SINAMICS S120 Load Slip at Start: Tuning Closed Brake Control

Field reports on the SINAMICS S120 in crane applications show a recurring commissioning failure mode: the suspended load slips downward for a fraction of a second at the moment the drive is enabled, even though the motor is configured for closed-loop speed control with encoder feedback (p1300 = 21) and the holding brake is functionally healthy. The drive ramps to the setpoint, the brake eventually releases, and the load is then held — but the initial drop is unacceptable on hoist, trolley, or jib axes and is treated as a safety-relevant defect under EN ISO 13849-1. This reference explains the root cause, the brake-control parameters that govern the start sequence, and the parameter changes required to start cleanly against a closed brake without sacrificing the smooth release that prevents load swing and brake-pad wear.

1. Problem Statement and Field Symptoms

The reported behavior on a SINAMICS S120 controlled crane axis is reproducible. When the operator commands "hoist up" or "hoist down" from standstill, the motor develops torque against the still-closed holding brake, the brake releases after a finite delay, and only then does the load accelerate up the ramp. During that delay the load is held solely by mechanical brake friction, which is insufficient if the load is heavy or if the brake torque is reduced by wear, contamination, or thermal cycling. The result is a transient load drop of typically 10–100 mm depending on load mass, brake response time, and the torque-vs-speed profile of the motor during the first cycles of current injection.

Reported parameter settings on the affected axis:

• p1300 = 21 — speed control with encoder
• Extended brake control with brake feedback contact — p1215 = 3
• Brake release time p1216 = 4.0 s — slow, suggesting a large or pneumatic thruster brake
• Ramp-up time p1120 = 10 s
• Voltage boost referenced at 50% — typically p1310 (continuous), p1311 (acceleration), or p1312 (startup)

None of these values is individually wrong. The problem is that the speed-controller pathway and the voltage-boost pathway are being asked to solve a problem that neither is suited to: holding a suspended load during the brake-release interval without commanding motion that the brake is not yet ready to release against. The correct pathway is the extended-brake-control block (p1215 = 3) with p1152 = 1 and a properly set brake-release threshold (p1220/p1221), optionally augmented by load-cell-derived supplementary torque (p1511/p1512).

2. Induction Motor Slip and Torque Fundamentals

Slip is the relative difference between the rotating magnetic field established by the stator (synchronous speed) and the mechanical speed of the rotor. Synchronous speed is defined by:

n_sync = 120 × f / p

where f is the supply frequency in Hz and p is the number of motor pole pairs. Rotor mechanical speed is n_mech = n_sync × (1 − s), where s is per-unit slip. At rest, s = 1; at no-load, s drops to roughly 0.005 (0.5%); and at rated load, s is typically 0.02–0.05 for an induction motor of NEMA Design B / IEC equivalent design. Torque is proportional to slip in the stable region of the torque-slip curve: T ∝ s / (R_r/s)² for small slip and the rotor resistance R_r dominated case.

For a SINAMICS S120 in vector control with encoder, slip is a controlled variable: the field-oriented model computes the slip frequency required to orient the rotor flux, and the current controller imposes the corresponding stator current vector. The drive does not "let" the rotor slip; it sets the slip. If the commanded torque exceeds what the rotor can develop against the still-closed brake, the speed-controller integrator will wind up, the output will saturate at current limit, and the drive will either (a) sit at zero speed with full current, or (b) rotate the motor briefly if the brake is partially released or if its holding torque is below the commanded torque — and this is the slip event operators observe at the load. The rotor does not slip electrically in the textbook sense; the load slips mechanically because the brake holding torque is below the developed motor torque.

At start, the load is stationary and the rotor slip is 1.0 per-unit by definition; the drive must produce the holding torque in the slip = 1.0 region of the torque-slip curve, which requires the highest current of the operating envelope. This is why start-against-closed-brake control is a dedicated function in the S120 firmware and not a parameter the user can set by accident.

3. Root Cause: Torque Build-Up Against a Closed Brake

The default factory setting for the "open internal holding brake" command in SINAMICS S120 is wired to the speed-setpoint status bit, i.e. p1152 = r0899.15. Under this default the drive waits for a non-zero speed setpoint before enabling the brake-release sequence, but the brake is commanded open and the torque is allowed to build simultaneously once the setpoint is present. If the brake is still mechanically closed when the controller output exceeds the brake's static holding torque, the load moves against the brake. Because the drive is in closed-loop speed mode, the controller will increase its output to recover the small speed error, pushing the motor further past the brake holding torque and accelerating the load downward (hoist) or upward (lower) before the brake fully opens.

