3RW4055-6BB44 Soft starter SIRIUS 3RW40 134A 400V

Siemens SIRIUS 3RW40 Soft Starters — The Standard for Protection with Symmetrical Polarity Balancing

The Siemens SIRIUS 3RW40 soft starter series is designed for three-phase induction motors from 5.5 to 250 kW. As a high-performance standard class starter, it combines integrated bypass contacts with full electronic motor protection. A standout feature of the 3RW40 is Siemens’ patented Polarity Balancing algorithm for two-phase thyristor control. This technology eliminates DC components in the motor phases during startup, significantly reducing vibration, noise, and motor thermal stress.

Typical Applications for Siemens 3RW40

• Inline and Deep Borehole Pumps: Provides an adjustable soft stopping ramp to control water deceleration, completely eliminating pipeline water hammer.
• Rotary Screw Air Compressors: Current limiting control minimizes line voltage drops and protects internal drive seals.
• Medium-Duty Industrial Conveyors: Delivers smooth torque buildup, eliminating gearbox backlash and belt slipping.

What Differentiates Siemens 3RW40 from Competitors

SIRIUS Polarity Balancing makes two-phase starting as symmetrical and balanced as premium three-phase systems, reducing mechanical wear on the rotor.

When Siemens 3RW40 is NOT Suitable

• For severe industrial heavy-starting cycles (e.g., loaded ball mills or heavy crushers) — upgrade to the premium three-phase Siemens 3RW44 range.
• If your process demands motor pre-heating via continuous DC injection to prevent condensation — choose the ABB PSTX.

Our System Commissioning Observations

The SIRIUS 3RW40 is our top-selling soft starter for mid-power wastewater and HVAC systems. It features clean diagnostic LEDs indicating trip faults (device overload, phase imbalance, or motor overheating). Be sure to configure the overload trip class (Class 10 vs 20) on the front dial depending on startup load severity.