SINAMICS G120C Current Limit Switching After Ramp Complete

The SINAMICS G120C is a compact, modular frequency inverter from the SINAMICS V family, available in frame sizes FSA through FSF covering 0.55 kW to 132 kW in 400 V and 0.55 kW to 18.5 kW in 230 V variants. A recurring application requirement on this platform is the ability to allow high inrush/acceleration current during the ramp phase (for example, 6 A) but to clamp or trip the drive at a much lower current once the motor has reached setpoint speed and is in steady-state operation (for example, 2.5 A). The factory firmware of the G120C (as of firmware V4.7 SP10 and later G120C-specific revisions up to V5.2) does not provide a single built-in parameter that performs this context-sensitive clamp automatically. The required behaviour must be engineered using one of three proven approaches:

1. Switching the active Drive Data Set (DDS) once the ramp completes so that a tighter current-limit profile becomes active.
2. Implementing custom trip logic using the drive's internal Free Function Blocks (FFB) and the BICO (Binector / Connector) interconnection system, generating an external fault via a digital output wired to an external fault input.
3. Moving the supervision into the supervising PLC (S7-1200, S7-1500, ET 200, or third-party controller) over PROFINET/PROFIBUS using cyclic PZD word r0027 (current utilisation) or r0035[0] (motor current actual).

This reference documents the parameter structure, BICO interconnect points, FFB block layout, and PLC-side code required to implement each approach on a production machine.

1. SINAMICS G120C Current Monitoring and Limiting Architecture

Before implementing context-sensitive current clamping, the engineer must understand how the drive internally generates, monitors, and limits current. The following parameters are central.

2. Why a Single Parameter Will Not Solve the Problem

The most common misconception when starting on the G120C is that p0640 "Current limit" can be modulated during operation. p0640 is read at run-up and during the current controller update; while it can be reparameterised via BICO or PLC, the SINAMICS current controller still applies it globally — there is no built-in state machine that says "use 6 A during ramp, switch to 2.5 A after ramp complete".

The drive does provide p2170/p2171 which generates an "I_act > threshold" binector that can be wired to a fault or warning. However, the user must explicitly select which operating mode (steady-state vs transient) the threshold applies to. The cleanest way to gate that binector is to AND it with the "speed reached within tolerance" status bit, which is what the FFB approach does.

3. Method A — Drive Data Set (DDS) Switching

The G120C supports up to four independent Drive Data Sets, indexed p0180 = 0, 1, 2, 3. Each DDS has its own copy of every motor control parameter including p0640 (current limit), p2170 (current threshold for warning), p2100/p2101 (fault configuration), and the ramp time p1120/p1121.

3.1 Configuring two DDS sets

1. Set p0180 = 0 (DDS0 active).
2. Program p0640[0] = 12.0 A (or 200 % of p0305) for the ramp phase. Set p1120[0] = 5 s for the desired ramp duration. Program p2170[0] = 50 % of p0305 as a soft warning during ramp.
3. Activate DDS1: set p0180 = 1, then program p0640[1] = 5.0 A (the steady-state clamp). Set p2170[1] = 60 % of p0305 to allow the 2 A nominal current but trip at 2.5 A using the F07801 / OFF1 configured in p2100/p2101 of DDS1. Set p1120[1] = 1 s so any unintended switch while running does not cause a jerk.
4. Set the second motor data set switching source. Use p0820 = "BICO select DDS bit 0" and wire p0820 to a digital input. With one DI (for example, DI3) configured as bit 0, LOW selects DDS0 (6 A allowed, ramping), HIGH selects DDS1 (2.5 A clamp, post-ramp).

3.2 Switching the DDS at end of ramp

The cleanest moment to switch is when the ramp generator output equals the setpoint, which corresponds to status bit r2197.7 "speed setpoint reached / ramp complete". Two implementation options exist:

• PLC-driven: PLC reads r2197 (status word ZSW1 in the PZD telegram, bit 7), waits for an additional debounce time (typically 500 ms after bit 7 goes high), then writes p0820 = 1 via acyclic PKW (PKE/IND/PWE) or sets the configured DI high.
• Drive-internal: use a Free Function Block AND gate with r2197.7 + ramp-time-elapsed to drive a digital output that is wired back to DI3.

