The Siemens 4EU2421-0AB00-4B is a 3-phase commutating choke (smoothing reactor) designed for use with line-commutated converters — the type that drives DC motors or serves as a DC-link supply in variable-speed drives. Its job is to limit the rate of rise of fault current (di/dt) during commutation and to reduce harmonic distortion on the AC line. Rated 91.1 A AC at 60 Hz, 400 V, with an inductance of 0.28 mH and a relative inductive voltage drop of 4% at rated current, it's sized for a specific converter output — not a general-purpose line reactor.
The 4% voltage drop at rated current is the key selection parameter: it tells you the choke will drop 4% of line voltage under full load, which the converter must be able to tolerate. At 91.1 A AC and 111.6 A DC, this choke handles the pulsed DC link of a converter rated for roughly 50–60 kW output — ballpark. The 80.2 W copper loss plus 46.6 W iron loss total about 127 W of heat that must be dissipated; with IP00 open-frame construction, it relies on cabinet airflow, not enclosure cooling. Thermal class H means the winding insulation withstands 180 °C hot-spot temperature — adequate for a 40 °C ambient with natural convection, but derate if your cabinet runs hotter.
Dimensions are 0.225 m wide × 0.21 m high × 0.136 m deep. Flat-type terminals for the main circuit — expect busbar or cable lug connections, not screw-clamp. IP00 means no enclosure; mount it inside a ventilated cabinet with clearance around the core for airflow. The 0.28 mH inductance at 60 Hz is a fixed value — no taps or adjustment. For a 50 Hz line, the same choke will saturate at a slightly lower current because the volt-second product is higher; verify with the converter manufacturer if your line frequency differs.