WIRING OF POWER FACTOR CAPACITORS TO SOFT-START STARTERS

Yes, you can use power factor capacitors with soft-start starters, you just have to apply them in a different fashion. First let's look at how soft-start starters operate to see why we must apply the capacitor a little differently. Note: Before applying capacitors to a soft start, refer to the starter manufacturer for recommended installation.

SOFT-START MODES

Basic Soft-Start Mode
The motor voltage is gradually increased during the acceleration ramp time.

Soft-Start with Kickstart Mode
A kickstart, or boost, at the beginning of the voltage ramp. This allows the motor to develop additional torque at the start. For loads which may need a boost to get started.

Current Limit Start Mode
Used when it is necessary to limit the maximum starting current.

Dual Ramp Start Mode
Used on applications that have varying loads or varying torque requirements.

As you can see, each type of starting controls the voltage to the motor. In order to do this, the soft-start chops up the sine wave, changing AC voltage to DC voltage and then back to AC voltage. As the sine wave is being chopped up, it is creating harmonic currents.

If we apply a capacitor to this motor circuit, the capacitor becomes the lowest impedance in the motor circuit. When this happens, most of the harmonic currents will flow to the capacitor, causing the capacitor to draw more current than it normally would. Consequently, the capacitor fuses will blow or the capacitor cell itself will fail. So how do we keep the capacitor from doing this?

When we look at soft-starters, the time it takes to reach full speed can be anywhere from 0 to 30 seconds. So what we need to use is a capacitor with a contactor (See figure below). When the startup is complete and the "up to speed" signal is energized, then we can energize the capacitor contactor. This will keep the soft-start harmonic currents from your capacitor.