For the line contactor of variable speed drive (ATV) & soft starter (ATS), What is its utilization category AC-1 or AC-3?
- The contactor feeding a VSD can be selected according to AC1. The input current of the drive is just the nominal current.
- While The contactor feeding a soft starter must be selected according to AC3. The input current of the soft starter can be 3-, 4- or 5-times the nominal current (depending on the settings).
Bypass or not, there is no difference for contactor feeding a soft starter. The starting current is always the same and the contactor must be AC3.
The bypass contactor itself can be selected according to AC1. This contactor is closed when the starting sequence is finished and the motor is running with nominal speed and nominal current.
The sequence of operation of soft starter is that the line and output contactor closes first ----> start command given to soft starter and the current start flowing through contactor----> The motor current reaches to full speed and bypass contactor is closed. In this sequence of operation, there shall be no inrush current druing contactor closing and opening, which is somehow similar to AC-1, but the contactor must be sized in accordance to the maximum current - in case of a VSD it is 150%, in case of a softstarter it is 400%.