The Siemens 3RT1945-5AR61 is a SIRIUS magnet coil for Size S3 contactors, terminated with screw terminals. It's wound for 400 V AC at 50 Hz and 440 V AC at 60 Hz, with an operating range of 0.8 to 1.1 times the rated voltage at 50 Hz — meaning the coil reliably picks up and holds across a line-voltage sag down to about 320 V and up to 440 V at 50 Hz. This is the replacement coil you'd order when the original on a Size S3 contactor fails or the panel spec calls for a 400 V AC control circuit.
This part is flagged as a phased-out product. The official successor is the SIRIUS 3RT2 series coil. If you're maintaining a BOM for an existing panel, the 3RT1945-5AR61 is still available through independent distribution — quote to order against an RFQ — but any new design-in should use the 3RT2 equivalent. No last-time-buy date is published on this record, so the surplus channel is your sourcing path for spares.
Mounts directly onto the Size S3 contactor frame via the standard screw-terminal interface. The AC coil draws its control voltage from the panel's control transformer; verify the tap matches 400 V AC (50 Hz) or 440 V AC (60 Hz) before wiring. No auxiliary contact block is integrated — the coil terminals are the two screw connections on the front face.