Problem Overview
The SIRIUS 3RW44 high-end soft starter family from Siemens is engineered for soft starting and soft stopping of three-phase induction motors up to 800 A (IN line-side, IEC). Units in the 3RW4425 size class (frame size FS IV, 250 A nominal) are commonly applied to fans, pumps, compressors, and conveyors where reduced inrush and controlled torque ramps are required.
On fan applications with constant mechanical load, the field report is reproducible: during continuous run, the operator panel or SIRIUS Soft Starter ES (TIA Portal) reports a bypass part fault (German: Bypass-Teil-Störung, frequently logged as fault code F03 / F04 / F08 depending on firmware revision). The fault typically latches the unit, dropping the motor offline and halting the process.
This article documents the root causes specific to 3RW44 25-1BC34 configurations on fan drives and walks through the parameter remediation, hardware verification, and commissioning checks that restore reliable operation.
3RW44 Hardware Reference
The 3RW44 is the flagship of the SIRIUS 3RW soft starter portfolio. The -1BC34 variant is a 3-phase controlled, fully digital unit with the following baseline characteristics:
| Parameter | Value (3RW4425-1BC34) |
|---|---|
| Frame size | FS IV |
| Rated operational current Ie (AC-53a) | 250 A |
| Operational voltage Ue | 200–600 V AC |
| Control supply Us | 110–230 V AC/DC (electronics); 200–460 V AC (control) |
| Motor power at 400 V | 132 kW (inline) / 250 kW (inside-delta) |
| Integrated bypass contacts | Yes (thyristor bypass on the main power section) |
| Parameter interface | RS-232 (service cable 3UF7940-0AA00-0), optional PROFIBUS / PROFINET (3RW4900-0PB00 / 3RW4900-0PN00) |
| Programming software | SIRIUS Soft Starter ES (TIA Portal) / legacy WinSoftStarter |
The 3RW44 hardware architecture uses two anti-parallel thyristor pairs per phase (one pair for run-up, one pair that is mechanically replaced by integrated bypass contacts once the motor reaches run state). The bypass contactor is closed by the unit's internal logic after the run-up time elapses and a confirmation of voltage stability is received.
Reference: SIRIUS 3RW5 / 3RW4 manual set (PDF).
Bypass System Architecture
During soft start, phase current flows through the power section (six SCRs in a 3-phase bridge). After the run-up ramp completes, the integrated bypass contact system closes, shorting the SCRs and carrying the running current through mechanical contacts. This reduces heat dissipation inside the enclosure (P = 3 · I2 · RSCR(on) drops to near zero) and extends SCR life.
The 3RW44 monitors the bypass section through:
- Bypass contactor coil voltage feedback (U at the contactor coil)
- Bypass contact status (auxiliary NO contact) feedback
- Phase current symmetry check during the transition window
- Thermal model of the bypass contacts (separate from the thyristor thermal model)
Any of the following triggers a bypass part fault:
- Coil voltage absent when commanded closed (contactor fails to pick up).
- Auxiliary contact fails to confirm closure within the configured bypass closing time (default 100 ms).
- Current imbalance > programmed threshold during transition.
- Thermal model of the bypass contacts exceeds the limit (overtemperature of the contact block).
- Phase loss detected post-bypass (one phase dropped, the other two carry the load → imbalance trip).
Root Cause Analysis
For fan applications, the dominant root causes observed in the field are listed in approximate frequency order:
| # | Root cause | Mechanism | Indicator |
|---|---|---|---|
| 1 | Insufficient start ramp / over-current trip carried into bypass | Excessive starting current on constant load exceeds 3RW44 programmed Ilimit; transition into bypass inherits the over-current and trips the bypass thermal model | Fault during first 30 s of run-up |
| 2 | Voltage ramp / torque control mis-tuned | Run-up time too short, current peak clipped to Ilimit, transition unstable | Fault at exact end of run-up |
| 3 | Bypass contact wear / welding | Excessive switching operations or DC component from half-wave starting | Fault after 2–3 years of service; contact resistance test fails |
| 4 | Control voltage drop | 110–230 V AC/DC at terminals A1/A2 sags below 0.85 · Us during motor starting (line dip), bypass coil does not hold | Fault correlated with upstream transformer / large load starts |
| 5 | Loose control wiring to bypass contactor coil | High vibration on fan skid; terminal screws on 3RW44 bypass terminal block (X21) loosen | Intermittent fault, disappears after retorque |
| 6 | Fan load torque characteristic mis-modeled | Centrifugal fan is a quadratic-torque load (T ∝ n2); 3RW44 is set to constant-torque profile by default | High starting current, slow ramp, Ilimit reached |
| 7 | Phase sequence or phase loss post-start | One supply fuse deteriorated; bypass closes on two phases only → overload of the remaining phases | Fault at random, motor continues to run on 2 phases if no overload |
Diagnostic Procedure
- Read the fault buffer. Connect SIRIUS Soft Starter ES (TIA Portal) over RS-232 using cable 3UF7940-0AA00-0. Read the last 8 fault entries including timestamp, fault code, current at trip (A), voltage (V), and thermal state (%).
