SINAMICS G120 p0640 Overcurrent vs F07801: Load Monitoring Setup

1. Problem Overview

A SINAMICS G120 inverter powered the motor of a chain-driven conveyor. After a chain failure, the drive did not raise a fault; the motor continued to turn into the jammed load and caused extended mechanical damage. Post-incident review of the parameter set showed:

• Motor nameplate current: 25.5 A
• p0640[0] = 38.25 A (1.5 × I_motor)
• No F07801 (motor overcurrent) in the alarm history
• No F30001 (power unit overcurrent) in the alarm history
• No torque-derived fault of any kind

The most common assumption when this happens is that p0640 should produce F07801 on a stalled load. That assumption is wrong. p0640 limits the current setpoint; it does not generate faults. During a stall the current regulator pushes the request toward p0640, the limiter clamps it, and the drive delivers sustained current at that value while the rotor sits still. From the inverter's perspective this is a normal condition.

The G120 protection model separates motor/inverter protection from mechanical load protection. For equipment where the mechanical drivetrain is weaker than the motor, the load-monitoring function (function diagram 8013) and external feedback are required.

2. SINAMICS G120 Protection Architecture

The G120 firmware separates two protection domains. Mixing them up is the single most common reason a mechanical failure goes undetected.

The middle three rows are limiters, not trip generators. They never raise a fault on their own; the controller simply stops requesting more current or torque. For a chain jam where the load demand rises within the limiter's capability, no fault is raised.

3. p0640 Demystified: Current Setpoint Limitation

p0640[0...n] defines the upper bound on the current setpoint issued to the closed-loop current regulator. The current regulator shapes the actual motor current to follow that setpoint within the inverter's hardware capability.

• Function: Setpoint clip. When the speed/torque controller requests more current than p0640, the request is clamped.
• Typical setting: 1.5 × I_motor_rated for constant-torque applications; 1.1 × I_motor_rated for variable-torque / fan / pump.
• No trip: p0640 itself does not raise F07801. F07801 is generated only by the inverter's hardware comparator.
• Three-phase formula reference: The torque-producing current capability of the drive at p0640 is approximately I_torque_max = p0640, which gives an apparent power of S = √3 × V_LL × I / 1000 kVA. A drive sitting at p0640 on a 400 V line is delivering up to 26.5 kVA on a 38.25 A setting.

4. F07801 and Internal Inverter Overcurrent Protection

Fault codes in the G120 family have firmware-dependent mapping. The two relevant hardware overcurrent codes are:

These are trip-on-event codes, not threshold-based. The internal comparator triggers when measured current exceeds an IGBT-dependent value well above any user parameter. Always cross-check the active firmware version's fault catalogue in the current SINAMICS G120 List Manual on the Siemens Industry Online Support portal before interpreting.

5. Torque and Power Limits (p1520, p1521, p1530, p1531)

Like p0640, torque limits do not generate faults. They cap the requested torque setpoint.

To retrieve actual torque for an external trip in a PLC, monitor:

• r0031 — Actual torque smoothed (% of rated torque)
• r0080 — Actual torque value (Nm if p2196 = 0, % if p2196 = 1)
• r0021 — Actual speed smoothed (RPM)

These can be mapped into a free PROFINET/PROFIBUS telegram (e.g., telegram 350 or 999) for cyclic read by the controller.

6. Load Monitoring Function (Function Diagram 8013)

The G120 load-monitoring function compares the current torque/speed envelope against configured thresholds and raises a configurable fault or alarm when the load deviates. Reference the SINAMICS G120 List Manual, function diagram 8013.

6.1 Operating Modes (p2181)

6.2 Key Parameters

6.3 Fault and Alarm Codes

Confirm exact codes and reactions against the List Manual for the active firmware version (4.7 SP10 / 4.7 SP13 / 6.x paths differ slightly).

7. Step-by-Step Configuration Procedure

7.1 Prerequisites

• SINAMICS G120 with CU240E-2, CU240E-2 PN, CU250S-2, or equivalent
• STARTER commissioning tool (V4.x) or TIA Portal (V15.1 or later) with SINAMICS Startdrive
• Motor nameplate data already entered into p0300-p0350
• Motor identification run completed (p1900 = 2 for static, p1960 = 1 for rotating)
• Speed encoder (if encoderless control, no extra wiring; if closed-loop with encoder, ensure correct direction in p0410)

7.2 Procedure

1. Baseline p0640. Keep p0640[0] at the motor-protection level. For a 25.5 A motor, leave it at 38.25 A or back off to 1.25 × I_rated (≈ 31.9 A) for a margin of safety on motor heating. This protects the motor windings, not the chain.
2. Capture reference torque. Run the conveyor under normal loaded conditions. In STARTER Trace, record r0031 (torque %) and r0021 (speed RPM) for at least 60 s of steady-state operation plus a full acceleration/deceleration cycle.
3. Set load-monitoring mode. p2181[0] = 1 for blockage detection, or p2181[0] = 3 for blockage and no-load.
4. Configure thresholds.
   • Peak r0031 under normal load = e.g. 85 %.
   • p2183[0] = 100 (blockage threshold = 100 % of rated torque; i.e., motor is requesting more than rated). Alternatively use the load torque reference approach: p2185[0] = 85, p2183[0] = 115 (blockage at 115 % of the load reference).
   • p2182[0] = 10 (no-load threshold = 10 % of rated torque with motor running).
5. Set delay time. p2184[0] = 500 ms (start conservative; shorten after bench testing). p2186[0] = 500 ms for torque-channel debounce. p2188[0] = 200 ms for speed-channel debounce.
6. Set speed threshold. p2187[0] = 100 RPM (only enable load monitoring above 100 RPM, so acceleration transients do not trigger a false blockage).
7. Wire fault reaction. Use p2100[0] and p2101[0] to set the stop reaction for F07935 / F07937. Default is OFF2; for a chain conveyor, OFF1 is usually preferred to allow controlled ramp-down.
8. Save. p0977 = 1 to copy RAM to ROM.

