The Siemens 3RT7045-1AN20 is a size S3 contactor from the SIRIUS family, built for switching three-phase motor loads up to 690 V AC-3. It carries three normally-open main poles and screw-type terminals on both the main and auxiliary circuits. Fastening is by screw or snap-on onto 35 mm or 75 mm standard mounting rail, with generous mounting-position tolerance — ±180° rotation on a vertical surface and ±22.5° tilt forward or backward.
AC-3 duty and switching cycle limits
What matters for a motor-starting application is the switching frequency: the contactor is rated for 800 operations per hour under AC-3 (or AC-3e) duty, but that drops to 250 operations per hour under AC-4 — the heavier inching/plugging duty. If your application cycles a motor more than once every 14 seconds under AC-3, or once every 14 seconds under AC-4, this contactor will exceed its mechanical design rate. The holding power of the DC magnet coil is 0.27 W, with a closing surge of 0.62 W — low enough to keep the control transformer small.
The screw terminals accept two conductors per clamp: solid or stranded from 0.5 to 1.5 mm², or 0.75 to 2.5 mm². Finely stranded wire with ferrule is handled the same way. The auxiliary switch block is field-addable — the product extension for auxiliary switches is supported.