SIRIUS 3RW soft starters – as versatile as your application

The 3RW soft starters should always be designed on the basis of the required rated operational current of the motor. The motor ratings listed in the selection and ordering data are rough guide values and designed for basic starting conditions (CLASS 10). For other starting conditions we recommend the Simulation Tool for Soft Starters (STS).

Motor rating data in kW and hp is based on IEC 60947‑4‑1.

At an installation altitude above 2 000 m, max. permissible operational voltage is reduced to 480 V.

Installation altitude for SIRIUS 3RW soft starters

The selection and ordering data were determined for the following constraints (stand-alone installation without additional fan)



High Performance

General Performance

Basic Performance

SIRIUS soft starters

3RW55 / 3RW55-F






Maximum starting time





Maximum starting current in % of motor current



Maximum number of starts per hour




Simulation Tool for Soft Starters (STS)

The Simulation Tool for Soft Starters (STS) provides a convenient means of designing soft starters using a simple, quick and easy-to-use interface. Entering the motor and load data will simulate the application and prompt suggestions for suitable soft starters.

Link to a free download of the Simulation Tool for Soft Starters (STS), see

  • Simple, quick and user-friendly operator interface
  • Detailed and up-to-date Siemens motor database, including IE3/IE4 motors.
  • Simulation of heavy starting up to CLASS 30
  • Update-capable (e.g. motors, load types, functions)
  • Fast simulations with minimum input data
  • Immediate, graphical curve charts of start operations with limit values
  • View in table form of suitable soft starters for the application

Everything at a glance: Simulation and results list

Circuit concept

Three-phase controlled SIRIUS 3RW soft starters can be operated in two different types of circuit:

  • Inline circuit
    The controls for isolating and protecting the motor are simply connected in series with the soft starter. The motor is connected to the soft starter with three leads.
  • Inside-delta circuit
    The wiring is similar to that of wye-delta starters. The phases of the soft starter are connected in series with the individual motor windings. The soft starter then only has to carry the phase current, amounting to about 58% of the rated motor current (conductor current).

Comparison of the types of circuit

Inline circuit: Rated current Ie corresponds to the rated motor current In, three cables to the motor

Inside-delta circuit: Rated current Ie corresponds to approx. 58% of the rated motor current In, six cables to the motor (as for wye-delta starters)

Which circuit?

Using the inline circuit involves the lowest wiring outlay. If the soft starter to motor connections are long, this circuit is preferable.

The wiring complexity is twice as high when using the inside-delta circuit, but a smaller device can be used with the same rating. Thanks to the choice of operating mode between the inline circuit and inside-delta circuit, it is always possible to select the most favorable solution.

The braking function is only possible in the inline circuit. The inside-delta circuit cannot be used in 690 V line supplies.


The solid-state 3RW soft starters are designed for normal starting. In case of heavy starting or increased starting frequency, a larger unit may need to be selected. The 3RW52 soft starters may be used in isolated supply networks (IT systems) up to 600 V AC and the 3RW55 soft starter even up to 690 V.

For long starting times, we recommend a PTC sensor or temperature switch in the motor. This also applies for the “torque control”, “pump stop” and “DC braking” stopping modes, because during the stopping time in these modes, an additional current loading applies in contrast to coasting down.

No capacitive elements are permitted in the motor feeder between the SIRIUS 3RW soft starter and the motor (e.g. no reactive-power compensation equipment). In addition, neither static systems for reactive-power compensation nor dynamic PFC (Power Factor Correction) must be operated in parallel during starting and stopping of the soft starter. This is important to prevent faults arising on the compensation equipment and/or the soft starter.

All elements of the main circuit (such as fuses and controls) should be dimensioned for direct-on-line starting, following the load short-circuit conditions. Fuses and switching devices must be ordered separately. The harmonic component load for starting currents must be taken into consideration for the selection of motor starter protectors (selection of release). Please observe the maximum switching frequencies specified in the technical specifications.

Discover Softstarter

Basic - 3RW30 - without overload
Basic - 3RW30 - without overload
Basic - 3RW40 - with overload
Basic - 3RW40 - with overload
General - 3RW52 - 200-480V
General - 3RW52 - 200-480V
General - 3RW52 - 200-600V
General - 3RW52 - 200-600V
High - 3RW55 - 200-480V
High - 3RW55 - 200-480V
High - 3RW55 - 200-600/690V
High - 3RW55 - 200-600/690V
3RW44 - with comms option
3RW44 - with comms option

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