What it is and what it does
The Siemens 3RT1045-1BF44 is a SIRIUS power contactor in frame size S3, designed for switching motor loads and resistive loads in industrial control panels. It is rated for 37 kW at AC-2 duty at 400 V, which covers slip-ring motor applications, and carries a 66 A rating at AC-4 at 400 V for reversing or inching duty where the contactor makes and breaks stalled rotor current. The contactor uses screw-type terminals on the main current circuit, accepting solid conductors up to 2x 4 mm² and stranded conductors up to 2x 50 mm². It mounts via screw and snap-on onto 35 mm or 75 mm standard mounting rail, which is the typical DIN-rail integration in a panel enclosure. The front face carries an IP20 rating with the cover or box terminal installed, meaning it is finger-safe for panel wiring. The terminal area itself is IP00, so the contactor must be housed inside a rated enclosure for washdown or outdoor environments.
Key ratings and what they mean for fit
The AC-2 rating of 37 kW at 400 V is the motor-switching figure that governs most slip-ring motor applications — it is the rating to match against your motor nameplate, not the higher AC-1 resistive rating. The AC-4 rating of 66 A at 400 V tells you the contactor can handle the high inrush of reversing or jogging duty without welding contacts. Mechanical life is typical at 10 million operations. The contactor is rated for up to 1,000 switching cycles per hour at AC-3 duty. The coil is rated at 110 V, with a pick-up threshold of 0.8 x rated value and a drop-out of 1.1 x rated value. Operating temperature range is -25 to +60 °C, with storage from -55 to +80 °C. Pollution degree 3 means it is suitable for industrial environments with conductive pollution.