The Siemens SIRIUS 3RT1944-5BB42 is a magnet coil for Size S3 contactors, rated 24 V DC and terminated with spring-loaded connection technology. It is the coil that pulls in and holds the contactor armature — the part that actually switches the load. The SIRIUS platform is Siemens' modular contactor and motor-starter family, so this coil mounts directly onto the S3 frame, which covers the 38–65 A motor current range in AC-3 duty.
Sourcing reality — phased out, but replaceable
This part is flagged as a phased-out product. Siemens' official successor is the SIRIUS 3RT2 series, which uses the same Size S3 frame and 24 V DC coil voltage.
What the 24 V DC rating means for fit
The 24 V DC coil is the control voltage — this is what the PLC output or control relay must supply to energize the contactor. DC coils hold in with continuous DC, unlike AC coils that hum and require a different latching characteristic. Size S3 means the coil physically fits SIRIUS contactors in that frame class; verify the mating contactor's order code includes the S3 designation.