7.3 Verification on the Bench

1. Run the motor unloaded. Verify A07938 (no-load alarm) appears within p2184 ms of the speed crossing p2187.
2. Mechanically lock the output shaft for > 1 s with the drive running. Verify F07935 (blockage) appears and the configured stop reaction fires.
3. Measure timing: from threshold exceedance to drive pulse inhibit should be ≤ p2184 + 2 × current-controller cycle time (typically 4 ms at 4 kHz IGBT, 8 ms at 2 kHz).
4. Review the alarm history (STARTER → Drive → Diagnostics → Alarms). All four events should appear with timestamps.

8. TIA Portal PLC Integration for External Trip Logic

If the load-monitoring function is not sufficient — for example, when the application has multiple operating states with different torque profiles — implement a PLC-side trip. The drive's actual torque and speed are read cyclically; the PLC compares them against a state-dependent threshold and issues OFF2 if exceeded.

8.1 Cyclic Data Setup

• Add the G120 to the PROFINET topology in TIA Portal.
• Insert standard telegram 1 (control word 1 + speed setpoint + status word 1 + actual speed) as a baseline.
• For free telegrams (350/352/999), append r0080 (actual torque) and r0031 (torque utilization) to the receive words.

8.2 SCL Trip Block

// Inputs
i_torquePct      : REAL;   // r0031 from drive, % of rated
i_speedRPM       : REAL;   // r0021 from drive
i_runEnable      : BOOL;   // bit 2 of status word 1
i_stateAllowTrip : BOOL;   // application state machine permission

// Constants
T_THRESHOLD_PCT  : REAL := 115.0;
T_NO_LOAD_PCT    : REAL := 5.0;
T_MIN_RPM        : REAL := 100.0;
T_DELAY_HIGH     : TIME  := T#500ms;
T_DELAY_LOW      : TIME  := T#2s;

// Filter / debounce
tonHigh          : TON;
tonLow           : TON;

tonHigh(IN := (i_runEnable
               AND i_stateAllowTrip
               AND i_speedRPM > T_MIN_RPM
               AND i_torquePct > T_THRESHOLD_PCT),
        PT := T_DELAY_HIGH);

tonLow(IN := (i_runEnable
              AND i_stateAllowTrip
              AND i_speedRPM > T_MIN_RPM
              AND i_torquePct < T_NO_LOAD_PCT),
       PT := T_DELAY_LOW);

// Trip output — clear bit 1 of STW1 to issue OFF2 (coast to stop)
IF tonHigh.Q OR tonLow.Q THEN
    i_stw1.%X1 := FALSE;
ELSE
    i_stw1.%X1 := i_runEnable;
END_IF;

8.3 Control Word Bit Map (STW1, Siemens standard)

9. External Speed and Motion Feedback

Torque-only monitoring misses certain failure modes:

• Chain break — torque drops to near zero but the motor keeps running. This is detected by the no-load branch of load monitoring.
• Chain slip on the sprocket — torque is partially normal but motor speed exceeds expected driven speed.
• Misalignment after a stall — high torque with a phase-shifted mechanical response.

For these conditions, add a feedback device that observes the chain or driven sprocket directly:

Compare the measured chain speed against the motor speed divided by the gear ratio. A deviation greater than 5-10 % indicates slip; a zero measured chain speed with the motor running indicates a break. Implement the comparison in the PLC using a periodic high-speed counter with a time-window check.

10. Commissioning Verification

1. Baseline trace. Record r0031, r0021, r0022 (output frequency) for a full normal cycle in STARTER Trace. Save the recording.
2. Blockage test. With the drive running under load, mechanically lock the output shaft for 1 s. Verify F07935 (or A07936 if alarm-only) raises and the configured stop reaction executes. Measure the time from threshold exceedance to pulse inhibit.
3. No-load test. With the drive running, mechanically disconnect the load. Verify F07937 or A07938 fires after p2184 + p2186.
4. Acceleration transient. Run a full acceleration cycle from 0 to rated speed. Confirm that F07935 does NOT trigger — the speed threshold p2187 should be set above the speed at which acceleration torque peaks.
5. PLC logic test. Use a simulated torque word injected via a watch table to trigger the SCL TON block. Verify OFF2 is issued and that the drive pulses inhibit.
6. Documentation. Save the parameter set with STARTER (Upload to PG) or with TIA Portal (Upload device as new station). Save the trace recordings for future reference.

11. Troubleshooting Matrix

12. Worked Example: 25.5 A Motor, Chain Conveyor

Apply the procedure to the original scenario:

1. Motor: 25.5 A, 400 V, 4-pole, 50 Hz.
2. Working torque reference: from STARTER Trace under normal loaded running, r0031 peak = 92 % of rated, average = 70 %.
3. Blockage threshold: p2185[0] = 70 (reference), p2183[0] = 120 (i.e., trip at 120 % of reference = approx. 84 % of rated, well above working peak of 92 % × rated, with a margin).
4. No-load threshold: p2182[0] = 5 (% of rated).
5. Delay: p2184[0] = 500 ms; p2186[0] = 500 ms.
6. Speed enable: p2187[0] = 200 RPM.
7. Reaction: F07935 and F07937 → OFF1 via p2101.

With this configuration a chain seizure (torque rising past 120 % of reference for > 500 ms) raises F07935 and ramps the drive down via OFF1. A chain break (torque dropping below 5 % of rated while speed > 200 RPM) raises F07937 and ramps down via OFF1. Neither condition relies on p0640 or F07801.

FAQ