Siemens Soft Staters, SIRIUS 3RW soft starters are a cost-effective alternative to using direct or wye-delta starters for starting three-phase motors. They avoid undesirable side effects such as mechanical bumps in the machine and mechanics or voltage drops in the mains supply. The soft start in control cabinets can be implemented in almost any application in a simple and practical way with our complete soft starter range.

Advantages at a glance

  • Soft start and soft stop
  • Smooth starting, without steps
  • Reduced current peaks
  • Avoidance of line voltage fluctuations during start-up
  • Reduced load on the power supply system
  • Reduced mechanical load in the drive
  • Considerable space savings and reduced wiring compared to other starters
  • Maintenance-free switching
  • Ease of handling
  • Perfectly matched with SIRIUS devices for the control cabinet


The SIRIUS 3RW44 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.


Rated current Ie corresponds to the motor rated current In


  • 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 current of the motor (conductor current).


Rated current Ie corresponds to approx. 58 % of the motor rated current , 6 leads to the motor (as with wye-delta starters)



Soft Staters Configuration


The 3RW44 solid-state soft starters are designed for normal starting. In case of heavy starting or increased starting frequency, a larger device must be selected. For long starting times it is recommended to have a PTC sensor in the motor. This also applies for the ramp-down modes smooth ramp-down, pump ramp-down and DC braking, because during the ramp-down time in these modes, an ditional current loading appliesin contrast to free ramp-down. In the motor feeder between the SIRIUS 3RW soft starter and the motor, no capacitive elements are permitted (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 ramp-down 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 starting, following the local short-circuit conditions. Fuses, controls and overload relays must be ordered separately. A bypass contact system and solid-state overload relay are already integrated in the 3RW44 soft starter and therefore do not have to be ordered separately.