Siemens SIRIUS 3RW50 Soft Starter | 143–570 A | Stoklink

About the SIRIUS 3RW50

The SIRIUS 3RW50 represents Siemens' updated standard-tier soft starter platform, purpose-built to handle the demanding starting and stopping requirements of medium-to-large industrial motors. Spanning eight current ratings from 143 A to 570 A, the family addresses motor powers from 75 kW up to 315 kW at 500 V, making it a natural fit for applications where direct-on-line starting is mechanically or electrically unacceptable but variable-frequency drive complexity is unwarranted.

At its core, the 3RW50 delivers a three-phase voltage ramp soft start and soft stop, progressively increasing or decreasing motor voltage to limit inrush current and reduce mechanical shock on couplings, gearboxes, and driven equipment. The built-in bypass contactor — present across the entire current range — eliminates the need for an external bypass contactor in the panel, reducing component count, wiring effort, and enclosure footprint. Once the motor reaches full speed, the bypass contactor closes and the thyristors are taken out of the power circuit, minimising heat generation and improving overall system efficiency.

The adaptive control system differentiates the 3RW50 from simpler ramp-only starters. Rather than applying a fixed voltage profile, the device monitors motor behaviour during starting and continuously adjusts the ramp to match the actual load characteristic — whether that load is a centrifugal pump, a loaded conveyor, or a high-inertia fan. The result is repeatable, optimised starts without trial-and-error parameter tuning on site.

Optional communication modules extend the 3RW50 into networked automation environments, supporting PROFINET, EtherNet/IP, and Modbus TCP. When deeper motor diagnostics and asset management are required, the 3RW50 is compatible with SIMOCODE motor management, enabling integration into condition monitoring and energy management strategies.

Where SIRIUS 3RW50 Fits in the SIRIUS Portfolio

Siemens structures the SIRIUS soft starter range across several tiers to match application complexity and budget:

• 3RW30 — Entry-level, basic voltage ramp, limited current range, no bypass.
• 3RW40 — Standard-tier previous generation, established field-proven platform, widely installed base.
• 3RW50 — Standard new generation (this family): built-in bypass for the full range, adaptive control, optional fieldbus. The direct evolution of the 3RW40 with enhanced functionality and a wider current coverage.
• 3RW52 — Advanced tier with expanded protection functions, integrated energy measurement, and broader communication options as standard.
• 3RW55 — High-feature tier, extended diagnostics, full parameter set, designed for critical process applications and motor management integration at the device level.

The 3RW50 sits at the crossroads of value and capability: it delivers features — particularly the built-in bypass and adaptive ramp — that previously required a higher-tier product, at a price point aligned with standard panel build budgets. For engineers migrating from 3RW40 installations, the 3RW50 offers a direct upgrade path with a broadly compatible footprint and connection arrangement.

Technical Specifications

Family-level specifications applicable across all eight SIRIUS 3RW50 models. Always verify individual model datasheets for precise dimensional and wiring data.

• Current range: 143 A – 570 A (8 ratings: 143, 171, 210, 250, 315, 370, 470, 570 A)
• Motor power range: 75 kW – 315 kW at 500 V AC
• Main circuit voltage: 200 V to 480 V AC, 3-phase
• Mains frequency: 50 / 60 Hz
• Control supply voltage: 110–230 V AC (refer to individual order number suffix for exact control voltage variant)
• Starting method: Three-phase voltage ramp (soft start); three-phase voltage ramp (soft stop)
• Bypass: Built-in bypass contactor, full power range
• Adaptive control: Yes — automatic ramp profile optimisation per load type
• Motor protection: Integrated electronic overload protection (thermistor input supported)
• Communication: Optional via plug-in communication module — PROFINET, EtherNet/IP, Modbus TCP
• Motor management compatibility: SIMOCODE (for advanced applications)
• Degree of protection: IP00 / IP20 (device); panel builder responsible for enclosure IP rating
• Ambient operating temperature: -25 °C to +60 °C (derating may apply above +40 °C depending on installation conditions)
• Standards: IEC 60947-4-2 (AC semiconductor motor controllers and starters)
• Approvals: CE, UL, cUL (verify current approvals on Siemens SIOS documentation portal)
• Mounting: DIN rail or screw mounting; vertical installation standard

