Whenever necessary the starting of a three-phase electric motor of squirrel cage, it should be direct in the network or full voltage, by means of vacuum contactors and / or circuit breakers. We should know that for a given motor alemite 8072-a the curves of conjugates and current are fixed regardless of the difficulty of starting, for a constant voltage. In cases where the motor starting current is high, harmful consequences such as high voltage drop in the mains supply system can occur, which will cause interference in equipment installed in this system, the power system (cables, switches, protection, transformer ) should be about dimensioning, increasing costs, determination of the electric utilities that limit the voltage drop of the network, as well as the limitation in case of isolated networks operating with generator sets.
If direct starting in the network is not possible, because of the problems mentioned above, we can use indirect starting system to decrease the starting current. These indirect starting, low voltage, frequency variation, and resistance systems are the star-delta key, the compensating or self-locking switch, the static or soft start starter, the frequency inverter and the rheostat for motors rings.