Resolving SINAMICS G120 Setpoint Not Reached Under Load Issue

1. Problem Description

A 90 kW (factory-rebadged 75 kW / 139 A / 400 V / 100 Hz / 2800 RPM) two-pole asynchronous motor driven by a Siemens SINAMICS G120 inverter on a commercial meat cutting machine fails to reach commanded setpoint speed when cutting load is applied. With a fixed setpoint of 800 RPM the drive reports a steady-state speed of 100-300 RPM, drawing 150-250 A, and behaves as if speed is a function of load rather than a fixed reference. At 2800 RPM the speed only ramps upward once the load is removed (ingredients fully pureed). The factory requirement is: any switch position must drive the motor to the corresponding setpoint regardless of mechanical load.

2. System Configuration

2.1 Drive And Motor Hardware

• Inverter: SINAMICS G120 with CU240E-2 control unit (or compatible CU)
• Power Module: PM240 or PM250 in the 75-90 kW class
• Motor: 2-pole induction motor, factory-rewound, rewired delta, 75 kW, 139 A, 400 V, 100 Hz, 2800 RPM, 1 pole pair
• Load: cutting bowl/blade assembly with high start-up torque demand

2.2 Parameter Snapshot Before Fix

2.3 Observed Behaviour

3. Root Cause Analysis

3.1 Load-Dependent Setpoint Modification (Primary Fault)

The driving defect is the combination of P15 = 3 (fixed setpoint macro) with P501 = 1 (load-dependent setpoint modification). With P501 = 1 the inverter superimposes a load-proportional correction onto the fixed setpoint: as motor current increases, the speed reference is reduced. Under mechanical load on a cutting machine the correction dominates and the drive settles at a fraction of the commanded setpoint - exactly the symptom observed. For a constant-torque cutting process the load-dependent function is fundamentally wrong; the drive must hold the setpoint and use its current/torque headroom to push through load transients.

3.2 Field Weakening From 50 Hz To 100 Hz

The motor is a 50 Hz design (1500 RPM) operated at 100 Hz (2800 RPM). Above base frequency the V/f ratio collapses when terminal voltage is held constant, the air-gap flux falls approximately as 1/f, and maximum available torque falls by the same factor:

T_max(f) ≈ T_rated · (f_base / f)

At 100 Hz with a 50 Hz base the same motor on the same 400 V supply produces:

T_max(100 Hz) ≈ T_rated · (50 / 100) = 0.5 · T_rated

This is the physical reason the original 90 kW rating is unattainable. The factory rewinding/rewiring is the corrective measure. The new design point is 75 kW / 139 A / 400 V / 100 Hz / 2800 RPM, which reflects the new torque and power envelope at the higher base frequency.

3.3 Low V/f Operating Point (50 V / 10 Hz)

For a 2-pole motor at 800 RPM, the synchronous speed corresponds to f = p·n/120 = 1·800/60 ≈ 13.3 Hz (plus 1-2 Hz slip for an induction motor). The drive was observed at 50 V / 10 Hz (V/f = 5 V/Hz). The expected linear V/f for a 400 V / 100 Hz base is 4 V/Hz, so the drive was actually slightly above the linear V/f - confirming it is not following the 800 RPM setpoint (which would be 13.3 Hz at ∼53 V); it is following a load-dependent equilibrium near 10 Hz. The 150-250 A current at this reduced operating point also exceeds the 139 A rated current, so the drive is operating in current limit while the setpoint controller has effectively given up.

3.4 Motor Nameplate Integrity

The motor has been rewired by the factory and the original nameplate no longer reflects the rewound configuration. Without correct P304-P311 entries (motor rated voltage, current, power, power factor, speed, frequency, pole pair number) the V/f controller cannot compute correct flux, slip compensation, or boost. Any residual error here compounds the P501 misconfiguration.

4. Required Parameter Changes

4.1 Disable Load-Dependent Setpoint Modification

4.2 Enter Verified Motor Nameplate Data

4.3 V/f Profile And Boost

5. Commissioning Procedure

5.1 Pre-Commissioning Checks

1. Verify mains voltage at the drive line terminals: 380-480 V ± 10 % three-phase.
2. Verify motor insulation with a 500 V megohmmeter: ≥ 10 MΩ phase-to-phase and phase-to-ground.
3. Confirm motor connection: 400 V delta for the rewound stator.
4. Confirm mechanical decoupling path exists between motor and cutter head (for P1900 = 2).
5. Save a backup of the existing parameter set via BOP-2 / IOP or SINAMICS Starter / TIA Portal.

5.2 Parameter Reset (Optional)

1. Set P10 = 30 (drive commissioning parameter filter = expert).
2. Set P97 = 1 (factory reset, all parameters to defaults).
3. Wait for completion; the drive restarts.

