What it is and what it does
The Siemens SIRIUS 3RT1935-5AH21 is a magnet coil for Size S2 contactors in the SIRIUS family. It provides the electromagnetic actuation for the contactor's main and auxiliary contacts when the coil is energized at its rated control supply voltage. The coil is rated at 48 V AC at both 50 Hz and 60 Hz, with a holding power draw in the range of 0.8 to 1.1 VA (the spec lists "at 50 Hz 0.8... 1.1", which is the apparent-power holding range at rated voltage). This means the coil's steady-state consumption is under 1.1 VA once the contactor is sealed — a light load on the control transformer, but the inrush during pickup will be higher (the holding figure is what governs continuous thermal load on the coil). The coil terminates via screw terminals, matching the SIRIUS Size S2 contactor base. It is a direct-replacement coil for any SIRIUS contactor in the Size S2 frame that requires a 48 V AC control voltage.
What the 48 V AC rating means for fit
The 48 V AC control voltage is a common industrial control voltage in some regions, particularly for safety circuits or panels where 24 V DC is not the standard. The coil is designed for AC supply only (Type Of Voltage Of The Control Supply Voltage = AC). Applying DC to this coil will not pull in reliably and may overheat the winding — the impedance of an AC coil is frequency-dependent, and DC operation lacks that limiting reactance.