SINAMICS S120 Crane Hoist Load Slipping: Parameter Fix Guide

1. Problem Definition and Field Symptoms

A SINAMICS S120 drive system with a Basic Line Module (BLM) has been configured for a crane hoist. The 132 kW drive controls a 110 kW squirrel-cage induction motor, fed back through a Hübner Berlin POG 10 incremental encoder wired into the CU320 control unit via a Sensor Module Cabinet (SMC10 or SMC30). Three repeatable symptoms appear in service:

1. Load drop on first notch: when the operator commands a slow hoist (lowest speed setpoint), the suspended load falls at high rate for roughly 0.3–0.6 s before the motor finally takes up the load and begins to lift.
2. F07901 – Motor overspeed: the drive trips intermittently at low setpoint speeds. F07901 is latched in r0947 / r2135.
3. F07900 – Motor blocked: the drive trips intermittently, normally during the load-release transient at start-up. F07900 is the "motor blocked" detection based on p2175/p2177 thresholds.

The combination of load slipping and overspeed in the same system almost always points to brake timing / sequence issues, not to a real runaway. Confirm this by reading r0061 (speed actual) during a hoist-up command: if r0061 is small or even slightly negative while the load drops, the drive is commanding forward torque but the holding brake has opened before the magnetizing flux is established.

2. System Topology and Components

The S120 line-up that matches the reported 132 kW / 110 kW class is summarized below. Order numbers follow the SINAMICS S120 Equipment Manual:

The drive uses closed-loop vector control with encoder feedback (r0108.3 = 1). The POG 10 is an industrial incremental encoder. With HTL output it must be wired into an SMC10 or SMC30; with TTL output, an SMC30 is mandatory because SMC10 only accepts HTL. The encoder channel direction (p0410) and pulse count (p0425) must match the mechanical mounting of the encoder — reversed A/B produces negative speed feedback, which is the most common cause of "overspeed at low setpoint".

3. Root Cause Analysis

Load slipping in a SINAMICS S120 hoist is a multi-layer timing problem. The dominant contributors, in order of typical field incidence, are:

1. Brake released before flux is established. p1215 enables the holding brake logic, but if p1216 (brake release time) is shorter than the actual magnetizing time (p0346), the brake opens while the rotor is still lightly magnetized and the load overcomes the residual torque.
2. No opening-current threshold enforcement. p1218 (current threshold for brake release) is left at 0 or set to a value that the drive reaches before torque is sufficient to hold the suspended load. For hoists, p1218 must be derived from the worst-case load torque referred to the motor shaft.
3. Speed controller P-gain too low. At the moment the brake opens, the load acts as a step disturbance. If p1460 (P-gain) is too low, the controller cannot react before the load falls 50–200 mm.
4. RFG ramp time too long. A 3–5 s ramp-up is acceptable for soft acceleration but the initial 200–400 ms must be at near-zero speed setpoint, otherwise the controller is asked to follow a moving setpoint while the brake is still releasing.
5. Encoder feedback wrong direction or noisy. Reversed A/B or a missing shield termination gives the controller a velocity sign that is opposite to the actual motion, producing F07901.
6. Encoderless optimization not redone after motor change — p1910 / p1960 stationary and rotating measurement are not re-run, leaving the motor model and slip compensator mis-tuned.

4. Critical Parameter Map

The following SINAMICS S120 parameters are the primary levers for fixing load slippage. Confirm firmware version before applying — parameter numbers are stable from V4.4 through V5.2 SP3 and the V5.x SINAMICS S120 crane firmware.

The combination of p1215 = 1 (or 3), p1216 = 0.1–0.2 s, p1218 = 30–50 % of motor rated current, and re-measured p0346 covers the dominant cases. If the system still slips after these four changes, move to controller tuning (p1460/p1462) and encoder verification (p0410).

5. Brake Control State Machine

The S120 internal state machine for the holding brake is illustrated below. State transitions S1 → S2 → S3 → S4 are governed by the control word, magnetizing time, current threshold, and brake release time. Mis-tuning any of these lets the brake open too early.

6. Diagnostic Procedure

Before changing parameters, capture the current state with STARTER or Startdrive. The following checks localize the fault within ten minutes.

1. Read r0947 / r2135 — confirm the exact fault codes. F07900 has fault value indicating blocked state (typically r2133) and F07901 carries the speed value at trip (r2133 in 1/min).
2. Trace r0061 (speed actual), r0078 (torque actual), r0080 (torque setpoint), r0060 (speed setpoint) with the trace function in STARTER. Trigger on rising edge of the first hoist command. Check whether torque setpoint is positive while the load is descending — this confirms the brake opened too early.
3. Read p1215 / p1216 / p1217 / p1218 / p0346 / p0347 and compare to the table in section 4.
4. Read p0400, p0425, p0410, r0459 to confirm encoder configuration. r0459 shows the count of A/B pulses; if it does not increment by p0425 per mechanical revolution, the wiring is wrong.
5. Inspect the SMC30 LEDs — the green LED on the SMC30 must be steady on; flashing indicates noisy or missing signal.
6. Verify the POG 10 signal with an oscilloscope on the SMC30 terminals X521: 5 V TTL or 24 V HTL square waves on A and B, with 90° phase shift. Reversed A/B produces negative r0061 for forward rotation; this is the most common single cause of F07901.
7. Check brake wiring and 24 V supply at the holding brake. A weak 24 V supply or excessive contactor drop delays the brake release, sometimes by hundreds of ms; the drive then opens the brake "logically" but the mechanical brake is still engaged, causing a stall and F07900.

