Troubleshooting CU250S-2 Vdc-Max Controller in Hoist Lowering

Troubleshooting Siemens SINAMICS G120 CU250S-2 Vector Control: Vdc-Max Controller Interaction in Vertical Lowering Applications

1. Problem Summary

2. How the Vdc-Max Controller Works on the G120 / CU250S-2

The Vdc-max controller (also called the DC-link voltage controller) is a built-in firmware function on the SINAMICS G120 / CU250S-2 that prevents the DC-link voltage from exceeding the hardware over-voltage threshold during regenerative operation. It does this by dynamically adjusting the torque or speed reference to limit the rate at which mechanical energy is converted into electrical energy and returned to the DC link.

Two variants exist:

• Vdc-max controller (p1240): Active during regenerative operation (lowering, fast deceleration). It intervenes to limit the speed or torque so that the DC-link voltage does not exceed the activation threshold.
• Vdc-min controller (p1230): Active during motoring operation when the line supply fails or is weak; it injects additional torque to keep the DC link charged. This controller is not the issue in this case.

In Vector Control, when the Vdc-max controller activates it normally reduces the output torque to absorb less regenerative energy. The drive will tolerate a brief DC-link rise but will not allow sustained regeneration beyond the resistor's dissipation capacity. This behaviour is documented in the SINAMICS G120 / G120C / G120D with CU250S-2 Control Units operating instructions (Section on "Vector control / Vdc control").

In a hoist application the lowering phase is fundamentally regenerative: gravity drives the motor above synchronous speed, and the motor becomes a generator. The energy flow is:

Gravitational PE  →  Motor (over-synchronous)  →  DC link  →  Braking resistor (P dissipation)

If the regenerative power exceeds the resistor capability or if the Vdc-max controller's intervention level is too aggressive, the drive cannot absorb the energy at the commanded lowering speed. The motor then either accelerates above the speed setpoint (load pulls through) or the firmware clamps the torque and the speed profile becomes non-linear.

3. Why the Fault Appears Only Above 1 m/s

Lowering speed and regenerative power scale linearly with mechanical speed at fixed load. With:

• Effective load radius (chain sheave + reeving) → motor speed n
• Gravitational force F = m · g ≈ 7 500 kg · 9.81 m/s² ≈ 73.6 kN
• Linear speed v = π · D · n (for a single-part reeving, where D is the sheave diameter)

The regenerative power returned to the DC link is approximately:

P_reg  ≈  F · v  =  73 600 N · v (m/s)  [W]

Above ~1 m/s the DC-link voltage during lowering rises faster than the braking resistor can dissipate it, and the Vdc-max controller is forced to intervene. From the SINA_POS side the drive still reports actual speed = setpoint speed at the regulator level (because the speed controller compensates), but the position integrator sees a steady-state error that builds during the deceleration ramp. When the controller finally commands stop at the absolute target, the load has accumulated extra kinetic + potential displacement, and the brake is asked to absorb everything at once. The result is the observed >1 m overshoot and an overspeed fault.

4. Diagnostic Procedure

The following procedure should be executed on the commissioning tool (STARTER or Startdrive) connected to the CU250S-2 via PROFINET or PROFIBUS.

4.1 Identify whether the Vdc-max controller is active

1. Open the drive online.
2. Navigate to the expert parameter list.
3. Read r1242[0] – Vdc-max controller status word. Bit 0 = controller active, bit 1 = controller reached intervention threshold.
4. Compare with the trace during a 1.25 m/s lowering cycle. If r1242 toggles between inactive and active during deceleration, the Vdc-max controller is interfering with the SINA_POS profile.