- Decode the fault. Cross-reference the fault code with the 3RW44 manual's fault table. Codes prefixed F03 and F04 indicate bypass-coil / contactor issues; F08 indicates thermal overload of the bypass section.
- Capture the run-up trace. Use the Soft Starter ES oscillograph feature to record U, I, and P across the run-up window. Verify that Ipeak ≤ programmed Ilimit and that the transition to bypass is monotonic (no current spike at tbypass close).
- Measure control voltage at A1/A2 with a true-RMS DMM during a real start. Voltage must remain ≥ 0.85 · Us throughout the ramp.
- Check thermal state. If the bypass thermal state shows > 90 % at trip, the issue is repeated over-current not a contactor problem.
- Inspect the bypass contactor resistance (de-energised). L1-L1', L2-L2', L3-L3' on the load side should measure < 5 mΩ per pole (cold). Values > 30 mΩ indicate contact erosion.
- Retorque control terminals on the front of the 3RW44 (X21 block) to 2.5 Nm. Vibration on fan skids is the most common single cause of intermittent bypass faults.
Parameter Tuning for Fan Applications
Centrifugal fans follow the quadratic-torque law T ∝ n2. The 3RW44 should be set to a voltage ramp or torque control starting mode with parameters sized for the fan curve, not the default constant-voltage profile.
| Parameter | Default (constant-torque) | Recommended for fan | Notes |
|---|---|---|---|
| Starting mode | Voltage ramp (U) | Torque control (T) or voltage ramp + current limiting | Torque control matches the quadratic curve; reduces peak current |
| Run-up time tR | 10 s | 15–25 s | Lengthen to let the fan accelerate smoothly; reduces Ipeak |
| Starting voltage Ustart | 30 % | 40–50 % | Higher initial voltage to overcome bearing stiction and bearing preload |
| Current limit Ilimit | 4 · Ie | 3.0–3.5 · Ie | For 250 A unit on a 132 kW fan, set 750–875 A |
| Bypass closing delay | 100 ms | 200–300 ms | Allows the thermal model to stabilise before contact closure |
| Soft stop time toff | 0 s | 10–20 s | Prevents water-hammer on ducted systems; reduces reverse rotation in axial fans |
| Breakaway pulse | Off | On, 80 % U, 200 ms | Releases stuck impeller after a long shutdown period |
SIRIUS Soft Starter ES Parameterization
Parameter changes can be applied via the operator panel on the front of the 3RW44, via the integrated Soft Starter ES in SIRIUS Soft Starter ES (TIA Portal) V19 SP1 or later, or via the legacy WinSoftStarter. The PC connection is the most reliable method.
- Install SIRIUS Soft Starter ES (TIA Portal). The COM port on the 3RW44 is the lower 9-pin SUB-D on the front; set DIP switch S1 to RS-232.
- Connect 3UF7940-0AA00-0 cable between the 3RW44 and the PC.
- Open the project, go online. The unit identifies as 3RW44 25-1BC34.
- Navigate to Parameters → Motor → Starting. Enter the values from the table above.
- Navigate to Parameters → Bypass. Set Closing delay and Open delay to 200 ms. Enable Auto-bypass.
- Navigate to Parameters → Protection → Bypass thermal model. Verify the model class matches AC-53a (in-line) or AC-53b (inside-delta) for your wiring. For -1BC34 wired in-line to a 132 kW motor, AC-53a is correct.
- Save to the unit and to the project. Cycle control power to commit.
Hardware Verification
After parameter updates, perform the following checks before returning the drive to service:
- Insulation test (megger): 500 V DC between phases and ground, > 100 MΩ.
- Control wiring retorque: 2.5 Nm on X21, X22, and PE terminals.
- Bypass contactor aux contact: NO contact closes within 50–80 ms of coil energisation; NC contact opens within the same window. Use a continuity tester on terminals 13/14 and 21/22 of the integrated aux block.
- Voltage at A1/A2 under load: must remain ≥ 0.85 · Us through motor start. If not, install a control-voltage buffer relay or a separate 24 V DC supply for the electronics.
- Thermal baseline: with the unit de-energised, the heat sink temperature should be within 5 °C of ambient. Persistent ΔT > 15 °C indicates blocked cooling fins or undersized enclosure ventilation.
Test Run Procedure
- Disable the upstream contactor (L1/L2/L3 off) to motor, software-bypass active only.
- Command a start from the PLC. Monitor I, U, cosφ, and P on the Soft Starter ES oscillograph for the full 30 s window.
- Verify the bypass contactor closes at the programmed delay. The Bypass closed flag in the diagnostics register (bit 5 of byte 0 in the cyclic process image) should assert within ±50 ms of the expected time.
