Siemens SIRIUS 3RW55 Soft Starter | 13–570 A | Stoklink

About the SIRIUS 3RW55

The SIRIUS 3RW55 is Siemens' flagship soft starter series, sitting at the top of the SIRIUS motor-management hierarchy. It was engineered for applications where controlled starting and stopping alone are insufficient — where the process demands continuous motor monitoring, functional safety compliance, and seamless integration into modern industrial networks. Typical deployment sectors include critical pump and fan stations, offshore and marine machinery, mining conveyors and hoists, power-generation auxiliaries, and any process line where unplanned downtime carries significant operational or safety consequences.

At its core, the 3RW55 delivers phase-angle controlled soft starting and soft stopping through an adaptive control system that continuously measures and adjusts torque throughout the acceleration ramp. A dedicated pump-stop algorithm suppresses pressure surges on deceleration, protecting pipeline infrastructure without requiring external dampening equipment. Once the motor reaches full speed, the built-in bypass contactor — fitted across the entire 13–570 A range — closes automatically, eliminating thyristor conduction losses and keeping the enclosure thermal budget low during steady-state operation.

Motor protection is handled internally without external relay modules. The 3RW55 monitors thermistor (PTC/NTC) input, residual current, ground fault, phase asymmetry, and supply voltage — delivering protection functions that previously required a separate motor protection relay. Optional integrated Safe Torque Off (STO) brings the unit into the functional safety domain, enabling SIL- and PL-rated safety architectures directly at the drive level.

Operator interaction is handled through a removable graphic operator panel with plain-text display and multi-language support, while Siemens' Soft Starter ES engineering software provides parameterisation, diagnostics, and data logging from a PC.

Where SIRIUS 3RW55 Fits in the SIRIUS Portfolio

The SIRIUS soft starter family spans five tiers, each targeting a distinct engineering requirement:

• 3RW30 — Entry-level, electronic motor starting for straightforward DOL replacement with ramp control only. No bypass, no fieldbus.
• 3RW40 — Standard tier adding thermistor input and basic motor protection, suitable for general-purpose pumps, fans, and compressors.
• 3RW50 / 3RW52 — Mid-range, introducing PROFIBUS and PROFINET communication, enhanced protection, and inside-delta (three-phase inline) wiring capability for reduced frame size. The 3RW52 is the direct successor to the 3RW44 and represents a significant step up in connectivity.
• 3RW55 — Top tier. Full motor protection suite, adaptive torque control, pump-stop algorithm, built-in bypass across all sizes, integrated STO safety, and five fieldbus protocols. Specified when the application cannot tolerate gaps in protection, communication, or safety coverage.

Engineers migrating from a 3RW40 or 3RW52 to the 3RW55 gain functional safety, ground-fault and residual-current monitoring, and multi-protocol fieldbus flexibility without changing the external wiring footprint in most frame sizes.

Technical Specifications

The following specifications apply at the family level across all 18 SIRIUS 3RW55 models. Always verify individual model datasheets for frame-size-specific derating, mounting orientation, and accessory compatibility.

• Rated operational current: 13 A to 570 A
• Motor power range: 5.5 kW to 315 kW (at 500 V, 3-phase)
• Main circuit supply voltage: 200–480 V AC, 3-phase, 50/60 Hz
• Control supply voltage: 110–230 V AC (external control supply, depending on variant)
• Frequency: 50 / 60 Hz
• Control method: Phase-angle control with adaptive torque / current control
• Bypass: Integrated bypass contactor — standard across all current ratings
• Motor protection functions: Thermistor (PTC/NTC), residual current monitoring, ground-fault detection, phase-asymmetry detection, voltage monitoring, overload (electronic)
• Safety function: STO (Safe Torque Off) — optional; SIL 2 / PLd capable
• Communication interfaces: PROFINET IO, EtherNet/IP, PROFIBUS DP, Modbus TCP, Modbus RTU (protocol depends on communication module fitted)
• Operator interface: Removable graphic operator panel; Soft Starter ES PC software
• Standards: IEC 60947-4-2, UL 508, CSA
• Degree of protection: IP00 / IP20 (device); suitable for installation in IP54+ enclosures
• Ambient operating temperature: −25 °C to +60 °C (with derating above +40 °C depending on model)
• Storage temperature: −40 °C to +80 °C
• Relative humidity: ≤ 95 %, non-condensing
• Mounting: DIN rail (smaller frames) or panel/chassis mounting (larger frames)

Key Features

• Adaptive Control System, Torque Control & Pump-Stop Algorithm

  The 3RW55 continuously monitors motor behaviour during the start ramp and adjusts thyristor firing angles in real time to maintain the programmed torque profile. This prevents current spikes caused by mechanical load variations and extends motor and driven-equipment service life. The dedicated pump-stop algorithm controls the deceleration ramp to suppress water hammer in pipeline systems — eliminating the pressure transients that cause pipe fatigue and valve damage without additional hydraulic equipment.