The classic signature is a brief load drop on hoist-up, where gravity is fighting the desired motion and the brake is the only thing holding the load before torque has built. Setting p1152 = 1 (constant 1, i.e. enable immediately) is the first step toward a correct start-against-closed-brake sequence, but it is not sufficient on its own. The drive will now generate torque against the closed brake as soon as it is enabled, and the brake-release threshold p1220/p1221 must be configured to gate the actual mechanical release. The extended-brake-control block also gates the operation-enable signal r0899.2 during the brake release interval to prevent the motor current from exceeding safe limits and to prevent the motor torque from damaging the brake.

The "50% voltage boost" reported in the original problem is a V/f-mode compensation that does not deliver additional holding torque in closed-loop vector control. In p1300 = 21 the current controller, not the voltage controller, commands torque. Voltage boost applied in vector control is either ignored (because the current controller dominates) or destabilizing (if it perturbs the current-controller output). It does not fix the load-slip problem; the parameters that fix it are p1152, p1220/p1221, and supplementary torque.

4. SINAMICS S120 Extended Brake Control Architecture

The SINAMICS S120 holding brake is controlled by the "Extended Brake Control" function block, which is configured by p1215:

For crane applications with a feedback contact, p1215 = 3 is the recommended setting. The "extended" mode is what enables the start-against-closed-brake sequence described in the SINAMICS S120/S150 Function Manual, including the p1220/p1221 torque threshold, the p1216/p1217 timing, and the gating of the operation-enable signal r0899.2 during the brake release interval. The standard brake control (p1215 = 1 or 2) does not gate the operation-enable signal and does not provide the threshold-based release sequence; it commands the brake open at the same moment the drive is enabled, which is exactly the failure mode that produces load slip on a hoist.

The following state diagram summarizes the extended brake control sequence for a hoist axis:

5. Starting Against Closed Brake: p1152 and Threshold Configuration

p1152 defines the signal that commands the holding brake to open. Factory default is p1152 = r0899.15 (speed-setpoint enable from sequence control). For start-against-closed-brake operation this must be changed to a constant 1:

• p1152 = 1 — brake is commanded to open as soon as the drive is enabled, regardless of setpoint state. The drive is allowed to develop torque against the still-closed brake.

This change alone is necessary but not sufficient. The actual mechanical release of the brake is gated by the brake-release threshold, which is configured by p1220 (signal source) and p1221 (threshold value). p1220 selects whether the threshold is evaluated as a current or as a torque:

• p1220 = 0 — current threshold. The brake is allowed to open when the motor current exceeds p1221 % of rated motor current (r0207 / p0305).
• p1220 = 1 — torque threshold. The brake is allowed to open when the motor torque exceeds p1221 % of rated motor torque. The torque is the smoothed value from the field-oriented model.

For hoist applications, the torque threshold (p1220 = 1) is preferred because it is load-independent: a heavy load and a light load both produce the same brake-release decision at the same percentage of rated torque. The current threshold is sensitive to motor power factor and to the rotor time constant at standstill, which makes it less repeatable across load conditions. The torque threshold value p1221 should be set above the gravitational holding torque of the worst-case load, plus a small safety margin. Typical values are 10–30% of rated torque; with load-cell-derived supplementary torque, the threshold can be set lower (5–10%) because the supplementary torque guarantees the holding torque is met before the brake opens.

6. Brake Sequence Timing: p1216, p1217, and Operation Enable Gating

The extended brake control manages three time intervals:

1. Brake release time p1216 — the time the brake takes to mechanically release after the open command. During this interval the operation-enable signal r0899.2 is gated (held low) so the motor current does not exceed the drive's permissible limit and the motor torque does not damage the brake pads or coil. The drive is developing torque against the brake, but the speed setpoint is suppressed.
2. Brake closing time p1217 — the time the brake takes to mechanically close after the close command. During this interval the drive decelerates along p1121 and parks at zero speed before the brake fully closes. The ramp-down time p1121 should be coordinated with p1217: a typical pairing is p1121 ≥ p1217 so the drive reaches zero speed before the brake applies.
3. Brake feedback monitoring — when p1215 = 3, the brake feedback contact is monitored continuously. A fault is generated (F07935) if the feedback state does not match the expected state within the timing window.

The reported p1216 = 4.0 s is plausible for a large pneumatic thruster brake but must be verified against the brake manufacturer's data sheet. Setting p1216 longer than the actual release time extends the operation-enable gating window unnecessarily and leaves the load held only by motor torque against a brake that has already released. Setting p1216 shorter than the actual release time causes the drive to release the operation-enable signal before the brake is mechanically open, producing a brief overspeed as the controller picks up a load that is suddenly free. Both are detectable on a properly configured trace recording.