4.2 Read regenerative and motoring power

1. Open the function generator / trace tool.
2. Record the following signals during a full lowering cycle:
   • r0079 – torque setpoint (Nm)
   • r0080 – actual torque (Nm)
   • r0031 – actual torque smoothed
   • r0082 – actual active power (kW). Negative = regenerative.
   • r0026 – actual DC-link voltage (V)
   • r0063 – actual speed smoothed (rpm)
   • r0021 – actual speed smoothed (rpm, filtered)
3. Set the trigger on r0026 > 760 V (for 400 V class devices; for 690 V class use 1100 V) to capture the exact instant the Vdc-max controller begins to clamp.

4.3 Check the braking resistor sizing

1. Read p0219 – braking resistor power rating.
2. Read p0220 – braking resistor continuous power.
3. Read p0221 – braking resistor peak power.
4. Compute peak regenerative demand at 1.25 m/s with full load (≈ 92 kW per Section 3).
5. If peak demand > p0221, the resistor is undersized for the new operating point and the Vdc-max controller will be forced to intervene to protect the IGBT chopper.

4.4 Verify the regenerative power limit (p1531)

Parameter p1531[0] limits the regenerative power in vector control. The community hint in the field report pointed to "p1531 not enough." Validate:

1. Read p1531[0] – current setting.
2. Compare to the actual regenerative power observed in r0082 during the failing cycle.
3. If r0082 (negative peak) exceeds the value of p1531, the firmware clamps the torque and the speed profile no longer matches SINA_POS.

4.5 Confirm SINA_POS configuration

1. In the TIA Portal project, open the SINA_POS instance DB.
2. Verify:
   • Mode = 2 (absolute positioning).
   • Position = absolute target in LU.
   • Velocity = scaled setpoint in LU/min (1.25 m/s ≈ 480 LU/min per the source).
   • Jerk / acceleration – the ramp time should be ≥ 1.0 s; too aggressive a ramp with an over-driven Vdc-max controller makes the overshoot worse.
3. Confirm that the scaling matches: 1 m/s → 400 LU/min per the source means the LU-to-metre conversion is 2 400 LU/m (or equivalent on the user-specific axis scaling).

5. Resolution Path

Two paths resolve the issue; the user's site chose option A because no hardware change was possible.

5.1 Option A – Disable the Vdc-max controller (workaround)

1. In STARTER / Startdrive, navigate to p1240[0].
2. Set p1240[0] = 0 (Vdc-max controller disabled).
3. Save the project to the CF card / non-volatile memory (RAM-to-ROM).
4. Perform a controlled lowering cycle at 1.25 m/s and confirm the drive reaches the absolute target without overshoot.

5.2 Option B – Properly size the regenerative path (recommended)

1. Verify the braking resistor continuous rating against the worst-case duty cycle (1.25 m/s lowering × typical cycle time). A rule of thumb is to size the resistor so that:

   P_resistor_continuous  ≥  P_reg  ×  duty_factor

   For an axis that lowers for 60 s out of a 180 s cycle, duty_factor ≈ 0.33, so the resistor must dissipate ≥ 30 kW continuously with ≥ 92 kW peak.
2. If the existing resistor is undersized, install a larger one and update p0219, p0220, p0221 accordingly.
3. Keep the Vdc-max controller enabled with a slightly elevated intervention level if available (firmware-dependent). On CU250S-2 with V4.7 SP3 and later firmware the dynamic response of p1240 is improved; consider an upgrade if the unit is on older firmware.
4. If hardware is fixed, raise p1531[0] so that the regenerative power limit does not activate during normal lowering (e.g., p1531 = 1.5 × rated motor power).

6. Parameter Reference Table

7. Verification Steps

After applying the change, perform the following:

1. Static test. Disable the drive output, then enable via SINA_POS in mode 2 (absolute positioning). Confirm the drive accepts the new speed setpoint and reports zero following error at standstill.
2. No-load lowering at 1.25 m/s. Verify that the drive reaches a configured mid-travel position (e.g., 10 m) and stops cleanly. Record r0063, r0080, r0082, r0026 in a trace.
3. Full-load lowering at 1.25 m/s. Repeat step 2 with the rated 7.5 t load. Confirm the drive reaches the target within the SINA_POS positioning window (typically ±2 LU).
4. Full-load lifting at 1.25 m/s. Asymmetry between lifting and lowering is informative. If lifting works but lowering does not, the regenerative path is the bottleneck – confirms the Vdc-max diagnosis.
5. Overspeed fault check. Monitor r0791 during the cycle. Confirm no F07901 (overspeed) and no F30002 (DC-link overvoltage) appears.
6. Brake test. With the motor at standstill, command the holding brake via p1215 and confirm the brake holds the static load with no drift.

