Semiconductor soft starter 3RW3 8.2.1 Physical principles Starting current Three-phase asynchronous motors have a high on current I (start) . Depending on the version of motor, this vary between 3 and 15 times the rated operat- ing current. 7-8 times the rated motor current can be assumed as a typical value.. I I Start I Rated I 0 (Idle) n Rated n Syn n M M Breakaway M Motor M Start M Pull-up M Load M Acceler M Rated M Motor M Load Fig. 8-2: Typical current and torque profiles of a 3-phase asynchronous motor n Rated n Syn n Note This starting current must be taken into consideration appropriately when designing the feeding network, e.g.
among other things by adapting the supply (high heat development) and also the fusing (inadvertent tripping of the fuses). Reducing the starting current There are various possibilities of reducing the starting current: by star-delta starter by frequency converter by soft starter 8-6 Stand 08/99