What it is and what it fits
The Siemens SIRIUS 3RT1945-5AH21 is a magnet coil for Size S3 contactors in the SIRIUS family. It provides the electromagnetic actuation for the contactor's main and auxiliary contacts, converting electrical control signals into mechanical switching action. The coil is rated for 48 V AC at both 50 Hz and 60 Hz, drawing 0.8 to 1.1 A at 50 Hz. It terminates via screw terminals, matching the standard wiring practice for panel-mounted contactor assemblies.
Lifecycle and sourcing reality
This part is a phased-out product. The manufacturer lists its lifecycle stage as current, but the description explicitly marks it as phased-out with the successor being the SIRIUS 3RT2 series. For a BOM line that requires this exact order code, sourcing goes through independent distribution channels. Availability and current pricing are confirmed at quote time against an RFQ. The official successor is the SIRIUS 3RT2 family, but a direct drop-in replacement for the 3RT1945-5AH21's specific mounting and coil voltage should be verified against the 3RT2 catalog.
Integration into a SIRIUS contactor panel
This coil mounts onto a Size S3 SIRIUS contactor frame. The screw terminals accept the control wiring from the PLC or control relay. The 48 V AC control voltage is common in North American and European panel designs that use a control transformer. When replacing a failed coil on an existing S3 contactor, the 3RT1945-5AH21 is the direct service part. For new builds or panel retrofits, the 3RT2 successor series should be evaluated for form, fit, and function compatibility.