The Siemens 3RW4028-2BB14 is a SIRIUS soft starter in frame size S0, designed to ramp induction motors up to 38 A at 40 °C (34 A at 50 °C, 31 A at 60 °C) on 50/60 Hz supplies. The 38 A figure is the full-load motor current it can start and run — size your motor nameplate against this, not the line breaker. Control supply is 110–230 VDC, which means it can be wired directly into a standard DC control bus without an extra transformer or AC-to-DC converter. The integrated bypass contactor closes after the start ramp, taking the thyristors out of circuit — so the soft starter dissipates no heat during continuous running, and the bypass carries the full motor current. Motor overload protection is built in, with adjustable current limitation and a minimum load threshold of 20%. No external overload relay is needed for most applications — the starter handles both soft start and overload in one device. The red fault signal display gives a visible trip indication on the panel.
Mounts via screw or snap-on to DIN rail. The S0 frame footprint is 45 mm wide × 150 mm high × 155 mm deep. Clearance: 60 mm above, 40 mm below, 15 mm at the sides. With an additional fan, the mounting surface can be vertical and rotated ±90° or tilted ±22.5° front-to-back; without a fan, rotation is limited to ±10°. Main current circuit uses spring-loaded terminals accepting 2× (0.25–2.5 mm²) solid or stranded. Auxiliary and control circuits also use spring-loaded terminals, wire size 2× (24–14 AWG). The front clamping point accepts 1× 8 AWG or 2× (16–10 AWG). Maximum wire length to the motor is 300 m.
Derating starts at 40 °C — the 38 A rating is at 40 °C; at 50 °C it drops to 34 A, at 60 °C to 31 A.