The Siemens 3RW4028-1TB04 is a SIRIUS soft starter in the S0 frame size, designed to ramp three-phase induction motors up to 38 A at 40 °C (34 A at 50 °C, 31 A at 60 °C) on a 400 V line. It's a current-production part — no lifecycle concerns for a new BOM line. The 45 mm width and screw/snap-on mounting mean it drops into a standard DIN-rail panel without extra bracketry, and the integrated bypass contact system pulls the thyristors out of the circuit once the motor is at speed, so you're not dissipating heat from the semiconductors during continuous run.
The S0 frame keeps the footprint compact — 45 mm wide, 125 mm high, 155 mm deep — and the 60 mm upward / 40 mm downward / 15 mm side clearance leaves room for adjacent devices in a dense panel.
Above that, derate to 34 A at 50 °C and 31 A at 60 °C. The 20 % minimum load means it won't regulate properly below that motor current, so it's not a fit for fractional-horsepower motors. The 600 V rated insulation voltage (pollution degree 3 per IEC 60947-4-2) covers 400 V and 480 V systems with margin. Control voltage is 24 V AC/DC at 50/60 Hz.
The auxiliary contacts accept 2x conductors from 20 to 14 AWG. The main power terminals accept 2x solid conductors from 0.5 to 2.5 mm². Maximum wire length to the motor is 300 m — keep that in mind for remote installations.