Problem Description
A SINAMICS S150 drive (CU320 control unit, vector control) controlling an asynchronous motor with encoder on a test bench has two functional speed-control modes but exhibits an unexpected speed limitation when switched to closed-loop torque control. The drive saturates the speed at approximately 225 rpm even though the relevant maximum speed parameters are configured far above this value. The unexpected limit is invisible in the parameter view because it is mechanical (load-side), not a configured drive limit.
Initial observations:
- Speed control modes operate correctly.
- Torque control selected via p1501.
- Motor speed clamped at ~225 rpm in torque mode regardless of torque setpoint.
- Parameter p1084 (positive speed limit) shows 3600 rpm; p1087 (negative) shows -3600 rpm.
- Setting p1084 = 150 rpm yields a measured limit of 195 rpm (i.e. +45 rpm offset above the parameter value).
- Setting p1084 = 150 rpm on the negative side (p1087 = -150) yields a measured -195 rpm.
- No drive fault is raised; the speed is regulated down internally to maintain the torque setpoint within the available mechanical margin.
Why the Speed Limitation Is Not Always a Parameter
Many engineers begin by inspecting the standard speed clamps:
- p1082 Maximum speed
- p1083 Speed limit, positive direction (torque-control mode)
- p1084 Speed limit, positive direction (speed-control mode)
- p1085 Speed limit, positive direction (with ramp-function generator)
- p1086 Speed limit, negative direction (torque-control mode)
- p1087 Speed limit, negative direction (speed-control mode)
These parameters do not behave as the limiting authority when the drive is in closed-loop torque control with a low-load test bench. In torque mode, the speed setpoint is internally derived from the torque setpoint divided by the estimated load. If the load cannot accelerate further (e.g. because the applied torque is consumed by friction), the speed simply does not rise, irrespective of p1082/p1083/p1084/p1086/p1087.
Parameter Snapshot From the Affected Drive
| Parameter | Description | Value | Interpretation |
|---|---|---|---|
| r0056 | Status word, closed-loop control | 15 | Closed-loop control operating (bits 0/1/2/3) |
| r0060 | Speed setpoint before filter | 0 rpm | Internal setpoint from torque mode |
| r0061 | Actual speed, unfiltered | 195 rpm | Measured mechanical speed |
| r0062 | Speed setpoint, filtered | 0 rpm | Filtered internal setpoint |
| r0063 | Actual speed, filtered | 195 rpm | Filtered measured speed |
| r0064 | Speed controller system deviation | 0 rpm | No deviation (torque mode) |
| r0066 | Output frequency | 6.49 Hz | 195 rpm / 30 = 6.5 Hz, consistent |
| p0079 | Identified motor type / order number | 5.5 → 11.4 | Asynchronous induction motor, 5.5 to 11.4 kW range |
| r0108 | Drive object functional scope | 20070104 | Standard drive object, vector control |
| p0807 | Save parameter set to non-volatile | 0 | No save in progress |
| p1170 | Speed setpoint filter selection | 0 | No filter active |
| p1406 | Closed-loop control configuration (dec) | 1000 → bin 0011 1110 1000 | Bit 12 not set in this snapshot |
| p1407 | Configuration, closed-loop control | 0x000C → bin 0000 0000 0000 1100 | Bit 2 and Bit 3 set = speed and torque active simultaneously |
| p1408 | Current controller configuration | 0x0001 | Bit 0 = current model active |
| p1084 | Speed limit, positive (speed mode) | 150 rpm | Reduced during test |
| p1087 | Speed limit, negative (speed mode) | -150 rpm | Reduced during test |
| p1438 | Lower torque limit, regenerative | 0 Nm | No lower torque clamp |
| p1478 | Speed controller integral component enable | 0 | Integral component inhibited |
| r1480 | Torque setpoint (PI output) | 11.47 Nm | Active torque demand |
| r1481 | Torque setpoint unfiltered | 0 rpm | Speed component in torque mode |
| r1482 | Torque setpoint, filtered | 11.47 Nm | Filtered torque demand |
| r1508 | Torque setpoint before filter | 11.47 Nm | Source torque demand |
| p1526 | Upper torque limit | 1500 Nm | Effectively unlimited vs 11.47 Nm demand |
| p1527 | Lower torque limit | -1500 Nm | Effectively unlimited |
| p1530 | Power limit, motoring | 100 % | Nominal power limit |
| p1531 | Power limit, regenerative | -100 % | Nominal power limit |
| p1551 | Power limit enable | 1 | Power limit active |
| p1538 | Torque limit upper (motoring, alternative) | 1500 Nm | Not the active ceiling |
| p1539 | Torque limit lower (regenerative) | -1500 Nm | Not the active ceiling |
| p0640 | Current limit | 417 A | Far above actual operating current |
| p0641 | Current limit, scaling factor | 100 % | Full current available |
All configured torque, current, and power limits are an order of magnitude above the operating torque of 11.47 Nm. Therefore, the active limitation must be mechanical.
