SINAMICS G120C Setpoint Not Reached: p0640 Current Limit Fix

Problem Description

A SINAMICS G120C compact inverter driving a 1.8 kW, 400 V, 3.5 A, 60 Hz, 1750 rpm squirrel-cage induction motor in a Bitzer CO2 refrigeration compressor fails to reach its commanded speed setpoint. The drive current saturates at the motor current limit (p0640[0] = 5.25 A, equivalent to 1.5 × motor rated current) and the motor runs at approximately 18 % of rated speed (≈315 rpm). At this reduced speed the self-ventilated motor cannot dissipate the I²R losses, the I²t motor model trips an overtemperature fault (F30011), and the compressor never reaches the process setpoint. Raising p0640[0] above 1.5 × FLA restores normal acceleration. The same root cause and remedy apply symmetrically to both inverters when the two compressors operate in parallel and one is pressurising the common suction header before the second starts.

Affected Hardware and Configuration

The reported configuration consists of two identical SINAMICS G120C drives wired in a star (Y) motor configuration with the following nameplate and commissioning data:

The published SINAMICS G120C Operating Instructions (09/2017 edition) document the relationship between p0305, p0640[0] and the drive's I²t protection, and confirm that the G120C delivers up to 150 % rated motor current for 60 s and 200 % for 3 s before the I²t power-unit model intervenes (F30005).

Root Cause Analysis: Current Limit vs. Load Torque Envelope

The drive did not reach the speed setpoint because the available torque at the current limit was insufficient to overcome the load torque at the operating point. Two physics-driven constraints apply:

1. Current-limited torque ceiling. For a given flux (set by V/Hz in open-loop V/f mode or by the magnetising current in SLVC), motor torque is approximately linear in stator current. Clamping current to 5.25 A in a 3.5 A motor caps steady-state torque at roughly 1.5 × rated torque. A Bitzer CO2 compressor starting against a pressurised suction header (≈18–25 bar in subcritical CO2 plants) typically demands 4–6 × rated torque during the first 1–3 s. The required current would therefore be 14–21 A, but the drive is allowed to deliver only 5.25 A.
2. Stall at 18 % speed. With current at the limit and slip at maximum, the motor's torque curve intersects the load curve at a very low speed. The V/Hz profile produces 0.18 × 60 Hz ≈ 11 Hz of stator frequency, but the rotor cannot follow and slip remains saturated. Shaft power (P = T × ω) collapses to 0.18 of the value achievable at full speed, exactly matching the 18 % shaft power reported.

The thermal consequence is unavoidable: a 1.5 × FLA stator current in a self-ventilated TEFC motor at 18 % speed is dissipating ≈2.25 × rated I²R while moving ≈18 % of rated air through the frame. The motor's I²t time constant (model parameter p0611, default 100 s for a 1.8 kW motor) is consumed in approximately:

t ≈ p0611 × [1 − p0305² / I²_actual]⁻¹ ≈ 100 / (2.25 − 1) ≈ 80 s to F30011 trip

This is consistent with the original report: the motor heats up and displays an overtemperature error within a minute or two of being commanded to start.

Diagnostic Parameters and Live Read Values

Connect to the drive via the BOP-2 (basic operator panel), IOP-2 (intelligent operator panel), or Startdrive / STARTER over PROFINET, USS, Modbus RTU, or USB. Capture the following read-only parameters with the drive running and the speed setpoint at 1750 rpm:

If r0027 pegs at 5.25 A while r0021 stalls and r0046 bit 4 = "1", the current limit is the active restraint and the corrective action below applies. If r0027 shows a value well below p0640[0] but r0021 still stalls, the restraint is mechanical (compressor seized, jammed suction valve, liquid slug) and p0640[0] alone will not help.

Resolution: Raising p0640[0] for Starting Torque Headroom

The remedy is to raise p0640[0] so the drive can deliver the brief, high current required to accelerate the compressor. A value of 2.0–3.0 × p0305 (7.0–10.5 A in this installation) is typical for subcritical CO2 compressor duty. The drive's own I²t model and overload ratings will still bound sustained current and trip on genuine faults.