5.3 Motor Data Entry And Identification

1. Set P10 = 1 (commissioning).
2. Enter the verified nameplate data into P304-P311 (see Section 4.2).
3. Set P96 = 0 (no automatic identification) initially; identify motor data manually first.
4. Set P501 = 0 (disable load-dependent setpoint modification).
5. Set P1900 = 2 (rotating motor identification) only after confirming the motor is decoupled from the load.
6. Issue an ON command; the drive performs the static and rotating measurement and writes P350-P367 with the identified values.

5.4 Setpoint Source And Fixed-Setpoint Macro

1. Set P15 = 3 (fixed setpoint macro). With this macro, DI0-DI5 select one of seven fixed setpoints.
2. Configure the seven setpoint values in P1001-P1007 (e.g., 0, 800, 1500, 2000, 2400, 2700, 2800 RPM).
3. Set P1000 = 3 (fixed setpoint as primary source) and P1003 (additional source) as needed.
4. Wire DI0-DI5 to the operator's selector switch (note: one input combination = 0 RPM / coast to stop).

5.5 V/f Boost Tuning

1. Run the motor unloaded at 5 Hz and observe r0027 (actual current).
2. If current exceeds 30 % of rated (~42 A) and speed is unstable, increase P1310 in 5 % steps.
3. If voltage at low frequency is excessive (motor saturates, current rises with the square of voltage), decrease P1310.
4. Repeat at 10 Hz, 20 Hz, 30 Hz to map the boost curve.

5.6 Slip Compensation Tuning

1. Calculate rated slip: n_s = 120·f/p = 120·100/2 = 3000 RPM. Rated slip = (n_s - n_rated) / n_s = (3000 - 2800) / 3000 = 6.67 %.
2. Set P1335 to a value near 6.67 % initially; fine-tune to minimise speed droop on a step load.

6. V/f Curve Verification

The V/f curve must reflect the 100 Hz base. A linear V/f from 0 to 100 Hz at 400 V produces V/f = 4 V/Hz. This is half the 50 Hz V/f of 8 V/Hz, which is correct for the rewound motor but means the same flux at 100 Hz requires more current per Nm. Verify with the drive running unloaded:

7. Speed-Torque Characteristic In Field Weakening

The shaft torque capability at rated current and different operating points:

T_rated = P / ω = 75,000 W / (2π · n / 60)

For 100 Hz operation at the rewound design point (V/f = 4 V/Hz, half flux):

T_100Hz = 75,000 / (2π · 2800 / 60) = 75,000 / 293.2 ≈ 255.8 Nm (theoretical rated torque at rated current)

For a 50 Hz machine with the same 400 V supply operated at 100 Hz (no rewinding, field-weakened):

T_{100Hz,field-weakened} ≈ T_50Hz,rated · 0.5 = 477 · 0.5 ≈ 238 Nm

8. Parameter Reference Table

9. Fault Code Reference

For the official SINAMICS G120 fault list, see the Siemens Industry Online Support list manual (article ID 109751902 or equivalent for the firmware in use).

10. Verification Checklist

11. Field Notes And Lessons Learned

1. The load-dependent technology functions (P500/P501) are designed for energy-saving applications such as pumps and fans where reducing speed at light load saves energy. They must not be enabled on constant-torque loads such as conveyors, mixers, hoists, and cutting machines.
2. A 50 Hz induction motor operated at 100 Hz on the same 400 V supply has half the available torque (field weakening). The factory rewinding/rewiring is necessary to restore full torque. Always verify the rewound nameplate rating before commissioning.
3. Motor identification (P1900 = 2) must be performed with the motor decoupled from the load. On a meat cutting machine, this means physically disconnecting the motor shaft from the gearbox or cutter head.
4. A V/f drive (P1300 = 0) without encoder cannot regulate speed tightly under load step. If the application demands tight speed regulation (e.g., < 2 % droop on heavy load), consider switching to sensorless vector control (P1300 = 20) once motor data is correct. This typically halves the speed droop on load steps.
5. The 90 kW / 75 kW discrepancy highlights the importance of nameplate verification. Always read the nameplate on the motor as physically installed, not the rating the user reports from memory or documentation.
6. If the drive still cannot reach setpoint after P501 = 0 and motor data correction, check r1538 / r1539 (torque limit) and r0067 / r0068 (current limit). The drive may be in torque limit due to insufficient current headroom; consider a higher-power Power Module.
7. Always keep the parameter backup (BOP-2 dump or TIA Portal upload) of the working configuration. The user later confirmed "It's working now" after re-entering the corrected nameplate data, which validates the parameter path documented here.

12. Related Drive Parameters For Tightening Speed Regulation

If after the parameter fix the speed regulation is still loose (5-10 % droop on heavy load), upgrade the control mode and consider pre-control:

For vector control with encoder feedback (highest performance), use P1300 = 22 and verify the encoder wiring to the CU250S encoder interface.

13. FAQ