What is this part?
The Siemens 3RK4320-3GQ51-1BA3 is a SIRIUS reversing motor starter designed as a motor starter MCU for AS-Interface communication, handling motor loads up to 2.5 A at AC-3 duty. It's a reversing starter, so it can switch a motor forward and reverse from the same unit — useful for conveyors, gates, or any application needing direction change without a separate contactor pair.
Rated 2.5 A at AC-3 — that's the motor-switching current for squirrel-cage motors, not the resistive load rating. If your motor draws more than 2.5 A under full load, this starter won't hold. That's standard for standard-start induction motors; it won't nuisance-trip on normal starts but protects against sustained overload. Control supply is 20.4 to 26.4 V DC — a standard 24 VDC control bus works, but the tolerance means it stays held even if your DC rail sags a couple volts. Operating voltage range 340 to 440 V AC at 50/60 Hz covers 400 V nominal systems with typical tolerance. Won't work on a 480 V line without a transformer. IP54 enclosure keeps dust and splashing water out — fine for a dry control panel or a covered machine bay, but not for washdown zones. Ambient temperature range -25 to +50 °C covers most indoor plant floors without derating, though you'd want to check the thermal curve if mounting next to a heat source.
Dimensions 245 mm wide × 215 mm high × 205 mm deep — measure your enclosure depth before committing; 205 mm is deeper than a typical contactor block. Communicates over AS-Interface, not PROFINET or PROFIBUS, so it needs an AS-i master in the network. Two digital inputs are built in for local sensors or pushbuttons. Includes a motor brake output for 400 V AC braking — if your application needs a mechanical brake, this unit handles the coil directly.