Siemens Introduces First Soft Starters with Integrated Safe Torque Off Functionality

The Sirius 3RW55 Failsafe soft starters from Siemens has an integrated fail-safe digital input that directly connects to an emergency stop pushbutton.

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  • Integrated motor protection function ensures fast, reliable shutdown;
  • Restart interlock feature and failsafe digital input help ensure user safety; and
  • Products are SIL 3 and PL e compliant and ATEX and IECEx certified.

The Sirius 3RW55 Failsafe soft starters are the first to feature the integrated Safe Torque Off (STO) function. This ensures that the motor comes to a complete standstill if an operator initiates an emergency shutdown. The motor then remains in this state until the restart interlock is used to restart it.

Another important safety feature of the 3RW55 Failsafe soft starter is the integrated fail-safe digital input that directly connects to an emergency stop pushbutton. This covers SIL 1 PL c applications and even achieves a SIL 3, PL e rating when a safety contactor and relay are applied.

The STO function facilitates risk-free working, even with the protective door open, making it popular in processing machine applications and systems such as agitators or belt conveyors. These soft starters are also certified according to ATEX/IECEx directives, making them ideal for starting explosion-proof motors and for use in applications at airports, gas stations, or other work areas where explosions might occur. 

In addition to their safety benefits, the Sirius 3RW55 Failsafe soft starters provide savings on components and wiring, making them a popular option for control panel engineers and designers. Control panel manufacturers no longer require using several individual components to have this function, which reduces the time and effort for wiring and maintenance and saves space in the control cabinet. In addition, the switching time of the integrated solution is significantly shorter than for conventional, electromechanical components.

With their innovative torque control, the Siris 3RW55 Failsafe soft starters can be used to start up to 400 hp at 480VAC for inline circuits or 750 hp at 480VAC for inside-delta circuits. Learn more about these soft starters’ comprehensive functionality at

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