Power Factor Correction: can it be used with soft starters? 

Individual power factor correction capacitors can be used with soft starters, provided that they are installed on the input side of the soft starter and switched in using a dedicated contactor when the motor is running at full speed. The contactor should be AC6 rated for the motor full load current.

Connecting power factor correction capacitors to the output of a soft starter will cause equipment failure due to severe overvoltage. This overvoltage is created by resonance between the inductance of the motor and the power factor capacitance.

PFC capacitors can be sized using the following formula:

kVA (Cap) = √3 x Vline x 0.8 x motor no load current