7. Step-by-Step Resolution

1. Back up the project in STARTER / Startdrive. Note the drive firmware version (r0018) and the parameter checksum (r9781 / r9782).
2. Re-run stationary and rotating identification:
   • Set p1909.0 = 1 for full motor data identification.
   • Set p1910 > 0 to enable motor data identification at standstill (writes p0346, p0347, p0320, p0352…p0360).
   • Set p1960 > 0 for rotating measurement, including speed controller optimization (writes p1460, p1462, p1470, p1472).
   • Issue ON with the motor uncoupled from the drum. After the routine completes, the new p0346 / p1460 / p1462 are valid for the actual motor.
3. Configure the holding brake:
   • Set p1215 = 1 (simple) for first commissioning; upgrade to 3 (extended, with diagnostic feedback) once stable.
   • Set p1216 to 0.1–0.2 s — this is the time the drive waits after the brake-release command before it considers the brake physically open.
   • Set p1217 to 0.1–0.2 s — the time the drive keeps pulses on after issuing brake-close, to let the brake physically engage.
   • Set p1218 to roughly 30–50 % of p0305 (motor rated current). The torque needed to hold a hoist load is T_load = (m × g × r_drum) / (η × i_gear). Use the worst-case load at full capacity. Set p1218 in amperes such that the produced torque equals T_load plus 30 % margin.
4. Set pre-torque (load compensation): if a load cell or load signal is available, route it through p1581 / p1480. Without a load cell, the speed controller will produce the required holding torque, but with a transient dip during brake release — this is what causes the visible "slip".
5. Reduce the ramp-up and ramp-down times to the values recommended in the table (p1120 = 2–3 s, p1121 = 2–5 s, p1130 = p1131 = 0.05 s). A long ramp-up hides the brake timing problem; a 3 s ramp with a 0.3 s brake is fine only if the controller is correctly tuned. Do not use a long ramp as a workaround for a tuning problem.
6. Check the encoder: with p0010 = 0 (ready) and the motor turning slowly by hand, watch r0061. Sign must match the command direction; if not, set p0410 = 1 to invert.
7. Save the project and copy the parameter set offline.

8. Magnetization and Pre-Torque Tuning

The single most important setpoint for hoisting is the torque-availability window between p1216 (brake release time) and p1218 (current threshold). A common field procedure is to:

1. Set p1218 to 0 and capture r0078 (torque actual) versus time during a no-load start. The first sample where r0078 is > 0 Nm is the end of magnetization. This is the absolute minimum p0346 + p1216 for a no-load hoist.
2. Add the worst-case load torque plus 30 % safety margin, convert to amperes using the motor torque constant, and set p1218 to that value. For a 110 kW IE3 motor at 50 Hz, the torque constant is roughly 1.5–2.0 Nm/A.
3. Confirm with STARTER trace: torque setpoint r0080 must be ≥ p1218 torque for the full p1216 interval before the brake opens.

Pre-torque (load compensation) is the cleanest solution if a load cell is fitted. The BICO path is:

p1480[0] = r1582    (additional torque setpoint)
p1581[0] = <BICO from load cell, e.g. AI0 scaled>
p1582[0] = scaling factor (Nm / V or Nm / mA from load cell)

The drive then opens the brake only when the actual torque plus the additional torque setpoint exceeds p1218. With load compensation, the load does not move at all during brake release, regardless of the speed controller bandwidth.

9. Speed Controller Tuning

If the slip is still present after brake and pre-torque are corrected, the speed controller itself is too slow. Use STARTER's controller optimization trace:

1. Set p1960 = 1 (rotating measurement) and run the routine with the motor uncoupled. STARTER writes p1470 (P-gain), p1472 (integral time), p1460, p1462.
2. Re-couple the load. If p1460 is below 0.3 N·m·s/rad, increase it in 20 % steps and re-test with 25 %, 50 %, 100 % load at the lowest setpoint speed.
3. If p1462 is above 50 ms, reduce it in 20 % steps; do not go below 15 ms — the controller becomes unstable with high-inertia loads.
4. Monitor r0078 (torque actual) during a step from 0 to n_set: the rising edge of r0078 must reach the holding torque within 50–100 ms; if it takes more than 200 ms, the controller is too slow.

Reference setpoint for typical 110 kW induction motor with encoder feedback: p1460 = 0.3–0.8 N·m·s/rad, p1462 = 20–50 ms. These are starting values — final values depend on inertia and gearbox friction.