- Verify that the heat sink temperature does not rise more than 8 °C during the run-up. Higher delta indicates SCRs are still conducting appreciable current after the bypass closes — check contact wear.
- Run for 30 minutes under load, then issue a soft stop. Confirm the soft stop ramp reaches 0 V within the programmed toff + 10 %.
- Reset the fault buffer and log the values for the commissioning record.
Field-Proven Fault Matrix
| Symptom | Likely cause | Action |
|---|---|---|
| Bypass fault at t = 0 s | Control voltage missing on bypass coil | Check A1/A2 wiring; measure voltage under start |
| Bypass fault at t = ramp end | Current limit set too low; motor stalls at end of ramp | Raise Ilimit to 3.5 · Ie or extend tR |
| Bypass fault after 10–30 min run | Bypass contact thermal overload | Check contact resistance; replace contactor module if > 30 mΩ |
| Intermittent bypass fault, no pattern | Loose terminal on X21 | Retorque 2.5 Nm; apply vibration-rated wire ferrules |
| Bypass fault only when upstream load starts | Line dip → control voltage drop → coil release | Add 24 V DC UPS to A1/A2; decouple from same feeder as large loads |
| Bypass fault after firmware update | Parameter set wiped to factory defaults | Reload project; re-tune for fan curve |
Prevention and Long-Term Reliability
- Apply a quarterly thermal-image scan on the heat sink and bypass contactor block. ΔT > 10 °C vs adjacent phases signals an imbalanced or high-resistance contact.
- Log the number of starts counter (register 0x0110 in the diagnostics map). Soft starter bypass contacts are rated for ~50 000 operations under normal load; replace at 80 % of this count for critical fans.
- Use vibration-rated control wiring (stranded, ferrule-terminated, support every 150 mm) on fan skids.
- Keep the SIRIUS Soft Starter ES project file under version control so that any field parameter drift can be detected against the commissioned baseline.
- Replace the 3RW44 cooling fan (where fitted, FS IV) every 5 years or 30 000 hours, whichever comes first. Cooling-fan failure on a 250 A unit during run can degrade the bypass contact block to thermal limit within 20 minutes.
When to Escalate to Siemens
Escalate to [email protected] with the full Soft Starter ES export (parameter set, fault buffer, oscillograph traces) and the motor nameplate data (kW, FLA, LRA, cosφ, efficiency class) when:
- The bypass fault reoccurs within 24 h of a clean parameter reset.
- Contact resistance measurements show > 50 mΩ on any pole after a single start attempt.
- Multiple 3RW44 units on the same bus report simultaneous bypass faults — indicates a power-quality issue upstream.
- Inside-delta wiring is in use and the fault appears only in that configuration — requires SIRIUS Soft Starter ES V18+ to access inside-delta protection parameters.
Related Siemens Soft Starter References
For new installations or migrations, the current-generation 3RW50 and 3RW55 soft starters provide a parameter structure very similar to the 3RW44, with native TIA Portal integration. The 3RW50 27-1BB14 (32 A, 15 kW) is the entry-level SIRIUS unit commonly used on smaller fans. Hardware manuals and firmware notes for the 3RW5 family are available at the official support portal referenced above.
What does a bypass part fault mean on a SIRIUS 3RW44?
It means the integrated bypass contactor section has either failed to close when commanded, failed to confirm closure through the auxiliary contact within the configured time (default 100–200 ms), or exceeded its thermal model limit. Common field causes are loose control wiring, undersized control supply, and contactor wear from repeated over-current starts.
How do I tune a 3RW44 for a centrifugal fan?
Use torque-control or voltage-ramp mode, set run-up time to 15–25 s, starting voltage to 40–50 %, and current limit to 3.0–3.5 × motor FLA. Extend the bypass closing delay to 200–300 ms and enable a 10–20 s soft stop to prevent reverse rotation of axial fans.
Which software do I use to parameterise the 3RW44?
SIRIUS Soft Starter ES (TIA Portal) V19 SP1 or later for units on firmware V2.4+. Older units use the legacy WinSoftStarter. Connection is via the front RS-232 port using cable 3UF7940-0AA00-0.
What is the correct current limit for a 132 kW fan motor on a 3RW4425-1BC34?
For a 132 kW, 400 V, 50 Hz motor with FLA ~ 250 A, set Ilimit in the range 750–875 A (3.0–3.5 × FLA). Do not set below 4 × FLA without first confirming the motor LRA from the nameplate, or the 3RW44 will report a run-up timeout.
How do I contact Siemens for 3RW44 technical assistance?
Email [email protected] with the Soft Starter ES export of the parameter set, the last 8 fault-buffer entries, and the motor nameplate data. For warranty replacement of the bypass contactor block, include the serial number and MLFB (3RW4425-1BC34) engraved on the unit's nameplate.