4. Method B — Free Function Blocks (FFB) Trip Logic

The G120C firmware ships with a fixed library of Free Function Blocks in the runtime group "Free Blocks". Each runtime slot (sampling time p20030[0..15]) can be configured to one of: AND, OR, XOR, NOT, NAND, NOR, ADD, SUB, MUL, DIV, LIM (limiter), INT (integrator), DIF (differentiation), PLG (polyline), PT1 (1st order lag), comparator CMP, signal-switching SW, signal-tracking ST, RS flip-flop, D flip-flop, MMA (motorised potentiometer), NSIN (sine), COSL (cosine lookup), ABS (absolute), ANA (analog multiplexer), DIVMOD.

4.1 Block diagram (steady-state current trip)

4.2 Parameter assignments

1. Configure the runtime group (p20030[0] = 4 ms) — every block runs at 4 ms; suitable for a 500 ms debounce.
2. Configure CMP_1 (comparator) in the FFB editor of STARTER / SINAMICS Startdrive: input X = r0035[0] (BICO source 0:53, scaled to A), input Y = p2170 reference (2.5 A scaled). Output drives binector b10.
3. Configure AND_1: input 1 = CMP_1 output, input 2 = r2197.7. Output drives binector b11.
4. Configure DLY_1 (signal tracking / delay): use the FFB "SW" or "PLG" block; on rising edge of b11, start 500 ms timer; if b11 remains high for 500 ms, output to binector b12 = TRUE.
5. Wire binector b12 to p2103 (BiCo source of external fault 1). Set p2100 = 0 (no fault number — but actually set p2100 to 7810 to use the standard "External fault 1"), and set p2101 = OFF2 (response: coast-to-stop) or OFF3 (fast ramp stop) per safety analysis.

5. Method C — PLC-Side Supervision over PROFINET

For machines already integrated with a SIMATIC controller, the most maintainable approach is to monitor r0027 (current utilisation %) or r0035 (current A) in the PLC and trigger a controlled stop.

5.1 Telegram and PZD mapping

Standard telegram 1 (STW/ZSW + NSOLL/IST) does not include r0027 or r0035. Use one of:

• Telegram 350 (free PZD configuration) — assign PZD3 output = r0027, PZD4 output = r0035[0]. Configure via p0922 = 350, then assign p2051[2] = r0027, p2051[3] = r0035[0]. The PZD type must be normalised word (16-bit) and r0027 will be reported in 0.01 %/100, r0035 in 0.01 A.
• Telegram 20 — Siemens standard extended telegram including status word r2089[0] and a selectable word that can be assigned to r0035 directly.

5.2 SCL / LAD sample (TIA Portal, S7-1200/S7-1500)

// Variables
iRawCurrent  : INT;        // PZD3 from drive, r0027 scaled 0.01%/100
iRawAmps     : INT;        // PZD4 from drive, r0035 scaled 0.01 A
rPctCurrent  : REAL;
rAmps        : REAL;
bRampDone    : BOOL;       // From PZD1 ZSW bit 7 (r2197.7)
bTrip        : BOOL;

// Scale PZD words
rPctCurrent := INT_TO_REAL(iRawCurrent) / 100.0;     // %
rAmps      := INT_TO_REAL(iRawAmps)  / 100.0;      // A RMS

// Steady-state trip
IF bRampDone AND (rAmps > 2.5) THEN
    bTrip := TRUE;
END_IF;

// Drive stop via control word
IF bTrip THEN
    iDriveControlWord.%X1 := TRUE;    // OFF2 (coast)
END_IF;

5.3 Debounce and ramp-detection

A 500 ms ON-delay is recommended to avoid nuisance trips on momentary transients during the last 10 % of the ramp where torque ripple can briefly exceed 2.5 A. Implement an SFB "TON" with PT = T#500ms.

iTON_001(
    IN := (rAmps > 2.5) AND bRampDone,
    PT := T#500MS,
    Q  => bTripRequest);

// Drive accepts OFF1/STW bit 0 = FALSE for ramp stop, OFF3 (bit 2) for fast ramp
IF bTripRequest THEN
    iDriveControlWord.%X0 := FALSE;   // ramp-down via OFF1
    iDriveControlWord.%X2 := TRUE;    // OFF3 (fast ramp)
END_IF;

6. Implementation Comparison Matrix

7. Step-by-Step Commissioning (Method B — FFB)

7.1 Prerequisites

• SINAMICS G120C with Control Unit CU240S2 PN or CU240S2 DP (firmware ≥ V4.7 SP3).
• STARTER V5.1 SP1 / SINAMICS Startdrive V15.1 (or later) commissioning tool installed.
• Motor nameplate data entered; quick commissioning completed; motor identification run successful.
• STARTER project online; drive in STOP / READY state.

