz bandwidth or precise torque dynamics.
• A real encoder (resolver, EnDat, sin/cos) is fitted. Sub-mode 20 (sensorless Servo) is rarely a good fit for scroll compressors.

6. Drive and Motor Specification Snapshot

The DC bus voltage, switching frequency and motor thermal model should be re-validated using the SINAMICS S120 commissioning manual after switching control modes, because the Servo and Vector controllers use different motor-equivalent-circuit models.

7. Related Faults and Parameter Setpoints

Warnings A07901 should be enabled during commissioning (p2100 / p2101) so that the operator can see the overspeed approaching before it becomes a fault.

8. Commissioning Step Sequence (Reference Procedure)

1. Wire motor (U2 / V2 / W2), protective earth and encoder (if used). Confirm motor insulation with a 500 V megohmmeter; minimum 100 MΩ phase-to-phase and phase-to-ground.
2. Power up the Smart Line Module or Active Line Module, then the Motor Module.
3. In STARTER, perform Restore factory settings on the drive object to start from a clean configuration.
4. Run the configuration wizard. Select Vector control mode p1300 = 20 for sensorless vector, or Servo p1300 = 11 if PMSM / encoder is fitted.
5. Tick Extended setpoint channel in the configuration options.
6. Enter motor data from the ZP51K5E-TFD-830 nameplate (p0304 = 460 V, p0305 = 11 A, p0310 = 60 Hz, p0311 = 3500 rpm, p0307 = 3.7 kW).
7. Run Motor identification -> Stationary followed by Rotating measurement. The rotating measurement automatically updates the equivalent circuit and identifies the speed-controller gain.
8. Set ramp-up / ramp-down times p1120 = p1121 = 10 s (scroll compressors typically need 5–15 s to avoid slugging).
9. Set OFF3 ramp time p1135 = 5 s.
10. Open the control panel, click Enable, then issue a small setpoint. Monitor r0021, r0022 and r0031 (active current) in the trace.
11. If F07901 still trips, raise p1120 / p1121, lower the first setpoint to 5 % of rated speed, and verify r1084 (speed actual extended) remains below the F07901 trigger.

9. Troubleshooting Matrix

10. Field-Proven Caveats

• Don't use Servo sensorless (p1300 = 11, sub-mode 20) for scroll compressors. The induction-motor model inside the Servo observer is tuned for high-bandwidth PMSM dynamics and routinely diverges on compressor load steps. Vector-sensorless is the engineering choice.
• Always configure OFF3 (p848 / p849) before energizing the drive. Even with RFG disabled, OFF3 acts as a hard current-zero gate. If p848 / p849 are left at the factory default (0), the drive will refuse to release pulses regardless of the setpoint channel.
• Check r1084 vs r0063. r1084 is the "extended actual speed" used internally for F07901 monitoring, while r0063 is the smoothed display value. A divergence > 5 % indicates a speed-observer problem that will trip F07901 even with the RFG enabled.
• Power modules and Control Unit firmware versions must match. SINAMICS V5.x firmware has cross-version compatibility restrictions documented in the compatibility tool. Mixed V4.x and V5.x firmware on a multi-axis S120 line-up will produce inconsistent F07901 trigger thresholds.
• For scroll compressors, ramp times are application-critical. Too short a ramp causes liquid slugging; too long causes the drive to drop out of compressor capacity range. Use 10 s for ZP51K5E, 15 s for larger ZP models.

11. FAQ