SINAMICS G150 Master-Slave Torque Oscillation: Tuning Guide

1. Problem Overview

A 1 km belt conveyor is driven by two mechanically coupled 500 kW induction motors, each fed by a dedicated SINAMICS G150 cabinet drive. The first drive operates as master and receives its speed setpoint from a PLC; the second drive operates as a slave and receives its torque setpoint from the master via SINAMICS Link. The configuration is a textbook load-sharing arrangement in which the master closes the speed loop and the slave closes the torque loop.

At 100% nominal speed the regulation is reported as "generally fine", but the trace of the slave drive (captured in STARTER / Startdrive at full speed) shows a clearly periodic oscillation in torque setpoint and torque actual value. Key facts about the disturbance:

• Period: 350–400 ms (≈ 2.5–2.85 Hz, i.e. approximately 3 Hz)
• Amplitude: visible in the trace on both setpoint and actual value
• Frequency invariant to changes in speed/current controller gains and to changes in filter time constants with units in milliseconds
• Frequency present only when both drives run together; vanishes when the belt is driven by a single motor

The fact that the period is invariant to ms-scaled parameter changes is the single most important diagnostic indicator: it points to a mechanical or process-driven excitation that the closed-loop torque control is tracking, not a closed-loop instability.

2. System Architecture and Signal Flow

The system follows the standard Siemens load-sharing topology documented in the Load sharing for two coupled drives application description:

1. PLC issues a single speed reference to the master drive.
2. Master drive closes the speed control loop and produces a torque setpoint M*.
3. Master publishes M* over SINAMICS Link to the slave drive.
4. Slave drive closes the torque control loop and outputs the same mechanical torque as the master.
5. Both drives share the mechanical load on the common shaft.

Signal flow diagram (text form):

PLC  --[n*]-->  MASTER (G150)  --[SINAMICS Link: M*]-->  SLAVE (G150)
                          |                                       |
                          +-------[speed ctrl + current ctrl]------+-------[torque ctrl + current ctrl]--+
                          |                                       |
                          +-------[shaft / belt / 1 km span]-------+

Important: in this topology the speed controller of the master is the only PI that sees the 1 km belt span. The slave's speed controller is normally disabled (or set to torque mode), which means the slave contributes damping only through its torque/current loop. Any mode in the mechanical span that the master cannot damp with its speed PI will be visible as a torque oscillation shared by both drives.

3. Root Cause Analysis

Three candidate root causes must be separated by test:

3.1 Mechanical resonance of the belt span

A 1 km long belt is a low-stiffness mechanical structure with several natural modes. Lateral, longitudinal, and torsional modes of a long conveyor belt typically fall in the 0.5–5 Hz range, which matches the observed 2.5–2.85 Hz exactly. The invariance of the period to ms-scaled controller parameters is the decisive evidence: if the loop were unstable, changing speed-controller Tn (P1451[0]) or current-controller P/Tn (P1700, P1715, P1717) would shift the period.

3.2 Torsional mode of the common shaft

If the two motors are coupled to the same shaft (gears, fluid couplings, or pinion to a common drum), the shaft has its own first torsional mode. With a 1 km belt acting as additional inertia, the first torsional mode of the motor–shaft–drum–belt system can easily fall in the 2–5 Hz band. This produces the exact signature seen: oscillation present only when both drives are mechanically coupled and active, with no oscillation in single-motor operation.

3.3 Controller interaction (less likely but to be ruled out)

The slave's torque controller and the master's speed controller can interact through the SINAMICS Link telegram update rate. The default SINAMICS Link cycle is 1–4 ms depending on the bus configuration. With sub-ms communication, a sample-time-induced limit cycle would typically show at >100 Hz, not 3 Hz. To rule it out: increase the torque setpoint filter (P1517) on the slave and observe the period; if it does not change, the source is mechanical.