Root Cause: Friction-Curve Torque Gap
The root cause in this case is a non-linear friction characteristic of the mechanical drivetrain (bearings, coupling, gearbox, brake). The friction curve has a sharp jump (a Coulomb-friction discontinuity) at a specific speed. To traverse that jump, the available torque must exceed the higher post-gap friction value. Below that torque threshold, the motor stalls at the pre-gap speed indefinitely.
The observed "+45 rpm offset" when p1084 = 150 rpm produces 195 rpm measured speed is the steady-state operating point on the pre-gap friction plateau. The drive applies the requested torque, but the resulting acceleration is consumed by friction; net speed stays constant at the plateau speed. Setting p1084 to a higher value does not change anything because the parameter is not the active clamp.
Verification Procedure (How to Confirm the Mechanical Cause)
- Read r0079 (motor identification), r0330 (rated motor torque), r0331 (rated motor current) and verify that the rated torque is well above the operating torque of 11.47 Nm.
- Read r1538/r1539, p1520/p1521/p1526/p1527, p0640, p1530/p1531 to confirm that no torque, current, or power clamp is binding. If any of these equals the corresponding operating quantity, that is the active limit.
- Read r1480 (torque setpoint at speed controller output) and r1482 (filtered torque setpoint) and compare to r0080 (actual torque). They should match within a few percent.
- Read p1407 bits 2, 3, and 17. Bit 2 = speed controller active, Bit 3 = torque controller active, Bit 17 = speed limit reached. If Bit 17 toggles, the drive itself is enforcing the clamp and you are looking at a parameter issue. If Bit 17 stays 0, the drive is not clamping; the speed is settling below the clamp due to mechanical balance.
- Read p1406 Bit 12. Bit 12 = torque control active. If 0, torque mode was not actually selected.
- Force the torque setpoint higher (e.g. via the control word from Starter or the fieldbus scaling). If speed suddenly jumps after a threshold is crossed, you have located the friction-curve torque gap.
- Plot speed (r0061) vs. torque setpoint (r1508) and observe a step discontinuity. The plateau speed is the apparent "limit"; the post-plateau speed is the real ceiling.
Diagnostic Flowchart (Inline SVG)
Solution
- Confirm the friction-curve hypothesis by sweeping the torque setpoint from 0 to rated torque while monitoring r0061 in STARTER or via trace recording. A step in speed vs. torque is the signature.
- Raise the torque setpoint multiplicatively (often ×2 is sufficient to clear the gap) so that the operating point jumps past the discontinuity. In this test bench the user doubled the setpoint after identifying the gap.
- If repeated testing is required, configure a soft-start of the torque setpoint with a small ramp (use p1115 if integrating via analog input, or pre-process the fieldbus torque word in the PLC). This prevents repeated stalling at the plateau.
- For test benches, consider configuring the friction compensation model (p3820..p3829 family) to pre-load the static friction torque offset. Vector control supports a friction model that injects a feed-forward torque to overcome static friction.
- Once the speed resumes, verify p1407.17 = 0 and r0061 follows r0080 × mechanical constant without drive-induced clamp.
Verification Checklist
- p1406 Bit 12 = 1 confirms torque-control selected.
- p1501 (torque-control source) is set to a valid source (e.g. fieldbus or fixed setpoint).
- p1407 Bits 2 and 3 = 1 confirm speed and torque controllers are simultaneously active in the hybrid regulation path.
- p1407 Bit 17 = 0 throughout the run (no internal speed-limit hit).
- r1480 / r1482 / r1508 match within tolerance (controller not in saturation).
- r0061 tracks the expected speed for the applied torque and mechanical inertia.
- No alarm A07901, A07902 (drive-internal speed clamp) in the diagnostic buffer.
Function Diagram Mapping (FP6040, FP6060, FP6640)
For SINAMICS S120/S150, the torque-control path traverses the following function diagrams from the SINAMICS S120/S150 List Manual:
- FP6040 – Torque setpoint, limits. Inspect the chain from r1508 → p1526/p1527 → p1530/p1531 → torque controller.
- FP6060 – Torque limiting with power/current limit interaction. Confirm p0640, p1530/p1531 scaling.
- FP6640 – Closed-loop torque control with speed limitation. This diagram is the canonical view of how p1083/p1086 interact with torque mode. If p1083/p1086 are large (e.g. ±6000 rpm) but the speed does not climb, the diagram shows why: the speed limiter only clamps the setpoint; the controller has no authority to push the load beyond its mechanical capability.
