The Siemens 3RK1323-2AS51-1AA0 is an ECOFAST reversing starter — a self-contained motor-starting and reversing assembly in a plastic IP65 enclosure, rated for AC-3 duty up to 4 kW at 400 V. It integrates a mechanical switching contact, thermistor-based motor protection, and AS-Interface bus communication into one field-mounted unit, eliminating the separate contactor, overload relay, and enclosure that a conventional reversing starter would need.
The 4 kW AC-3 rating at 400 V tells you this starter is sized for a standard 4-pole induction motor up to that power on a 400 V three-phase line — common for small conveyors, pumps, or reversing drives. Motor protection is via a thermistor input (PTC sensor embedded in the motor windings), not a bimetallic overload. That gives better thermal replication of the motor's actual temperature, especially under frequent starting or reversing duty where a standard thermal overload might nuisance-trip or fail to protect. The reversing function is built in — two mechanically interlocked contactors inside the same housing prevent simultaneous closure.
AS-Interface integration and fieldbus constraints
This starter communicates via AS-Interface (AS-i), not PROFIBUS DP. That means it connects to an AS-i master (typically a gateway to a higher-level PLC or fieldbus) using a two-wire flat cable that carries both power and data. The AS-i protocol is deterministic at 167 kHz and supports up to 62 slaves per segment — adequate for distributed motor control on a single machine or small line. No PROFIBUS DP interface is present, so if your control architecture uses PROFIBUS, you need an AS-i/PROFIBUS gateway. The enclosure dimensions — 130 mm wide, 250 mm high, 91 mm deep — are compact for a reversing starter with integrated bus and protection. It mounts directly to a machine frame or wall plate via the plastic housing's mounting lugs; no DIN rail is specified. The 10-pole motor outgoing feeder and 9-pole power connection are pre-wired to the tab terminals inside.