A soft starter ramps motor voltage up on start and down on stop, cutting inrush current and the mechanical shock that breaks couplings, belts and pump impellers. The SIRIUS 3RW range runs from the compact 3RW30 for simple in-line starting up to the 3RW44 and 3RW55 for large pumps, fans and compressors with torque control and integrated bypass.
Size on motor current and starts-per-hour, not kW alone — repeated starting drives the thermal rating.
Confirm the control supply voltage separately from the main circuit voltage.
Check whether your application needs in-line or inside-delta connection; inside-delta allows a smaller starter for the same motor.
A soft starter is not a variable frequency drive — if you need speed control while running, look at SINAMICS instead.
Frequently asked questions
Soft starter or VFD?
Use a soft starter when you only need controlled starting and stopping of a fixed-speed motor — it is cheaper, smaller and more efficient at full speed. Use a VFD when the process needs variable speed or energy savings on part-load pumps and fans.