One diagnostic that is often overlooked: while the operation-enable signal r0899.2 is gated during p1216, the speed setpoint is suppressed but the torque is not. Trace r0899.2, r0079 (torque), and the brake feedback contact together to confirm that the operation-enable signal rises within p1216 of the brake feedback contact closing/opening, and that the torque is at the holding value (or above) at the moment r0899.2 rises.

7. Why Voltage Boost Fails on a Closed-Loop Hoist

Voltage boost in SINAMICS S120 is configured by three parameters:

• p1310 — permanent voltage boost (V/f mode only). Adds a fixed voltage offset to the V/f curve at low frequencies.
• p1311 — voltage boost during acceleration (V/f mode only). Adds a voltage offset while the drive is accelerating.
• p1312 — voltage boost at startup (V/f mode only). Adds a voltage offset for the first few cycles after enable.

All three are evaluated in the V/f open-loop control mode (p1300 = 0, 1, 2, …). In closed-loop vector control with encoder (p1300 = 21) the current controller, not the voltage controller, commands torque. The current controller operates on the d/q frame and imposes the stator current vector that produces the requested torque; the V/f voltage boost terms are not in the control path. Setting p1310, p1311, or p1312 in p1300 = 21 has no effect on the produced torque, because the current controller overrides any voltage perturbation within its bandwidth. If the voltage boost is large enough to perturb the current-controller output, the result is current ripple and torque pulsation, not increased holding torque.

The correct pathway for load holding is the supplementary torque, described in the next section.

8. Supplementary Torque Configuration for Load Pre-Load

Supplementary torque 1 (p1511, scaled by p1512) and supplementary torque 2 (p1513, scaled by p1514) are added to the speed-controller output before the current setpoint. The most common use on a crane hoist is to take a load-cell signal, scale it to a torque value equal to the gravitational torque of the suspended load, and add it as supplementary torque 1. The effect is that the moment the drive is enabled, the motor pre-loads exactly the torque required to hold the load, and the brake-release threshold p1220/p1221 is satisfied immediately, so the brake opens without the load moving.

Configuration steps:

1. Identify the load-cell analog input wired to a SINAMICS analog input (typically AI0 or AI1 of the Control Unit CU320-2, or a TM31 terminal module).
2. Scale the load-cell signal (0–10 V or 4–20 mA) to a torque value in Nm. Use p1512 to apply this scaling factor. The scale factor must be calculated for the specific load cell, gear ratio, and motor rated torque; the calculation chain is load-cell_signal × (load-cell_full_scale / analog_input_full_scale) × (gear_ratio / motor_rated_torque) = p1512.
3. Wire the scaled signal to p1511 via BICO interconnection (e.g. p1511 = r755[0] for AI0 of the TM31).
4. Verify with the trace function in SINAMICS STARTER or TIA Portal Startdrive that r1515 (total supplementary torque) equals the expected holding torque at zero motion. The trace must be taken with the load suspended and stationary, no motion command issued.

For cranes without a load cell, a constant supplementary torque can be applied by setting p1511 to a fixed BICO source, e.g. p1511 = r1515 with a fixed p1512. This is acceptable for single-load applications but does not adapt to variable load. For variable-load hoists (a hoist that lifts different masses during normal operation), use a load-cell-derived supplementary torque.

One important caveat: the supplementary torque is added to the speed-controller output. If the speed controller also commands a torque (because the operator has commanded motion), the two are summed. This means a heavy load with a fast ramp will see a total torque request that exceeds the drive's continuous rating; check r1539 (torque limit) and r1538 (current limit) under all expected load and motion combinations.

9. Complete Parameter Reference

The following table lists the SINAMICS S120 parameters that govern the start-against-closed-brake sequence. Refer to the SINAMICS S120/S150 List Manual on the Siemens Industry Online Support portal for the full description, BICO interconnection options, and dependencies of each parameter. Where parameters have motor-data-set indices (e.g. p1216[0], p1216[1]), the table refers to the active data set.

10. Step-by-Step Commissioning Procedure

Use this procedure to commission the start-against-closed-brake sequence on a SINAMICS S120 crane axis. The procedure assumes STARTER (or TIA Portal Startdrive V15 or later) is the commissioning tool, and that the drive has been commissioned for basic motor control (motor identification complete, encoder signed and verified, current limits set).

