Sinamics S120 Manual Speed Controller Tuning in SLVC Mode

Manual tuning of the speed controller on a SINAMICS S120 drive operating in Sensorless Vector Control (SLVC) with a third-party squirrel-cage induction motor is a recurring field task on conveyor applications where a closed-loop encoder is either unavailable or impractical. This reference consolidates the parameter sequence, the manual Kp/Tn adjustment procedure, and the F07902 fault root-cause analysis that a commissioning engineer needs to walk through when the drive trips on motor-speed deviation at first start with load.

1. Application Overview and Drive Topology

The reference topology for a coal-conveyor duty is a Line Module (Active Line Module or Smart Line Module) feeding a Motor Module (Booksize or Chassis) that supplies a 3rd-party TEFC induction motor through a sinus filter or du/dt filter. SLVC is selected because the motor shaft is not fitted with an incremental encoder or resolver, and the mechanical envelope will not allow retrofit.

For an overview of the control structure, refer to the SINAMICS S120 Drive Functions Function Manual, section "Vector control / Speed controller."

2. Prerequisites and Required Tools

1. Starter / Startdrive / Scout commissioning tool online connection via PROFIBUS, PROFINET, or Ethernet service interface (X127 on CU320-2).
2. Motor data sheet: rated voltage (p0304), rated current (p0305), rated power (p0307), rated power factor (p0308), rated frequency (p0310), rated speed (p0311), motor cooling type (p0335), and rotor time constant (p0623 if known).
3. Equivalent circuit data (preferred): stator/rotor resistance, leakage reactance, magnetizing reactance. Where unknown, p0620-p0628 can be left on default and identified automatically during the rotary measurement.
4. Mechanical data: total inertia referred to motor shaft (J_total in kgm²), gearbox ratio, conveyor belt length and load profile.
5. Trace license / 4-channel SCOUT trace for capturing r0061 (actual speed, encoderless) and r0062 (setpoint) during the step response.

3. Motor Data Entry and Identification Sequence

The first source of F07902 trips on a 3rd-party motor is incorrect motor model parameters. Perform the data entry and identification sequence in this order:

3.1 Enter motor nameplate data

3.2 Run stationary measurement (p1900 = 2)

Set p1900 = 2 and issue an ON command. The drive injects a controlled stator current to measure stator resistance (p0620), rotor resistance (p0621), and the IGBT dead-time compensation values. The motor is not rotating. The drive returns to "Ready to Run" upon completion.

3.3 Run rotary measurement (p1960 > 0)

Set p1960 = 1 (rotary measurement, full identification including saturation characteristic) or p1960 = 2 (only saturation characteristic). With p1960 = 1 the drive accelerates the motor to the configured speed (typically p1961 = rated speed) and measures the magnetizing reactance, leakage reactance, and the rotor time constant used by the SLVC observer.

4. F07902 Fault Root Cause Analysis

F07902 ("Drive: Motor speed deviation") is raised when the absolute difference between the speed setpoint and the speed actual value exceeds the threshold configured in p2162 for longer than p2163 ms.

The F07902 value 2 reported by the source content is the loaded-pull-out case. The corrective sequence is:

1. Verify p0620-p0628 (motor equivalent circuit) are populated from the rotary ID, not left at default.
2. Verify p1498 / p0341 / p0342 / p1496 (load inertia, motor inertia, friction). The total inertia is the dominant parameter for SLVC; the speed observer at low speed depends on the inertia model.
3. Reduce p1120 (ramp-up) and p1121 (ramp-down) to a conservative value, e.g. 20-30 s for a long conveyor, and re-test.
4. Increase p2162 (default 6 rpm) and p2163 (default 100 ms) only as a temporary measure to validate root cause. Do not leave relaxed; it masks the real issue.
5. Perform the manual Kp/Tn procedure in section 6.

5. Speed Controller Structure and Parameter Map

The SLVC speed controller is an IP controller with feed-forward torque. The relevant parameter map for manual tuning is:

The default SLVC values are calculated by the drive during identification and stored in p1470/p1472. They are typically conservative. Manual tuning overwrites these.

6. Manual Kp/Tn Tuning Procedure (With Load)

The conventional manual tuning approach for a 3rd-party induction motor in SLVC follows the engineering convention of starting from the slowest stable controller and progressively increasing Kp, then reducing Tn, while observing the actual speed response on a SCOUT trace.

6.1 Initial setup

1. Set p1470 = 0.005 Nm/rpm (a deliberately low Kp).
2. Set p1472 = 2000 ms (a deliberately high Tn, effectively disabling the I term).
3. Set p1496 = 0% to disable acceleration pre-control during the first pass.
4. Disable the setpoint filter: p1656 = 0.
5. Set the ramp p1120 = 30 s (long), p1121 = 30 s. Adjust once tuned.

6.2 Open-loop step response (no load)

Apply a small fixed setpoint step from 0 to 20% of rated speed using the control panel in Startdrive. Capture r0061, r0062, r0079 (torque setpoint), and r0080 (torque actual) on a 4-channel trace with a 4 ms sample time.

