What this part is
The Siemens 3RT1945-5AG61 is a SIRIUS magnet coil designed for Size S3 contactors. It provides the control voltage for the contactor's electromagnetic actuation, accepting 100 V AC at 50 Hz and 110 V AC at 60 Hz via screw terminals.
Sourcing reality — phased out with a clear path
This coil is a phased-out product. The official successor is the SIRIUS 3RT2 series. For a BOM line that needs this exact order code, sourcing runs through surplus or broker channels — quoted to order against an RFQ. There is no last-time-buy window; the part is already in the phase-out lifecycle stage.
What the control voltage ratings mean for fit
The coil is wound for 100 V AC at 50 Hz and 110 V AC at 60 Hz. That 10 V difference compensates for the higher impedance at 60 Hz, so the magnetic pull-in force stays consistent across line frequencies. If your panel runs 110 V AC at 60 Hz, this coil matches. If you have a 100 V AC 50 Hz supply, it also matches. The operating range of 0.8 to 1.1 times the rated voltage at 50 Hz means the coil will pick up reliably between 80 V and 110 V AC.
Integration note — Size S3 footprint
This coil mounts onto SIRIUS Size S3 contactors. The screw terminals accept the same wire ranges as the contactor's auxiliary circuit — typically 2x (0.5 to 1.5 mm²) solid or stranded. The coil draws its rated control voltage from the panel's AC control transformer; no DC rectification needed since it's an AC coil.