The Siemens 3RT5944-5AQ01 is a SIRIUS magnet coil, the replaceable solenoid that pulls in and holds the contactor armature. It is the control-side component that converts the AC control voltage into the magnetic force that closes the main and auxiliary contacts. The coil is rated for a control supply voltage of 380 V at 50 Hz, and it also accepts 400 to 440 V at 60 Hz, covering the common industrial line voltages across 50 Hz and 60 Hz regions.
What the voltage tolerance means for fit
The coil's operating range is specified at 0.8 to 1.1 times the rated voltage at 50 Hz. At the 380 V nominal, that means the coil will reliably pick up and hold between approximately 304 V and 418 V. This tolerance accounts for line sag under load and typical transformer regulation. If your control transformer is sized for a 380 V tap and the line drops during a large motor start, the coil stays sealed as long as the voltage stays above 304 V. Below that threshold, the contactor may drop out, so verify your control transformer sizing and secondary voltage under worst-case load.
Integration note
This coil is a direct replacement for the corresponding SIRIUS contactor frame. It snaps onto the contactor's magnet system without tools. The AC supply is wired to the coil terminals; verify the free-wheel diode or varistor polarity on the suppression module if one is fitted. The coil's apparent holding power is typically in the range of 5-10 VA for SIRIUS S00/S0 sizes, so the control transformer should be sized to handle the inrush during pick-up without excessive voltage drop.