Individual or ‘Static’ power factor correction capacitors can be used with soft starters provided they are installed on the input side of the soft starter and switched via a dedicated contactor only after the motor has reached full speed. The contactor should be AC6 rated for the motor full load current. Automatic or ‘Bulk’ power factor correction systems make use of a power factor controller to monitor changing power factor and automatically switch capacitors as needed. When used with a soft starter the automatic switching of capacitors should be inhibited until the motor is running at full speed. When a soft starter is installed in close proximity to a power factor correction capacitors
(less than 50m) and used without a main contactor, the switching of capacitors whilst the soft starter is not passing motor current can also lead to premature starter failure.The use of a main contactor is therefore recommended when;
• Multiple soft starters are installed along with static power factor correction capacitors.
• A soft starter is installed along with a bulk power factor correction system.
Connecting power factor correction capacitors to the output of a soft starter will cause equipment failure due to severe over voltage. This over voltage is created by resonance between the inductance of the motor and the power factor capacitance.