4. Diagnostic Procedure (Use the Slave Trace First)

1. Capture a high-resolution trace on the slave (the drive that receives the torque setpoint). Record at minimum: r0079[0] torque setpoint, r0080[0] torque actual, r0063[0] speed actual, r0021[0] speed setpoint, and r1538/r1539 upper/lower torque limit. Sample time ≤ 1 ms; trace length ≥ 10 s.
2. Repeat the trace on the master with the same signals plus r1482 speed controller I-component. Compare phase between the two traces.
3. Run the belt on a single motor (use the second DDS as described) and capture the same traces. The oscillation should disappear or reduce to noise. If it does, the excitation is shared-mechanical.
4. Disable the speed controller I-component on the master (set P1470[0] = 0 s or P1451[0] = 0% to remove I-action). If the oscillation vanishes, it is being excited by integral wind-up on a low-frequency disturbance (rare, but easy to test).
5. Add a torque setpoint filter on the slave (P1517[0] = 50–200 ms) and observe whether the period changes. If the period is unchanged but the amplitude is reduced, the source is mechanical and the filter only masks the symptom.
6. Measure the belt with an external tachometer or laser vibrometer on the belt mid-span if accessible, or use the motor encoder to compute torsional vibration.

5. Tuning Solutions in Priority Order

The remediation ladder below should be applied in the order shown. Each step must be verified by trace before moving to the next.

5.1 Step 1 – Activate the droop function (master and slave)

According to the official Siemens application description Droop function, droop adds a small speed reduction proportional to torque load, so that two drives sharing a load do not fight each other. It is the canonical first-line remedy for load-sharing instabilities.

Apply the same droop value on master and slave. Droop intentionally makes the speed of each drive slightly load-dependent so that the two drives settle on a stable load partition instead of fighting through the integral of the speed controller.

5.2 Step 2 – Disable or shrink the speed-controller I-part on the master

If the oscillation is amplified by integral wind-up on the 2–3 Hz mechanical disturbance, reducing the I-action reduces gain at low frequency and may stop the loop. Use P1470[0] (Tn, speed controller integral time) and P1451[0] (P-gain of speed controller). The trade-off is degraded steady-state speed accuracy, which is acceptable on a belt conveyor with a PLC trim loop.

5.3 Step 3 – Add a torque setpoint filter on the slave

A low-pass filter on the slave's torque setpoint (P1517[0]) reduces the bandwidth at which torque disturbances are rejected. A 50–200 ms filter will attenuate a 2.5–3 Hz ripple by 6–12 dB without measurably affecting conveyor dynamics, because the belt's mechanical response time is much slower than the filter.

// Typical safe values
P1517[0] = 0.100    ; 100 ms torque setpoint filter 1 (low-pass)
P1518[0] = 0.000    ; no second filter
P1520[0] = 1.05     ; upper torque limit (105 % rated)
P1521[0] = -1.05    ; lower torque limit (-105 % rated)

5.4 Step 4 – Activate the SINAMICS Load Sharing firmware option (DCC-based)

For coupled drives with strict load sharing, Siemens offers a Drive Control Chart (DCC) load-sharing block diagram. Reference applications:

• SINAMICS S: DCC Load Sharing (Entry ID 38470057)
• SINAMICS G: Load sharing for two coupled drives (Entry ID 109774232)

The DCC block provides:

• Active load balancing between master and slave with a configurable ratio
• Anti-windup logic that prevents the master's integral from accumulating during large torque transients
• Bus-failure fallback to speed-controlled operation

5.5 Step 5 – Use a torque overlay / additional setpoint on the slave

Independent of droop, a small proportional speed-error signal can be added to the slave torque setpoint to give the slave a stiffness against speed disturbances. This is the slave-side equivalent of droop and is implemented in the SINAMICS G150 function diagram 5030 (torque setpoint modification). Typical block: T_add = Kp_overlay × (n_master − n_slave) with Kp_overlay ≈ 0.5–2 % rated torque per rpm.

6. SINAMICS Parameter Reference

The following parameters are relevant on the SINAMICS G150 chassis units used in this application. They are documented in the SINAMICS S120/S150 List Manual and the G150-specific Operating Instructions (the G150 parameter set is a subset of S120, so the same numbers apply).

7. Mechanical Resonance Verification

If the trace on the slave shows a clean 350–400 ms sinusoid, the next step is to confirm the mechanical nature of the excitation. Recommended methods:

1. Bump test at standstill. Apply a step of torque (10 % rated) from the master with the slave in torque mode, capture the speed response of the slave encoder, and look for a 2–3 Hz ringing component in r0063 with the FFT function of STARTER/Startdrive.
2. Coast-down vibration measurement. Run the belt at 50 % speed, command STOP OFF1, and record vibration with a handheld accelerometer on the motor feet, gearbox housings, and (if safely accessible) the belt structure. The 2–3 Hz peak will show as a clear resonance on the FFT.
3. Encoder-based torsional analysis. If both motors have encoders, log both r0061[0] (encoder actual speed) signals at 1 ms and compute the phase difference. A 2–3 Hz phase oscillation between the two encoders confirms a torsional mode in the shaft.

