a)Inrush current

Inrush currents exceeding the making capacity of contactors:


Current peaks often occur on the primary side of frequency converters and are caused by DC-link capacitors during the charging process. The magnetic force of such peak currents is sufficient to open the contacts of an unsuitably sized contactor. During opening an arc occurs that fluidises the contact material. Since the magnet system of the contactor is still energized, the contacts close again and weld together. Inrush current calculated formula given below


Inrush current=2 root 2*Rated current (Ie)





Fig:Inrush current




Solution: Only solution is to use oversized contactor having making capacity of a contactor given by formula.

Making Capacity=10 * Ie (AC-3)* 1.41

Ie- Rated current carrying capacity of contactor



.....
AC 3 is a type of duty of contactor.