What this 281 A soft starter does on the line
The Siemens 3RW4436-2BC44 is a SIRIUS soft starter rated 281 A at 40 °C, 251 A at 50 °C, and 217 A at 60 °C, with an insulation voltage of 690 V and operating frequency of 50 to 60 Hz. Above that, you derate: at 50 °C you lose about 11% of the rated current, at 60 °C about 23%. If the panel runs hot — common in a crowded MCC or near a dryer section — size the motor FLA against the derated number, not the headline 281 A.
The unit measures 200 mm high by 170 mm wide by 270 mm deep and uses screw fixing to a vertical mounting surface. It can be rotated ±90° or tilted ±22.5° front-to-back, so it fits in a swing-frame or a tight backpanel without forcing a custom bracket. Clearance requirements: 100 mm above, 75 mm below, 5 mm at the sides — the side gap is tight enough that adjacent devices can sit nearly flush, but the vertical gap matters for convection cooling of the thyristor stack. Main circuit connection is via busbar; the terminal range accepts 25 to 120 mm² stranded or 4 to 250 kcmil. For auxiliary contacts, the terminals take 2x (0.25 to 1.5 mm²) solid or 2x (24 to 16 AWG). The maximum wire length from the soft starter to the motor is 500 m — beyond that, capacitive charging current can interfere with the thyristor firing and the internal bypass.
Protection and control features
The soft starter includes motor overload protection and intrinsic device protection — meaning it can protect both the motor and its own thyristors from overcurrent and overtemperature without an external overload relay. The adjustable current limitation lets you set the starting current between 8% and 100% of the rated current, which is useful for soft-starting pumps or conveyors where you need to limit inrush to avoid tripping upstream breakers or mechanical shock to the load. An external reset input is provided, so the starter can be reset remotely from a PLC or control room — no need to walk to the panel after a fault. The display version shows fault signals on the unit itself, which helps a technician on the floor identify the reason for a trip without a laptop. It supports inside-delta circuit wiring, which reduces the current through the thyristors to 58% of the line current — useful for derating the unit on a high-inertia load where the motor FLA exceeds the soft starter's rating.
Environmental and pollution rating
Storage temperature range is -25 to +80 °C; during operation the ambient limit is 60 °C. The derating temperature threshold is 40 °C, above which you must reduce the rated current as shown in the spec table.