7.2 Procedure

1. Open Online → Drive → Functions → Free Blocks.
2. Set runtime group 1 sampling time to 4 ms (p20030[0] = 4).
3. Insert CMP_1: assign X = r0035[0], Y = 2.50 (referenced against a constant set by p2170).
4. Insert AND_1: input 1 = CMP_1.output (b10), input 2 = r2197.7 (b5).
5. Insert DLY_1 (PT1 or PT2 with T = 500 ms): output goes to b12.
6. Wire p2103 = b12 (BiCo source). Set p2100 = 7810 ("External fault 1"). Set p2101 = OFF2.
7. Save to RAM → ROM, then perform "Copy RAM to ROM".

7.3 Verification

1. Force b12 = 0 via the trace tool; confirm drive is in RUN with motor at 1500 RPM and 2.3 A.
2. Briefly load the motor to 2.6 A by mechanical braking or by raising speed reference slightly above the mechanical capacity.
3. Verify the drive raises F07910 / F07810 within 500 ms ± 50 ms and responds with coast-to-stop (OFF2).
4. Reset the fault via digital input configured as p2104 = DI3 (acknowledge) or by toggling ON/OFF.
5. Capture the event with the STARTER trace at 4 ms sampling — confirm binector b10, b11, b12 transition sequence.

8. Edge Cases and Field-Proven Caveats

8.1 Motor current spikes on mains dips

If the supply network has frequent short dips, the inrush to maintain torque can produce 3 A spikes of < 100 ms that should not be considered a fault. Either (a) raise the debounce in DLY_1 to 1 s, or (b) use a PT1 filter with T = 200 ms on r0035 before feeding CMP_1.

8.2 V/f control mode vs Vector control

In V/f mode (p1300 = 0, 1, 2) the drive does not precisely control current; r0035 reflects the measured line current and can be lower than the true motor current at low speed. For tight 2.5 A trip accuracy, switch the drive to Sensorless Vector Control (p1300 = 20) and run motor identification. Without this, a trip threshold of 2.5 A may actually correspond to 3.0–3.5 A of true motor current.

8.3 Asymmetry on three-phase mains

r0035 is the RMS of phase U; the G120C does not report per-phase current unless you switch to r0069[0..5] (phase currents) which requires extending the PZD telegram. If the load is highly asymmetric, monitor the maximum of r0069[0..2] instead.

8.4 Behaviour at zero speed

Below 1 Hz output frequency the FFB comparator on r0035 can chatter if the resolver or encoder feedback is noisy. Add a low-speed mask: AND the trip condition with n_act > 50 RPM (use r0021[0]). Otherwise the drive may trip immediately after a STOP command.

8.5 PLC write race conditions

On PROFINET, p0820 (DDS select) writes via acyclic PKW channel can take 5–30 ms depending on controller scan. If you need deterministic <1 ms switching, drive-internal FFB (Method B) is required.

9. Parameter Quick Reference

10. Recommended Implementation Path

For a project where the drive is part of a larger S7-1500 system (typical for a SINAMICS G120C in a machine line), Method C (PLC supervision) is the most maintainable. The PLC already has the application logic, the HMI already shows drive state, and the alarm can be logged with operator-friendly text.

For a standalone retrofit with no PLC, Method B (FFB) is preferred because it survives a PLC failure (the drive will self-protect) and does not require commissioning of a separate controller.

Method A (DDS switching) is recommended when the actual goal is not to trip but to change operating envelope — for example, switch from "high-torque acceleration mode" to "low-torque hold mode" once at speed. It is the cleanest approach when the change is binary and intentional, not a fault response.

11. Related SINAMICS Documentation

• SINAMICS G120C Operating Instructions (Compact Operating Instructions, FW V4.7) — parameter list, terminal layout, commissioning flow.
• SINAMICS G120C List Manual (Firmware V4.7 SP10) — full BICO table including p2051 PZD assignments.
• SINAMICS S120/S150/G130/G150 Free Function Blocks Application Manual — block library reference for FFB.
• STARTER Commissioning Tool Online Help — FFB editor workflow.
• SINAMICS G120 Getting Started with Startdrive — TIA Portal integration.

12. FAQ