• Built-in Bypass Contactor Across Full Range

  Every 3RW55 model from 13 A to 570 A includes an internal bypass contactor that closes once the motor reaches rated speed. This removes thyristor losses from the thermal budget during normal running, reduces heat dissipation in the panel, and eliminates the need for an external bypass contactor and associated wiring — simplifying panel design and reducing installed cost.

• Full Motor Protection Package

  Thermistor input (PTC/NTC), residual current monitoring, ground-fault detection, phase-asymmetry detection, and voltage monitoring are all handled internally. This eliminates the need for a separate motor protection relay in most applications, reduces panel component count, and provides a single diagnostic source for all motor-related fault events.

• Integrated Safety Functions — STO (Safe Torque Off)

  The optional STO function meets IEC 62061 SIL 2 and ISO 13849 PLd requirements, allowing the 3RW55 to be incorporated directly into machine safety architectures. Safe Torque Off prevents the motor from generating torque without isolating the main power supply, enabling faster and more cost-effective safety circuit design compared to external safety contactors.

• Broadest Fieldbus Support in the SIRIUS Range

  PROFINET IO, EtherNet/IP, PROFIBUS DP, Modbus TCP, and Modbus RTU are all supported via plug-in communication modules. This makes the 3RW55 compatible with virtually every PLC and DCS platform encountered in process, marine, and energy applications, and future-proofs panel designs against network migrations.

• Removable Graphic Operator Panel & Soft Starter ES Software

  The detachable graphic panel allows local parameterisation, status readout, and fault diagnostics without a laptop. Soft Starter ES PC software extends this with offline parameter editing, upload/download, and data logging — accelerating commissioning and simplifying remote support.

Selecting Your SIRIUS 3RW55 Model

Correct soft starter sizing is based on the motor's full-load current (FLC) as stated on the motor nameplate — not on the motor's kilowatt rating alone. The kW figures in the lineup assume a 500 V, 3-phase supply; at lower voltages (e.g. 400 V or 230 V) the same motor draws higher current for the same power, so always work from FLC.

Step 1 — Identify motor FLC. Read the nameplate current at your supply voltage. If the motor is dual-rated (e.g. 400 V / 690 V), use the value corresponding to your network voltage.

Step 2 — Match to the next model at or above FLC. The 3RW55 range steps through 13, 18, 25, 32, 38, 47, 63, 77, 93, 113, 143, 171, 210, 250, 315, 370, 470, and 570 A. Select the lowest rating that equals or exceeds motor FLC. Do not undersize — operating a soft starter at or above its continuous current rating will shorten semiconductor life and risk tripping on thermal overload.

Step 3 — Apply derating for ambient temperature and start duty. The rated current figures assume ≤ 40 °C ambient and a standard start duty cycle (typically ≤ 3 starts per hour, depending on start duration). For higher ambient temperatures, more frequent starts, or extended ramp times (> 20 s), apply the derating factors from the relevant model datasheet. In practice, this often means selecting one model size up.

Step 4 — Consider inside-delta wiring for large motors. For motors ≥ 75 kW at 400 V in delta connection, inside-delta (three-phase inline) wiring allows a smaller-frame 3RW55 to be used because the thyristors carry only 58 % of line current. This reduces hardware cost and panel space. Consult the 3RW55 system manual for wiring diagrams and minimum motor power thresholds.

Step 5 — Select communication module. Specify the required fieldbus protocol (PROFINET, EtherNet/IP, PROFIBUS DP, Modbus TCP, or Modbus RTU) at order stage, as communication modules are factory-fitted or field-retrofittable but must be matched to the master PLC/DCS platform.

If you provide motor FLC, supply voltage, ambient temperature, and start duty, Stoklink's application team can confirm the correct 3RW55 article number before you order.