Closed-Loop Torque Control Operating Mode Selection
Torque control can be selected in two ways on the SINAMICS S120/S150 platform:
| Selection method | Parameter | Behavior |
|---|---|---|
| Direct selection via control word | p1501 = 1 | Torque control without ramp-function generator. p1406.12 = 1, p1407.2 = 1, p1407.3 = 1. |
| Implicit via open-loop/closed-loop selector | p1300 = 22 (torque control, without encoder) or 23 (with encoder) | Permanent torque control. The function diagrams describe: "A changeover cannot be made between speed and torque control if torque control is selected directly with p1300 = 22 or 23." |
Per the SINAMICS S120 Function Manual, closed-loop torque control section, p1300 = 22/23 locks the drive into torque mode and prevents dynamic changeover back to speed mode without a stop-and-restart. Use p1501 if you need runtime changeover.
Force/Torque Limiting on S7-1500T Axes (Related)
If the test bench controller is an S7-1500T positioning or speed axis, additional force/torque limits can be imposed by the PLC technology object. Always verify that no upper or lower force/torque limit on the TO is binding before suspecting the drive. The TO limit acts upstream of the drive's analog or PROFIdrive setpoint and can be the actual active ceiling if it is set lower than the drive's clamp.
Troubleshooting Matrix
| Symptom | Likely parameter | Diagnostic signal | Resolution |
|---|---|---|---|
| Speed saturates below p1084 | Friction torque gap | r0061 plateau vs r1508 | Raise torque setpoint past the gap |
| Speed saturates below p1084 | p0640 current limit | r0067 ≈ p0640 | Raise p0640 or reduce load |
| Speed saturates below p1084 | p1530/p1531 power limit | r0080 × r0061 ≈ p1530 | Raise power limit or reduce speed |
| Speed saturates below p1084 | p1526/p1527 torque limit | r0080 ≈ p1526 or p1527 | Raise torque limit |
| Speed limit offset (+45 rpm) | Encoder scaling p0408/p0432 | r0061 vs r0061[filtered] | Verify encoder resolution and p0400 |
| Speed zero in torque mode | Encoder not detected | F07935 encoder fault | Check encoder wiring and p0400 |
| Speed overshoots then clamps | Speed controller I component | p1478 = 1 | Disable I component for pure torque mode |
Preventive Recommendations for Test-Bench Applications
- Map and store the friction curve (r0061 vs r1508) during commissioning. Document the torque gap so future operators can pre-load the setpoint.
- Use the trace function in STARTER (or the drive's internal trace) to record r1508, r1480, r0061 over a torque sweep at the first commissioning.
- Configure a fixed minimum torque bias via p1610/p1611 (torque offset) to overcome static friction if the bench is operated frequently near zero speed.
- If repeated speed/torque mode changeovers are needed, use p1501 (runtime changeover) rather than p1300 = 22/23 (locked torque mode).
- Verify the S7-1500T technology object force/torque limits (if used) are set above the bench's worst-case static friction.
Why does my SINAMICS S150 limit speed to 225 rpm in torque control even though p1084 is 3600 rpm?
The active ceiling is mechanical, not parametric. A friction-curve torque gap (static Coulomb-friction jump) absorbs the entire applied torque, so the motor stalls at the plateau speed regardless of p1082/p1083/p1084/p1086/p1087. Inspect r0061 vs r1508 to confirm a step discontinuity.
Which parameters actually clamp speed in closed-loop torque control on SINAMICS S120/S150?
Torque-mode limits are p1526/p1527 (torque upper/lower), p1530/p1531 (power motoring/regenerative), p0640/p0641 (current). Speed clamps p1082..p1087 only truncate the speed setpoint and do not force motion; they do not act as active ceilings in torque mode.
How do I select torque control at runtime on the SINAMICS S150?
Use p1501 = 1 to enable runtime changeover between speed and torque control. The drive then sets p1406.12 = 1 and p1407 Bits 2/3 = 1. Direct selection via p1300 = 22 (without encoder) or 23 (with encoder) locks the drive permanently into torque mode and prevents changeover.
What is the meaning of the 45 rpm offset when I set p1084 = 150 rpm?
The offset is not a parameter error; it is the plateau speed at which friction balances the applied torque. The drive is correctly tracking torque, but the load cannot accelerate beyond the friction-curve step. Increase the torque setpoint past the gap to resume motion.
How can I confirm that no drive-side clamp is active?
Monitor p1407 Bit 17 (speed limit reached). If it remains 0 while r0061 is far below p1083/p1084, the drive is not enforcing the clamp – the limitation is on the load side. Also verify p1526/p1527, p1530/p1531, p0640 are an order of magnitude above r0080 and r0067.