1. Verify encoder feedback. Confirm p1300 = 21 (speed control with encoder). Check r0061 (actual speed) tracks the setpoint in motoring and the sign convention is correct. The actual speed must be positive for the positive setpoint direction defined in p1820 (direction of rotation reversal). Reverse the encoder channels A/B/ if needed; do not use sign-flip parameters because they can interact with safety functions.
2. Configure extended brake control. Set p1215 = 3. Confirm the brake feedback contact is wired to a SINAMICS digital input, and that the input is BICO-connected to p1223 (BI: holding brake feedback contact) — typically p1223 = r722.x. Verify the contact convention (NO or NC) against the brake vendor data sheet; a mismatched convention produces F07935 on the first enable.
3. Set brake timing. Enter the brake manufacturer's mechanical release time into p1216 and the brake close time into p1217. The reported 4 s is plausible for a pneumatic thruster brake but must be verified with the brake nameplate, not with the brake manufacturer's data sheet for a different model. Setting p1216 longer than the actual release time extends the operation-enable gating window unnecessarily and can mask other tuning issues.
4. Set p1152 = 1. This decouples the brake-open command from the speed-setpoint enable and enables the start-against-closed-brake sequence. The drive will now develop torque against the closed brake from the moment of enable.
5. Configure the brake-release threshold. Select torque threshold (p1220 = 1) for hoist applications. Set p1221 to 10–30% of rated motor torque, which is high enough to verify the motor is producing torque but low enough to release the brake without delay. For a load-cell-equipped hoist, the threshold can be set lower (5–10%) because the supplementary torque guarantees the holding torque is met.
6. Apply supplementary torque (if load cell is fitted). Wire the load-cell signal through p1511/p1512 as described in Section 8. Verify the actual holding torque on the trace.
7. Set ramp times. p1120 = 10 s for hoist ramp-up is conservative and acceptable. p1121 ramp-down should be matched to p1217 so the drive reaches zero speed before the brake closes; a typical pairing is p1121 ≥ p1217.
8. Remove voltage boost. Set p1310 = 0, p1311 = 0, p1312 = 0. In closed-loop vector control these are ineffective at best and destabilizing at worst.
9. Test under no-load. Enable the drive, command a small setpoint (e.g. 5% rated speed), and verify on the trace that the speed reaches setpoint without overshoot, the current stays within limits, and the brake feedback contact opens within p1216.
10. Test under full load. Apply a representative load (rated load or test load per the crane's load-test procedure). Command hoist-up and hoist-down. Verify the load does not move during the brake release interval. Adjust p1221 upward if the load drops before the brake opens.
11. Test the safety chain. Verify that the safety stop functions (STO, SS1, SLS as applicable) still gate the drive and the brake correctly. E-stop the drive and confirm the brake closes and the load is held.
12. Document the parameter set. Save the offline parameter set in STARTER/Startdrive with a unique project name, archive the trace recordings, and update the crane's commissioning record.

11. Verification and Acceptance Checks

After completing the tuning, the following checks must be performed and signed off by the responsible engineer:

12. Troubleshooting Matrix

13. Safety, Standards, and Recommended Change Set

The start-against-closed-brake sequence is a functional-safety-relevant operation: the brake is a safety-rated component, and the torque threshold parameters effectively determine the safety margin against load drop during normal operation. Commissioning changes to these parameters must be performed by personnel qualified to the crane's EN ISO 13849-1 PL and IEC 61508 SIL ratings, and the changes must be reviewed against the crane's original risk assessment. In particular:

• The brake-release threshold p1221 must always be set to a value that the drive can achieve against the closed brake with the available current. If the threshold is unreachable, the brake will never open.
• The supplementary torque p1511 must be sourced from a SIL-rated load cell (or equivalent) if it is part of the safety chain. A non-safety load cell is acceptable if the safety function relies only on the brake.
• Voltage boost parameters p1310/p1311/p1312 must not be used to compensate for inadequate current limit. If the drive cannot deliver the holding torque at p1221, the motor/drive selection is wrong, not the parameter set.
• Brake feedback contact wiring must be supervised by the drive's safety functions (typically via p1223 BICO and F07935 fault response).

Refer to the EN ISO 13849-1, IEC 61508, and IEC 61800-5-2 standards for the safety-integrity requirements applicable to the specific crane axis. These standards do not specify parameter values, but they do specify the methodology for verifying that the safety functions (STO, SS1, SLS, SOS as applicable) are achieved by the chosen parameter set. SINAMICS S120 safety functions are documented in the SINAMICS S120 Safety Integrated Function Manual.

Summary of recommended parameter changes for a SINAMICS S120 hoist axis exhibiting load drop at start in closed-loop speed control with extended brake control and feedback:

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