Observe:

• Rise time t_r to 90% of final speed.
• Steady-state error with p1472 = 2000 ms (should be obvious — that is the point of starting with a large Tn).
• Any oscillation. With Kp = 0.005 the response should be very damped.

6.3 Increase Kp

Raise p1470 in 50% steps (0.005 → 0.0075 → 0.011 → 0.016 → 0.024 …) and repeat the step. The drive becomes faster and stiffer; overshoot will appear. Continue until one of the following limit conditions is reached:

• Torque ripple on r0079 exceeds ~3% of rated torque (controller is driving mechanical resonances).
• Sustained oscillation on r0061 (the Kp has exceeded the open-loop gain margin).
• Acoustic noise increase in the motor.

Set p1470 to approximately 60-70% of the Kp value that produced the first sustained oscillation. This is the "engineering-acceptable" Kp.

6.4 Decrease Tn

With the chosen p1470, lower p1472 in steps: 2000 → 1000 → 500 → 333 → 250 → 200 → 150 → 100 ms. The drive becomes more aggressive in closing the steady-state error but also less damped. Stop at the Tn value where:

• The steady-state speed error in r0061 - r0062 collapses to within p2162 within 1 s of a step.
• Overshoot remains under 5% of the step magnitude.

A rule of thumb is p1472 ≈ 2-3 × t_r observed at the chosen Kp. For a 200 ms rise time, start with p1472 ≈ 400-600 ms and adjust from there.

6.5 Enable acceleration pre-control

Set p1498 to the total inertia referred to the motor shaft (motor + gear + belt + load). For a coal conveyor the belt dominates: a 200 m belt at 200 kg/m contributes ~6,400 kg of belt plus a nominal load of ~5 t, all scaled by the gearbox ratio cubed. Use the SIEMENS "Inertia Calculator" worksheet or compute directly:

J_total = J_motor + J_gear/η + (m_belt × r² / η) + (m_load × r² / η)

where r is the head pulley equivalent radius and η is the gearbox efficiency. Set p1496 = 100% to enable full feed-forward.

6.6 Repeat with the belt loaded

Reapply the step with full coal load on the belt. Kp/Tn may need to be re-trimmed by ±20% because the observer has to track a higher inertia. The F07902 value 2 trip should clear if the model and the gain/reset are correct.

7. Reference Tuning Example (90 kW Conveyor Motor)

Note that the tuned Kp is lower than the default. The default is computed from p0350 (motor moment of inertia) only; the manual procedure takes into account the load inertia and the observer stability at low speed.

8. Verification and Acceptance Tests

1. Step response test (no load): Apply 0% → 50% → 0% step in Startdrive control panel. Record r0061, r0062, r0079, r0080. Verify: rise time < 500 ms, overshoot < 5%, settling time < 1.5 s.
2. Step response test (loaded): Same test with the belt loaded. Verify the same numerical criteria and no F07902.
3. Reversal test: Issue 0% → +50% → -50% → 0% setpoint sequence. Verify no observer loss of synchronism (r1407 bit "Sensorless model valid").
4. No-load current: r0027 (motor current utilization) below 30% at zero load, 60-80% at full load.
5. F07902 fault memory: Confirm r0945 / r0947 = 0 after the acceptance run. Re-arm p0950 = 0 if required.
6. Long-term loaded run: 30 minutes at rated load with continuous monitoring of r0061 - r0062. Deviation should be less than p2162 (15 rpm) with p2163 (300 ms).

9. Coal Conveyor Application Specifics

Coal conveyors present four characteristics that complicate SLVC speed control relative to a pump or fan:

• High and varying inertia. The load can change by 5:1 between empty and fully loaded belt. The inertia parameter p1498 should be set to the average loaded value; the I-term absorbs the residual error.
• High starting torque. Coal on a start-up is not a static load — the coal mass shifts and settles. Kp must be high enough to track a 100% torque step, but the observer bandwidth at low speed (below p1755, default 5 Hz) is limited.
• Negative resistance to flow. On inclined conveyors the load pulls the belt downhill when the drive trips. The controller must remain stable at low speed with negative load torque — confirm the reverse direction test in section 8.
• Long cable runs and du/dt filters. The encoderless observer assumes a clean back-EMF measurement. du/dt filters add ~150 ns of rise time and may require p0230 (filter type) = 2 to be selected so the controller compensates.

10. Troubleshooting Matrix

11. Field-Engineering Notes

• Always save a backup of the parameters before any manual tuning. Use p0971 = 1 to write to the CF card on the CU320-2.
• Parameter r1407 bit 8 ("Sensorless model valid") should be 1 continuously when the motor is rotating above p1755. If it toggles, the observer is fighting the load and Kp/Tn are off.
• Where multiple Motor Modules are mechanically coupled (dual-drive conveyors), enable droop with p1475 ≈ 0.3 × rated speed for even load sharing.
• If the application demands SLVC below 5 Hz for sustained periods, do not use SLVC. Use V/f with slip compensation (p1300 = 3) for those sub-frequency operating points and switch to SLVC above p1755.
• For chassis-format S120 modules above 250 kW, the rotary measurement must be run in the "vector control" mode (p1300 = 20) and not in "V/f" (p1300 = 0); verify before issuing the ON command.

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