SIRIUS 3RW44 soft starter — motor starting in the panel
The Siemens 3RW4422-1BC34 is a SIRIUS 3RW44 series soft starter, designed to ramp three-phase induction motors up to speed and protect them during start. It uses thyristors to control voltage to the motor, reducing inrush current and mechanical stress on the driven load — conveyors, pumps, fans, compressors in a control panel.
Integrated bypass and adjustable current limit
An integrated bypass contact system shorts the thyristors once the motor is at full speed, eliminating the semiconductor heat load during run — the bypass carries the 50 A continuously without the 1–1.5 V forward drop of the SCRs. This keeps the enclosure temperature rise in check and lets the soft starter handle longer start cycles. Adjustable current limitation sets the maximum current the soft starter delivers during start, typically 2–5× the motor FLA depending on the load. The minimum load is 8% of the rated current — below that the motor current may be too low for the thyristor firing circuit to detect, causing a trip. Motor overload protection is built in, so no separate overload relay is needed for most installations. The intrinsic device protection covers the thyristors against overtemperature and phase loss.
Mounting, wiring, and clearance
Screw fixing to the back panel — the unit is 192 mm high, 170 mm wide, 270 mm deep. The mounting position allows vertical mounting surface with ±90° rotation and ±22.5° tilt front/back, so it fits in angled or space-constrained enclosures. Clearance around the device: 100 mm above, 75 mm below, 5 mm at the sides. Maximum motor cable length is 500 m — longer runs increase charging current that can nuisance-trip the soft starter. Pollution degree 3 per IEC 60947-4-2 means it tolerates conductive dust and humidity typical in industrial environments.