SIRIUS 3RT1045-1BM40 — what it is and what it does
The Siemens 3RT1045-1BM40 is an SIRIUS power contactor in frame size S3, designed for switching motor loads in control panels. Its AC-2 rating at 400 V is 37 kW — that is the motor power it can handle under starting and running conditions for slip-ring or wound-rotor motors. The AC-4 breaking current of 66 A at 400 V covers reversing and plugging duty, where the contactor must interrupt full motor current. These ratings define the real load it can carry, not just a theoretical maximum.
Mounting and integration
Mounts via screw or snap-on onto 35 mm or 75 mm DIN rail — typical for European-style panels. Side-by-side mounting is permitted, which matters when packing multiple contactors into a tight enclosure. The 70 mm width and 146 mm height fit the S3 footprint; depth of 152 mm includes the arc chamber and terminal clearance. Screw-type terminals on the main circuit accept 2x (10…50 mm²) stranded cable; control terminals accept 2x (0.5…1.5 mm²) solid or 2x (0.75…2.5 mm²) stranded. IP20 on the front with cover or box terminal; IP00 at the terminal itself — so the panel enclosure provides the overall IP rating.
What the ratings mean for fit
The AC-2 rating of 37 kW at 400 V is the headline number for motor starting — it certifies the contactor for slip-ring motors where the starting current is lower than direct-on-line. The AC-4 rating of 66 A at 400 V is the severe-duty figure; if your application involves frequent reversing or inching, that 66 A is the limit, not the AC-2 kW. The coil operates on DC with a pick-up time of 14…20 ms and an arcing time of 10…15 ms. Mechanical life is typical 10 million operations, but the electrical life depends on the switching category and load current — AC-3 allows up to 1,000 operations per hour; AC-4 drops to 300 per hour due to the higher arc stress.