Siemens SIRIUS Soft Starter Series | 1.5–315 kW | Stoklink

What Is a Soft Starter?

A soft starter is an AC semiconductor device that controls the voltage applied to a three-phase induction motor during start-up and, where required, during stopping. By ramping voltage up progressively rather than applying full mains voltage instantaneously, a soft starter limits mechanical shock to driven equipment, reduces electrical inrush current to the supply network, and extends the service life of motors, couplings, and driven machinery such as pumps, compressors, and conveyors.

Unlike a variable frequency drive (VFD), a soft starter does not regulate speed during continuous running. Once the motor reaches full speed, the internal bypass contacts close and the semiconductors are taken out of circuit, resulting in very low losses during steady-state operation. This makes soft starters the preferred solution when controlled starting and stopping is required but variable-speed regulation is not.

Soft starters comply with IEC 60947-4-2, the international standard for low-voltage switchgear and controlgear — specifically AC semiconductor motor controllers and starters — which defines performance, testing, and marking requirements.

The SIRIUS Soft Starter Family

Siemens manufactures the SIRIUS soft starter portfolio at its German production facilities. The range is structured in five product families, each addressing a distinct tier of application complexity, current rating, and feature requirement:

• 3RW30 — Compact Entry-Level: 13 models covering 3.6–106 A (1.5–55 kW at 500 V). Designed for space-constrained control panels and straightforward applications where basic soft-start functionality is the priority.
• 3RW40 — Standard: 9 models covering 12.5–106 A (5.5–55 kW at 500 V). Adds built-in bypass relays, improved motor protection functions, and optional Modbus RTU communication for broader industrial use.
• 3RW50 — Standard New Generation: 8 models covering 143–570 A (75–315 kW at 500 V). The current-generation standard platform for medium-to-large motors, supporting fieldbus connectivity via accessory modules including PROFINET, EtherNet/IP, and Modbus TCP.
• 3RW52 — Advanced: 18 models covering 13–570 A (5.5–315 kW at 500 V). Full-range advanced platform with integrated Modbus RTU as standard, expanded motor protection, and comprehensive fieldbus options. Compatible with SIMOCODE pro motor management systems.
• 3RW55 — High-Feature Top Tier: 18 models covering 13–570 A (5.5–315 kW at 500 V). The highest-specification family, adding PROFIBUS DP support alongside PROFINET, EtherNet/IP, and Modbus TCP, plus the broadest set of protection and diagnostic functions in the SIRIUS range.

Across all five families, Siemens maintains a consistent control supply voltage of 110–250 V AC (universal), simplifying panel design when mixing families within a single installation. All families are engineered for three-phase AC induction motor applications and comply with IEC 60947-4-2.

Engineering and commissioning is supported by Soft Starter ES software, which integrates with Siemens TIA Portal for parameterisation, diagnostics, and project documentation. On the 3RW52 and 3RW55, integration with SIMOCODE pro extends motor management capabilities to include comprehensive asset monitoring, load-feeder protection coordination, and plant-level reporting.

Choose Your SIRIUS Family

Selecting the correct SIRIUS family begins with three questions: What is the motor's full-load current? What level of motor protection and diagnostics does the application require? Does the installation need fieldbus communication to a PLC or DCS?

3RW30 — When to Specify

Specify the 3RW30 when the motor is 55 kW or smaller at 500 V, the control panel is space-constrained, and the application requires straightforward soft starting without integrated bypass relays. The 3RW30 covers 3.6–106 A across 13 models and operates on a 200–480 V AC supply voltage range. Note that the 3RW30 does not include a full built-in bypass relay in the same manner as higher families; bypass functionality is handled via dedicated bypass relays for inrush reduction. Communication and advanced protection functions are not available on this family.

3RW40 — When to Specify

Specify the 3RW40 for standard industrial applications up to 55 kW at 500 V where built-in bypass, improved motor protection, and optional Modbus RTU communication add operational value over the entry-level 3RW30. The 3RW40 covers 12.5–106 A across 9 models, operates on 200–480 V AC, and suits applications where a degree of network integration is desirable but full fieldbus connectivity is not mandated.

3RW50 — When to Specify

Specify the 3RW50 when the motor is between 75 kW and 315 kW at 500 V and a current-generation standard platform is required. The 3RW50 covers 143–570 A across 8 models and supports 200–600 V AC supply voltage. Fieldbus connectivity — PROFINET, EtherNet/IP, and Modbus TCP — is available via accessory communication modules, making this family suitable for modern networked machine or plant architectures without the overhead of the advanced protection suite found on the 3RW52.

3RW52 — When to Specify

Specify the 3RW52 when the full current range (13–570 A, 5.5–315 kW at 500 V) combined with advanced motor protection, standard Modbus RTU, and SIMOCODE pro compatibility is required. The 3RW52 is the preferred choice for demanding loads where protection functions such as thermistor motor protection, electronic overload protection, and phase-failure monitoring need to be configured and monitored from a plant control system. Fieldbus expansion modules support PROFINET, EtherNet/IP, and Modbus TCP.

3RW55 — When to Specify

Specify the 3RW55 for critical processes and high-feature requirements across the full 13–570 A range (5.5–315 kW at 500 V). The 3RW55 adds PROFIBUS DP connectivity — in addition to PROFINET, EtherNet/IP, and Modbus TCP — making it the correct choice for installations built around PROFIBUS infrastructure or requiring the broadest diagnostic and protection feature set in the SIRIUS portfolio. Like the 3RW52, it integrates with SIMOCODE pro and Soft Starter ES within TIA Portal.

