Siemens SIRIUS 3RW40 Soft Starter | 12.5–106 A | Stoklink

About the SIRIUS 3RW40

The SIRIUS 3RW40 is Siemens' workhorse soft starter for standard industrial motor applications. It applies a three-phase voltage ramp during start-up, progressively raising voltage to the motor and eliminating the inrush current spike and mechanical shock associated with direct-on-line switching. A corresponding voltage ramp on stop reduces water hammer in pump systems and belt stress in conveyors. Torque-controlled start and stop options are available for applications where a linear torque profile is preferred over a simple voltage ramp, giving engineers additional flexibility without requiring a higher-tier device.

A defining characteristic of the 3RW40 is its built-in bypass contactor. Once the motor reaches full speed, the thyristors are automatically bypassed, eliminating switching losses and heat generation during normal run. This makes the 3RW40 energy-efficient in continuous-duty cycles and simplifies thermal management in the control panel. Integrated electronic motor overload protection to Class 10 and Class 20 removes the need for a separate overload relay, reducing component count and wiring.

LED status indicators provide at-a-glance diagnostic feedback. For sites requiring network integration, an optional Modbus RTU accessory module can be fitted, enabling parameter read-back and status monitoring without re-wiring the power circuit.

Where SIRIUS 3RW40 Fits in the SIRIUS Portfolio

Siemens structures the SIRIUS soft starter range into clearly defined tiers, each addressing a different level of application complexity:

• 3RW30 — Entry-level, single-phase control supply, basic voltage ramp, no integrated bypass. Suited to the simplest starting tasks where cost is the primary driver.
• 3RW40 — Standard tier (this family). Adds a built-in bypass contactor, integrated overload protection, and torque-control options. The most common replacement for star-delta starters up to 55 kW.
• 3RW50 — Advanced tier. Extends the power range significantly, adds a full alphanumeric display, on-board fieldbus interfaces (PROFIBUS, PROFINET, Modbus), and more granular protection functions including thermistor input and phase-sequence monitoring.
• 3RW52 — Builds on the 3RW50 platform with enhanced safety functions and additional communication options, targeting machinery with functional safety requirements.
• 3RW55 — High-feature tier for demanding applications. Offers the widest protection suite, advanced diagnostics, and full integration with the TIA Portal engineering framework.

Choose the 3RW40 when you need reliable, efficient soft starting up to 55 kW, built-in overload protection, and optional Modbus connectivity — without the cost or complexity of the advanced tiers. If your application exceeds 55 kW, requires integrated fieldbus as standard, or demands expanded protection functions, evaluate the 3RW50 or above.

Technical Specifications

The following specifications apply at the family level to all nine SIRIUS 3RW40 models. Refer to the individual product datasheet for model-specific values.

• Current range: 12.5 A to 106 A (motor FLC)
• Power range: 5.5 kW to 55 kW at 500 V AC
• Main circuit supply voltage: 200–480 V AC, 50/60 Hz, three-phase
• Control supply voltage: 110–230 V AC, 50/60 Hz (internal from main supply on standard variants)
• Frequency: 50 Hz / 60 Hz
• Start/stop ramp time: Adjustable (voltage ramp and torque-controlled modes)
• Motor overload protection: Integrated electronic, IEC Class 10 and Class 20 selectable
• Bypass contactor: Built-in; activates automatically at end of ramp
• Communication: Modbus RTU via optional accessory module
• Status indication: LED indicators (no alphanumeric display)
• Degree of protection: IP00 / IP20 depending on terminal cover fitted; designed for installation in an enclosure
• Applicable standards: IEC 60947-4-2 (low-voltage switchgear and controlgear — AC semiconductor motor controllers)
• Ambient operating temperature: -25 °C to +40 °C (derating required above 40 °C up to 60 °C maximum)
• Storage temperature: -40 °C to +80 °C
• Altitude: Up to 1000 m above sea level without derating; derating applies above 1000 m
• Pollution degree: 3 (suitable for industrial environments)

Key Features

• Three-phase voltage ramp soft start and soft stop: The 3RW40 ramps supply voltage on all three phases simultaneously during acceleration and deceleration. This reduces inrush current, limits mechanical shock on couplings and driven equipment, and extends motor and load service life — a direct improvement over both direct-on-line and star-delta switching.
• Built-in bypass contactor: Once the motor has accelerated to full speed, an internal bypass contactor shorts out the thyristors. Heat dissipation during run is reduced to near zero, eliminating the need for forced ventilation or oversized enclosures in most applications and improving overall system energy efficiency.
• Integrated electronic motor overload protection (Class 10 / 20): A dedicated external overload relay is not required. The 3RW40 monitors motor current continuously and trips to protect against sustained overload. Class 10 suits standard motors; Class 20 accommodates motors with higher starting inertia. This reduces panel component count, wiring time, and potential fault points.
• Torque-controlled start/stop options: In addition to voltage ramp, the 3RW40 offers torque-controlled starting and stopping. This provides a more linear torque profile at the shaft, which is valuable for conveyor and crusher applications where abrupt torque changes cause mechanical damage or product disturbance.
• LED status indicators: Onboard LEDs give operators and maintenance personnel immediate visual feedback on device status, run state, and fault conditions without requiring a laptop or HMI connection. Diagnostics are fast and tool-free in the field.
• Modbus RTU via optional accessory module: Sites requiring SCADA or PLC integration can add Modbus RTU communication without changing the power circuit. Parameter monitoring and status read-back are available over the serial network, supporting predictive maintenance strategies and remote commissioning checks.

Selecting Your SIRIUS 3RW40 Model

Selecting the correct 3RW40 model requires matching the soft starter's rated current to the motor's full-load current (FLC) as stated on the motor nameplate. The soft starter current rating must be equal to or greater than the motor FLC — never size below nameplate current.

Step 1 — Determine motor FLC. Read the full-load current from the motor nameplate at the actual supply voltage. If your supply is 400 V, the FLC will be higher than the 500 V figure; use the nameplate value for your voltage, not the motor's rated power alone.

Step 2 — Select the next model at or above that FLC. The 3RW40 lineup steps through 12.5, 25, 32, 38, 45, 63, 72, 80, and 106 A. Choose the smallest model whose rated current meets or exceeds the motor FLC. Do not over-size unnecessarily, as overload protection accuracy improves when the motor current is close to the device's rated current.

Step 3 — Check duty cycle and starts per hour. The 3RW40 is rated for standard duty. If your application requires an unusually high number of starts per hour or an extended ramp time (heavy inertia loads), verify the thermal duty against the datasheet. Crushers and large compressors may require derating or selection of the next current size up.

Step 4 — Confirm supply voltage. All 3RW40 models support 200–480 V AC main supply. If your supply is 500 V or higher, consult the datasheet; the 500 V power ratings listed are at the upper end of the design envelope.

Step 5 — Identify accessories. If Modbus RTU integration is needed, specify the corresponding accessory communication module at time of order. This avoids retrofit work after installation.

For applications above 55 kW or where integrated fieldbus is mandatory, consider the SIRIUS 3RW50 family.