8. Why This Fault Mode Is Specific to Hoist / Vertical Axes

On a horizontal axis, regenerative energy only appears during fast deceleration and is transient. The Vdc-max controller can briefly clamp the torque and the position controller naturally corrects because the speed setpoint decays toward zero. The fault is rarely observed.

On a vertical axis, regenerative energy is continuous during the entire lowering phase, not just at deceleration. The Vdc-max controller is therefore active throughout the move, not just at the end. When SINA_POS then commands a precise stop at an absolute position, the following sequence unfolds:

1. SINA_POS commands deceleration from 1.25 m/s to 0 m/s.
2. The drive starts to ramp the speed down.
3. Regenerative power remains high because gravity is still doing work.
4. Vdc-max controller intervenes and reduces the available braking torque.
5. The actual deceleration is shallower than the commanded profile.
6. Position error accumulates; the drive passes the absolute target.
7. The holding brake is engaged, but the load's kinetic energy is higher than the brake can absorb in a single engagement – the load drifts past the target by an amount proportional to v² and to the excess deceleration deficit.
8. If the overshoot is large enough that the actual speed exceeds the configured limit (p2162 / p2163 threshold), the drive trips with F07901 overspeed.

This explains why the fault only appears above a threshold speed: below ~1 m/s the regenerative power stays inside the resistor's continuous dissipation range and the Vdc-max controller never intervenes. Above ~1 m/s, the Vdc-max controller becomes the limiting factor, not the resistor itself.

9. Related CU250S-2 Behaviour (Overload and Brake Control)

The CU250S-2 documentation describes how the drive reacts to overload conditions. In vector control, the drive reduces the output current; in U/f control, the drive reduces the speed. Once the overload condition has cleared, the drive returns to its setpoint. This is a separate mechanism from the Vdc-max controller but is governed by related parameters (p0640, p0290, p0292). See the operating instructions: SINAMICS G120 with CU250S-2 Control Units – Operating Instructions.

For hoist applications the combined motor holding brake control (p1215 through p1227) and the Vdc-max behaviour must be reviewed together. The extended brake control functions (p0505, p1215 advanced configuration) are designed to coordinate brake engagement with the DC-link energy state.

10. Fault Code Reference

11. Best-Practice Checklist for Vertical-Axis G120 / CU250S-2 Sizing

1. Compute regenerative power at the maximum required lowering speed with the rated payload before selecting the inverter and braking resistor.
2. Size the braking resistor for both continuous dissipation (worst-case duty cycle) and peak power (worst-case single event).
3. Configure the Vdc-max controller with a known intervention threshold and document it in the project functional specification.
4. When using SINA_POS over Vector Control on a vertical axis, perform commissioning at multiple lowering speeds (0.25, 0.5, 1.0, 1.25 m/s) and verify positioning accuracy at each step.
5. Record traces of r0026, r0063, r0079, r0080, r0082, r1242 during commissioning and store with the project archive.
6. Review the motor holding brake timing (p1216, p1217) against the load's worst-case free-fall energy – a brake that closes in 200 ms cannot absorb the kinetic energy of 7.5 t at 1.25 m/s.
7. Consider adding an external speed monitor / overspeed protection relay as a hardware safety layer independent of the Vdc-max controller.
8. Document the firmware version (r0018 / r0965) and keep a copy of the drive parameter set in source-controlled storage.

12. FAQ