If a mechanical resonance is confirmed, the electrical remedies above will only attenuate the symptom. A long-term fix requires one of the following:

• Soft-start ramp rate reduction on the master (P1120 / P1121) to limit excitation at start-up
• Mechanical detuning (tension, alignment, coupling stiffness) to shift the resonance out of the operating bandwidth
• Speed notch filter on the master (SINAMICS function block "Speed setpoint filter") tuned to the measured resonance frequency

8. Firmware, Option, and Compatibility Notes

• Load sharing via DCC requires the firmware option SINAMICS DCC on both drives. The block diagram is available for download from the Siemens Support entry linked above (38470057 for S120, 109774232 for G).
• SINAMICS Link must be configured on the Control Unit (CU320-2 DP or CU320-2 PN) with a consistent bus cycle. Mismatch between the master and slave cycle time will appear as a low-frequency jitter, not a 2–3 Hz oscillation.
• STARTER (legacy, V5.x) or Startdrive (TIA Portal) can be used to capture traces. For a 2–3 Hz phenomenon, a trace length of 10–20 s at ≤ 1 ms is appropriate.
• On 500 kW G150 units, the default current controller bandwidth is well above 100 Hz, so the 2.5–3 Hz oscillation cannot be caused by current-controller interaction.

9. Recommended Verification Checklist

Apply this checklist before declaring the issue resolved.

1. Capture a 10 s trace on the slave at 100 % speed: r0079, r0080, r0063. FFT must show < −20 dB at 2.5–3 Hz compared with the baseline trace.
2. Capture a 10 s trace on the master with the same signals. Both drives should now show clean torque traces with only stochastic noise.
3. Perform a 0 % → 100 % speed step on the master. Overshoot must remain < 5 % and settling time within the conveyor's mechanical time constant.
4. Apply 110 % load step for 5 s. Torque sharing error (computed from both r0080 values) must remain < 3 % of rated.
5. Command an emergency stop (OFF1) and confirm both drives ramp down in torque-limit, not current-limit, and that the speed traces do not diverge.
6. Disable SINAMICS Link and confirm both drives fall back to speed-controlled mode without fault (F08501 SINAMICS Link).

10. Common Pitfalls

• Adjusting current-controller parameters first. Current loop bandwidth is > 1 kHz; it cannot excite a 3 Hz ripple. Move to speed-loop and droop first.
• Looking at the master trace only. The slave trace exposes torque-link quality. The master trace shows speed-loop quality. They must both be analyzed.
• Setting the droop value asymmetrically. Equal droop on both drives is essential. Asymmetric droop biases the load toward one motor and does not improve stability.
• Forgetting to retrigger the trace after each parameter change. STARTER / Startdrive traces the r-values in real time, but a fresh trigger avoids including the transition in the FFT window.
• Assuming SINAMICS Link is the source. SINAMICS Link jitter is in the 1–4 ms range and cannot produce a 350–400 ms period.

11. Quick Triage Matrix

12. Summary

A 2.5–3 Hz torque oscillation between two SINAMICS G150 drives in a load-sharing configuration on a 1 km belt is almost always a mechanical mode of the belt/shaft being excited by the closed-loop torque control. The diagnostic decision path is:

1. Capture slave trace at 1 ms or better.
2. Run belt on a single motor – if oscillation disappears, the source is shared-mechanical.
3. Vary ms-scaled controller parameters – if the period is invariant, the source is mechanical.
4. Activate the droop function on both drives (P1485, P1486, P1487) as the primary electrical remedy.
5. If droop alone is insufficient, add a torque setpoint filter (P1517) and apply the DCC load-sharing block diagram from Siemens Support entry 109774232 / 38470057.
6. For long-term fix, address the mechanical mode by alignment, tension, or a speed setpoint notch filter.

The droop function, the DCC load-sharing option, and the published Siemens application notes together provide a complete, documented remediation path for this class of problem.