Soft start equipment with common current source inverter for several high voltage motors

Today, there is an evergrowing demand for frequency control and soft starting of three-phase induction motors with squirrel-cages. In addition to new installations, constant speed or hard starting (direct on line) drive systems, installed many years ago, can be adapted to the new operation requirements. In these cases, frequency control or soft start equipment is added to the motor. Mainly two different systems for soft starting can be applied: the three phase AC power controller; and the frequency converter. In the following, the design and application of control and power electronics for such special soft start equipment with frequency converter is described