1. Confirm drive sizing. Verify that the drive is rated to deliver the new current continuously. For a G120C 3 kW frame (rated output ≈7.3 A at 400 V), setting p0640[0] = 10.5 A is allowed because the drive will only sustain 1.5 × 7.3 = 10.95 A for 60 s, after which F30005 will trip if the overload is not cleared. Read r0209 to confirm the absolute ceiling of the power unit in the field.
2. Enter commissioning mode. On the BOP-2 navigate to the parameter menu, or in Startdrive / STARTER open the drive offline/online project. Set access level to "Expert" (p0003 = 3) to expose p0640[0] directly. Set p0010 = 1 (Quick Commissioning) only if a full reset is required; otherwise leave p0010 = 0 and write p0640[0] in run state.
3. Read p0305 baseline. Note p0305 = 3.50 A. Calculate the new limit as 2.5 × p0305 = 8.75 A. This is the recommended starting point for a 1.8 kW CO2 compressor on a 3 kW drive; 3.0 × p0305 (10.5 A) gives additional headroom for cold-start cases.
4. Write p0640[0]. Set p0640[0] = 8.75 (or 10.50 for headroom). Save to RAM only first (p0971 = 1) for a live test, then copy RAM to ROM (p0977 = 1) once the test passes.
5. Confirm the maximum current limit ceiling. The SINAMICS firmware will accept p0640[0] values up to the drive's "max current" (typically 2 × I_rated,inv). A setting above 14.6 A on a 7.3 A drive will be silently clamped by the firmware — read r0209 (rated power-unit current) to confirm the absolute ceiling.
6. Re-run the start sequence. With the compressor off-loaded (or with both compressor suction valves open to a vented receiver), command the setpoint and observe r0021, r0027, r0034. r0021 should rise to 1750 rpm within the ramp time (p1120), r0027 should peak at ≈8.75 A for ≈1–2 s and then decay to 2.5–3.5 A.
7. Test the on-load start. With the second compressor running and the common suction header pressurised to operating pressure, command the corrected drive to start. Confirm r0021 still reaches 1750 rpm, r0027 peaks below 10.5 A, and r0034 stays under 80 %.
8. Lock the parameters. Run "Copy RAM to ROM" (p0977 = 1) so the value survives a power cycle. Reset any active F30011 fault with the OFF1-OFF2-ON sequence or via the panel / PLC.

For installations requiring different starting behaviours (e.g., unload compressor before start, soft-loading the second machine), the current limit can be kept at a moderate value and a pre-opening suction bypass valve can be sequenced in the PLC, but raising p0640[0] is the lowest-effort and most robust fix.

Verification and Commissioning Checks

1. Steady-state current. With the compressor running at 1750 rpm and process setpoint achieved, r0027 should be 60–80 % of p0305 (≈2.1–2.8 A in this case). If r0027 sits at >90 % of p0305 indefinitely, the load is heavier than expected — re-check p0304 / p0305 / p0310 / p0311 against the motor nameplate and re-run motor identification (p1900 = 2 for rotating measurement).
2. Acceleration envelope. Capture r0024 (frequency) during a start from 0 to 60 Hz. The frequency should rise linearly with the ramp (p1120 = 10 s default) and reach 60 Hz at the end of the ramp. Any plateau at low frequency indicates a torque / current limitation.
3. Thermal model headroom. r0034 (motor I²t utilisation) must stay < 100 % continuously. A reading > 90 % after a start indicates the I²t model is being saturated — either the current limit is set too low (motor takes too long to accelerate) or the load is genuinely too large for the drive.
4. Fault log. Read the drive's fault buffer (r0945 / r0947 / r0949) and history (r0950). Confirm F30005 (power unit I²t) and F30011 (motor I²t) are absent after a full duty cycle.
5. Off-load vs. on-load start. Repeat the test with the suction valve closed (off-load) and open (on-load). The corrected p0640[0] value must succeed in both cases. A 2.5 × p0305 setting is generally sufficient; if the on-load start still stalls, the compressor's starting torque demand exceeds the drive's peak current capability and a larger drive is required.
6. PLC cross-check. If the compressor is controlled by an S7-1200 / S7-1500 over PROFINET, verify in the PLC trace that the drive word PZD2 (current / torque feedback) tracks the setpoint word PZD1 (speed setpoint) during the start. A constant PZD2 with a rising PZD1 confirms the drive is in current limit.

Related Fault and Alarm Codes

The G120C fault and alarm register includes several codes that interact with the current-limit / motor-thermal subsystem. Review each during commissioning:

Compressor-Specific Load Considerations

Reciprocating refrigeration compressors — and CO2 compressors in particular — are torque-dense, high-inertia loads with very different starting profiles from fans, pumps or conveyors:

1. Locked-rotor torque. Bitzer specifies 2.5–3.5 × rated torque for the LRT of standard 4-pole CO2 compressors, rising to 4–6 × for the heaviest models. This is the torque the drive must deliver in the first 0.5–2 s.
2. Breakaway torque. CO2 compressors with oil in the crankcase and refrigerant dissolved in the oil may exhibit a static breakaway torque of 4 × FLA, especially in cold installations. Crankcase heaters reduce this, but the drive must still be sized for the worst case.
3. Suction header back-pressure. In a parallel compressor bank, the second compressor starts against the discharge of the running machine. Subcritical CO2 suction is typically 20–40 bar(g) for low-temperature refrigeration, and discharge-side pressures of 80–100 bar are possible. The torque required to pull suction and compress to discharge is 3–5 × rated. Starting the second compressor "off-load" (suction valve closed) reduces this to 1.5–2 × rated but is rarely practical in a refrigeration process — the suction valve is the process control element.
4. Discharge valve unloading. Some Bitzer models (e.g., 4FC-3, 4GE-30 series) include capacity-control unloaders that hold the suction valves open during start. If present, the PLC should energise the unloader first, command the drive, and release the unloader only after r0021 = setpoint.
5. Soft-starter vs. inverter start. Across-the-line starting of a 1.8 kW compressor inrushes 6–8 × FLA from the supply, which is acceptable for a single compressor but causes unacceptable voltage dip when two start in parallel. An inverter with adequate p0640[0] headroom limits inrush to ≈3 × FLA and the supply dip is contained.
6. Crankcase heater interlock. Permit a drive start only when the crankcase-heater contact is closed (pump-down / cold-cycle protection) — CO2 refrigerant in liquid form entering the suction port is a primary cause of catastrophic liquid slugging and p0640[0] cannot save the compressor from that.

For a CO2 compressor, the recommended V/f mode is p1300 = 0 (linear V/f) with elevated starting boost (p1310 = 50–80 %, p1311 = 30–50 %). Sensorless vector control (p1300 = 20) provides higher starting torque and tighter speed regulation, but requires a one-time motor identification (p1900 = 2) and is sensitive to stator resistance drift (motor warm vs. cold). For a 1.8 kW compressor on a 3 kW drive, V/f with boost is the conservative choice; SLVC is preferable if the compressor has a high load-shock profile (e.g., cycling between loaded and unloaded).

Parameter Reference and Defaults

The following parameters govern motor data, current limit, and thermal protection on the SINAMICS G120C. The "Default" column shows the factory value; check the active value with the panel or Startdrive because commissioning tools frequently overwrite defaults during quick commissioning.

The default value of p0640[0] = 0 is interpreted by the firmware as "use 1.5 × p0305". When the value is non-zero, the firmware uses the written value as the absolute ceiling, in amperes. If a commissioning tool has written 5.25 explicitly, raising the value is the only way to get more headroom.

Best Practices for Parallel Compressor Installations

1. Size p0640[0] for the on-load case. Set p0640[0] to the maximum current the drive can deliver within its overload rating (1.5 × r0209 for 60 s or 2.0 × r0209 for 3 s), not to a "comfortable" 1.5 × FLA. Compressor inrush against back-pressure can briefly demand up to 2.5 × FLA; sizing for 1.5 × leaves no headroom.
2. Pre-spin the second compressor. Open the suction valve of the second compressor to atmospheric pressure before commanding the start, then close it once r0021 = setpoint. This reduces the inrush torque from 4–6 × rated to ≈1.5 × rated and makes the start robust with p0640[0] = 1.5 × p0305. The PLC interlock is straightforward.
3. Sequence the start. Always start the second compressor at least 5 s after the first has stabilised. Back-pressure ramps in 1–3 s after the first compressor starts, and a delayed start lets the discharge pressure equalise.
4. Monitor r0034 in the PLC. The motor I²t utilisation is a high-resolution, model-based signal that anticipates F30011 by 10–30 s. Trending r0034 over the start cycle is the cleanest way to verify p0640[0] is correctly sized.
5. Run motor identification on both drives. After commissioning p0304–p0311, run p1900 = 2 (rotating identification) on each drive. The motor data is re-measured at standstill and at 50 % of rated speed, which calibrates the I²t model, the SLVC torque model (if used), and the stator resistance compensation. A drive that has only been quick-commissioned (p0010 = 1) has default motor data, which is conservative for stator current and over-conservative for I²t headroom.
6. Keep p0601 = 0 for IEC motors without sensors. If the motor has no temperature sensor, leave p0601 = 0. Setting p0601 to a sensor type without a sensor wired will cause a sensor-open-circuit fault at every start.
7. Log r0027 in the historian. A 1-second sample of r0027 during a start gives a permanent record of the inrush profile. Comparing two consecutive starts on the same drive will detect bearing wear, suction valve leakage, or refrigerant overcharge long before they become field failures.
8. Re-check the second drive as well. The original report states the problem occurs on both G120C inverters. The most likely reason is that both were commissioned with the same conservative p0640[0] = 1.5 × p0305 default. Apply the fix to both drives, run motor identification on each, and document the final p0640[0] value in the plant asset record.

Following the steps above, the original symptom of "motor runs at 18 % speed, heats up, and trips overtemperature" is fully explained: the drive is delivering the maximum 5.25 A allowed by p0640[0] but the compressor's on-load starting torque exceeds what 5.25 A can produce at 11 Hz. Raising p0640[0] to 2.5–3.0 × p0305 allows the drive to deliver 8.75–10.5 A for the 1–2 s required to accelerate the compressor, after which the motor speed, airflow and thermal dissipation all normalise.

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