Key Features

• Three-phase voltage ramp soft start and soft stop: The 3RW50 controls all three phases simultaneously during acceleration and deceleration, ensuring balanced current limiting and smooth torque development. This eliminates single-phase ramp asymmetry that can cause vibration in larger motors, and the configurable soft stop ramp is especially valuable for pump applications where abrupt stopping would cause water hammer in pipeline systems.
• Built-in bypass contactor — full power range: Unlike many competitors where bypass is optional or limited to smaller ratings, the 3RW50 integrates the bypass contactor across all 8 current ratings up to 570 A. At full speed, the thyristors are bypassed, drastically reducing heat dissipation in the panel. Panel builders save wiring time, enclosure space, and a discrete contactor component cost.
• Integrated electronic motor overload protection: Class 10/20 thermal overload protection is built in, removing the need for a separate overload relay in most installations. The device monitors motor current continuously and trips on sustained overload, protecting the motor winding investment and reducing BOM complexity.
• Adaptive control system — optimised ramp profile per load: The adaptive algorithm monitors motor current feedback during starting and adjusts the voltage ramp in real time to match the actual load torque characteristic. Centrifugal pumps, high-inertia fans, and loaded conveyors each exhibit different torque-speed curves; the 3RW50 adapts automatically, reducing the need for site commissioning adjustments and delivering consistent starts across varying load conditions.
• PROFINET, EtherNet/IP, Modbus TCP via optional communication module: A single plug-in module unlocks integration with standard industrial Ethernet networks. Control word, status word, current, and fault diagnostics become available to the PLC or SCADA system without additional gateways, supporting predictive maintenance and remote monitoring strategies.
• Compatible with SIMOCODE motor management: For applications requiring detailed motor asset management — energy consumption logging, extended protection, load monitoring — the 3RW50 interfaces with SIMOCODE, enabling it to participate in a comprehensive motor management architecture without replacing the soft starter itself.

Selecting Your SIRIUS 3RW50 Model

Correct sizing of a soft starter is based on the motor's full load current (FLC), not directly on kilowatt rating, because voltage, power factor, and efficiency all influence actual current draw. Follow these steps:

Step 1 — Determine motor FLC. Read the full load current from the motor nameplate. If only kW and voltage are known, use the approximation: I = P / (√3 × V × η × PF), where η is motor efficiency and PF is power factor. For standard IE3 motors at 400–500 V, typical FLC values are published in IEC 60034 frame tables.

Step 2 — Apply an application derating factor. For normal duty (Class 10, ≤3 starts per hour, motor at ambient ≤40 °C), select the model whose rated current is equal to or greater than the motor FLC. For heavy duty or high starting frequency applications, apply a derating factor or consult Siemens SIZER software. The 3RW50 range spans 143 A to 570 A across eight models, providing close current steps to minimise oversizing.

Step 3 — Verify supply voltage. All 3RW50 models in the standard lineup are rated for main circuit voltages of 200 to 480 V AC. Confirm your nominal system voltage falls within this window with adequate margin for voltage tolerance (typically ±10%).

Step 4 — Select the order number. The 3RW50 lineup offers ratings at 143, 171, 210, 250, 315, 370, 470, and 570 A. Choose the smallest rating that meets or exceeds the motor FLC under your duty conditions. The 75 kW / 143 A model is the entry point; the 315 kW / 570 A model addresses the top of the family range.

Step 5 — Specify communication and accessories. If fieldbus connectivity is required, specify the appropriate communication module (PROFINET, EtherNet/IP, or Modbus TCP) at time of order. Consult the Siemens SIRIUS 3RW50 system manual for accessory compatibility by frame size.

Use the comparison table below to cross-reference model order numbers, current ratings, and power equivalents at 500 V before placing your order.