Typical Applications

SIRIUS soft starters are deployed across a wide range of industrial sectors. The correct family selection is driven not only by motor size but also by the operational demands of the driven load and the level of process integration required.

Small Pumps, Fans, and Conveyor Drives (3RW30)

The 3RW30 is well suited to small centrifugal pumps, axial fans, and light conveyor systems in building services, water treatment, and food processing where motor ratings are modest and panel space is at a premium. The primary requirement is reducing mechanical shock and limiting supply current peaks at start-up without the overhead of advanced protection electronics.

Standard Industrial Loads (3RW40 and 3RW50)

The 3RW40 addresses standard industrial applications including compressors, mixers, grinders, and medium-duty conveyor drives where built-in bypass and basic motor protection deliver reliable operation. The 3RW50 extends coverage to larger motors in the same application categories — pumping stations, HVAC air-handling units, and industrial fans — where the higher current range (143–570 A) and modern fieldbus connectivity via accessory modules align with current plant automation standards.

Demanding Loads Requiring Advanced Protection (3RW52)

The 3RW52 is specified for applications where motor protection integrity and network integration are both critical: large centrifugal pumps in water and wastewater infrastructure, compressors in oil and gas facilities, and heavy conveyors in mining and aggregate handling. Integrated Modbus RTU and fieldbus expansion support real-time monitoring and fault diagnostics from the plant SCADA or PLC.

Critical Process Applications (3RW55)

The 3RW55 is the appropriate choice for critical process applications in chemicals, power generation, and large-scale manufacturing where PROFIBUS DP infrastructure is in place or where the highest level of diagnostics, protection coordination with SIMOCODE pro, and TIA Portal integration are required to minimise unplanned downtime and support predictive maintenance strategies.

SIRIUS Common Features

While each SIRIUS family offers a distinct feature tier, several capabilities are consistent across the portfolio and represent the foundation of the SIRIUS soft starter design.

Universal Control Supply Voltage

All principal SIRIUS families accept a 110–250 V AC universal control supply voltage. This eliminates the need for separate control transformers when mixing soft starters of different sizes in a single panel and simplifies spares management across multi-site installations.

IEC 60947-4-2 Compliance

Every SIRIUS soft starter family is designed and tested in accordance with IEC 60947-4-2, the international standard governing AC semiconductor motor controllers and starters. This ensures consistent performance, safety, and interoperability with other IEC-compliant switchgear and controlgear in the motor control centre.

Three-Phase AC Induction Motor Application

SIRIUS soft starters are engineered specifically for three-phase AC induction motor soft starting and stopping. The controlled voltage ramp-up at start limits mechanical stress on couplings and driven equipment while reducing peak inrush current demand on the supply network.

Built-In Bypass Contacts

From the 3RW40 upward, all families incorporate built-in bypass contacts that short-circuit the semiconductor elements once the motor reaches full speed, eliminating semiconductor losses during continuous running and improving thermal performance of the overall installation.

Motor Protection Functions

The scope of motor protection scales with family tier. Advanced families (3RW52, 3RW55) offer electronic overload protection, thermistor motor protection, and phase-failure monitoring as standard. These functions reduce dependence on external protection relays and simplify panel architecture.

Communication Scalability

Communication capability scales progressively: optional Modbus RTU on the 3RW40; PROFINET, EtherNet/IP, and Modbus TCP via accessory module on the 3RW50, 3RW52, and 3RW55; Modbus RTU as standard on the 3RW52 and 3RW55; and PROFIBUS DP exclusively on the 3RW55. This tiered approach allows engineers to match communication investment to actual integration requirements.

TIA Portal and SIMOCODE pro Integration

Soft Starter ES engineering software integrates with Siemens TIA Portal, enabling parameterisation, online diagnostics, and project documentation within a unified engineering environment. On the 3RW52 and 3RW55, integration with SIMOCODE pro motor management extends protection coordination and asset monitoring capabilities.

How to Size a SIRIUS Soft Starter

Correct soft starter selection requires matching the device's current rating to the motor's full-load current, choosing the appropriate family for the required feature level, and verifying suitability for the installation's duty cycle and ambient conditions. Follow these steps:

Step 1 — Read the Motor Nameplate Full-Load Current

Locate the full-load current (FLC) value in amperes from the motor nameplate. This is the primary sizing parameter. Do not size on motor kilowatt rating alone, as FLC varies with supply voltage, power factor, and efficiency class. Always use the FLC corresponding to the actual installation supply voltage (e.g., the 400 V or 500 V column on the nameplate).

Step 2 — Select the SIRIUS Family by Feature Requirement

Use the tier guide above to identify which family — 3RW30, 3RW40, 3RW50, 3RW52, or 3RW55 — matches the application's protection, communication, and integration requirements. Upgrading to a higher family tier for features is always preferable to retrofitting protection relays externally.

Step 3 — Select the Model with a Current Rating at or Above FLC

Within the selected family, identify the model whose rated operational current (Ie) is equal to or greater than the motor nameplate FLC. Never select a soft starter with a current rating below the motor FLC. Each family's current range is documented in the sub-pillar page for that family, linked below.

Step 4 — Verify Duty Cycle and Ambient Temperature

Check the number of starts per hour and the ambient installation temperature against the soft starter's published duty cycle rating and derating curves. High ambient temperatures (above 40 °C) or frequent starting duty may require selection of the next model up in the current range. Verify the supply voltage falls within the family's specified range: 200–480 V AC for 3RW30 and 3RW40, or 200–600 V AC for 3RW50, 3RW52, and 3RW55.