The Siemens 3RW4028-1BB05 is a SIRIUS soft starter, size S0, rated 38 A at 40 °C. It uses thyristors to ramp motor voltage during start, reducing inrush current and mechanical stress on the load. The integrated bypass contact system bridges the thyristors once the motor is at full speed, eliminating heat dissipation from the semiconductors during run — that's the main reason this size S0 frame can handle 38 A continuously without an external heatsink.
The 38 A rating at 40 °C is the baseline; at 50 °C it derates to 34 A, and at 60 °C to 31 A. The minimum load is 20 % of the rated current, so it won't regulate properly on a motor drawing less than about 7.6 A. Insulation voltage is rated 600 V, and the operating frequency covers 50 to 60 Hz.
The S0 footprint is 45 mm wide, 125 mm tall, 155 mm deep. Clearance: 60 mm above, 40 mm below, 15 mm to the side. With an additional fan, the mounting surface can be vertical and rotated up to ±90° or tilted ±22.5° front/back. Without a fan, keep it within ±10° rotation and ±10° tilt. Main circuit and auxiliary/control connections are screw-type terminals; the main circuit accepts 2x (0.5... 2.5 mm²) solid or stranded. Maximum motor cable length is 300 m.
Protection and control features
Built-in motor overload protection and intrinsic device protection are standard. The adjustable current limitation lets you set the start current between the minimum load and the device's rated current — useful for matching the soft starter to the actual motor FLA without changing hardware. An external reset input is provided, and a red fault signal display is on the front. The control circuit is 24 V AC/DC (both 50 and 60 Hz). No internal delta circuit or motor brake output on this variant.