The Siemens 3RW4055-6BB44 is a SIRIUS soft starter, size S6, designed to ramp induction motors up to speed and protect them during start. It uses thyristors to control voltage, so the motor sees a gradual increase rather than a full-voltage slam. The unit is rated 134 A at 40 °C, 230 V at both 50 and 60 Hz, and carries motor overload protection and adjustable current limitation built in.
But if your panel runs hotter, derate: 117 A at 50 °C, 100 A at 60 °C. The minimum load is 20% of the rated current, so a motor drawing less than about 27 A won't trigger the internal protection correctly.
Mounting is screw fixing. The unit is 250 mm deep, 120 mm wide, 198 mm tall. Clearance: 100 mm above, 75 mm below, only 5 mm at the sides — tight side clearance means you can stack them close. The main current circuit connects via busbar; main contacts accept 4 to 250 kcmil. For the control circuit, solid wire 2x (0.5 to 2.5 mm²) or stranded 2x (25 to 120 mm²). Auxiliary contacts accept 2x (20 to 14 AWG). Maximum motor wire length is 300 m — keep that in mind for long cable runs.