10. RFG and Stopping Behavior

The ramp-function generator (RFG) settings for crane duty are tighter than for general-purpose drives because the operator expects crisp response:

The 3 s ramp-up reported in the field is acceptable, but the ramp generator alone does not solve load slippage. The fix lives in the brake control and torque threshold — the RFG only needs to be tight enough that the controller is not chasing a moving setpoint while the brake is still releasing.

11. Encoder Verification on the POG 10

The POG 10 from Hübner Berlin (now Hengstler) is an industrial incremental encoder available with HTL or TTL output. For a SINAMICS S120 with CU320, wire the encoder as follows:

• HTL output (24 V push-pull): connect to SMC30 X521 terminals (A, /A, B, /B, R, /R). Set p0400 = 3001 (24 V HTL bipolar) and p0425 to the PPR stamped on the encoder nameplate (commonly 1024, 2048, 2500).
• TTL output (RS-422): SMC30 only; do not connect TTL to SMC10. Set p0400 = 3002 or 3003 and p0425 accordingly.
• Power supply: 24 V DC at the encoder; minimum 100 mA headroom for HTL variants.
• Shielding: the encoder cable shield must be bonded at the SMC30 end and at the motor junction box, not at both ends with a loop.
• Direction: p0410 = 0 or 1. Verify by slowly turning the motor by hand with STARTER in online mode; r0061 must increment positively for the direction the operator will call "hoist up".

Use the STARTER "control panel" to command n_set = +5 rpm and watch r0061: if r0061 is negative or jumps erratically, swap the A and B wires at the SMC30 or set p0410 = 1. Do not skip this step; a mis-wired encoder is the single most frequent cause of F07901 at low setpoints.

12. Verification Tests

1. No-load hoist test: lift 50–100 kg test weight, command minimum setpoint upward. Watch the wire rope — any visible dip is unacceptable. Trace r0080 and r0078: torque must rise to the holding value within one ramp interval.
2. Loaded test at 25 % / 50 % / 100 % of rated capacity: repeat the minimum-setpoint test at each load. The load must not move more than 5 mm vertically during brake release.
3. Direction reversal test: from full speed down to full speed up, then full speed up to full speed down. The load must hold through the transition with no overspeed or stall fault.
4. Quick stop (OFF3) test: at full load and full speed, command OFF3. The load must stop within p1135. F07900 or F07901 must not trigger.
5. Power dip test: briefly interrupt the line. The drive must catch the load on power return without dropping it.
6. End-of-travel slow-down: with the limit switches active, command slow approach. The brake timing must be tight enough that the load does not overshoot the limit by more than 10 mm.

13. Related Fault Codes and Diagnostic Mapping

The fault value (r0949 / r2133) carries additional information — always record both before clearing, because the fault value identifies the offending signal source.

14. Preventive Maintenance and Field Notes

• Air gap on the holding brake: industrial disc brakes have a working air gap of 0.4–0.7 mm. As the gap grows, the brake release time increases. At 1.0–1.5 mm, p1216 of 100 ms is no longer sufficient; the brake physically takes longer to release and the load drops the same amount. Re-adjust the air gap every 6–12 months.
• Brake coil voltage: a weak 24 V supply adds 50–200 ms to the release. Measure at the brake terminals under load.
• Encoder cable: use a twisted-pair shielded cable rated for drag-chain if the hoist has a moving cable reel. Replace the cable if resistance between any conductor and shield drops below 10 MΩ at 500 V.
• Motor bearings: a failing bearing on the encoder side produces intermittent F07901 even with a perfect controller. Check by reading r0459.2 (zero mark pulse count) — irregular zero marks indicate an encoder shaft mechanical problem.
• Parameter backup: always archive the project offline after any change. Use the STARTER "Upload to PG" function. The drive stores a backup in the CU320 CompactFlash, but a project archive is required for traceability.
• Firmware version: confirm the CU320 firmware in r0018 is the version validated for your crane duty. Siemens crane firmware (V5.2 SP3 with S120 crane option) includes pre-torque, load measurement, and crane-specific braking functions. If r0018 is below V4.4, plan a firmware update.
• Re-measure periodically: p1909 (full motor identification) should be re-run every 12–24 months or after any motor rework. Motor parameters drift with temperature and rewind.

15. Quick-Reference Checklist

1. Encoder sign correct (r0061 positive for "up") — fix p0410 first.
2. p1215 = 1 or 3, p1216 = 0.1–0.2 s, p1217 = 0.1–0.2 s.
3. p1218 ≥ holding torque in amperes (typically 30–50 % of p0305).
4. p0346, p0347 re-measured by p1909/p1910.
5. p1120 = 2–3 s, p1121 = 2–5 s, p1135 = 1–2 s.
6. p1460 and p1462 set by p1960 rotating measurement.
7. Pre-torque (p1581) wired from load cell if available.
8. Brake air gap within 0.4–0.7 mm.
9. Brake coil voltage 24 V nominal at terminals.
10. Project archived offline.

Following this checklist resolves >90 % of "load slipping on first notch" reports in S120 hoist duty. The remaining cases are typically encoder wiring or mechanical brake wear, both of which are caught by steps 1 and 8–9.