The difference can be found in the following white paper.

Hardstart-Softstart Comparison.pdf Pdf, 4 MB.

For permanent split capacitor motors, the torque is dictated by:

1. The current magnitude (more current means more torque) and

2. The phase difference between the run and start winding currents (a cosine function, where the max. torque occurs when the start current leads the run current by 90°). Both hard-start kits and SureStart manipulate the phase and current to change the start-up torque.

Hard-start kits change the current by temporarily adding a start capacitor in parallel with the run capacitor. This extra capacitance increases the current and phase angle on the start winding. The net result is a larger torque, but due to its simplicity, it does not regulate nor limit the current drawn by the motor. This results in a larger start-up current.

SureStart uses a start-capacitor to increase the phase difference but also regulates and limits the current flowing through the run winding. To accommodate for the torque lost due the smaller current, the soft-starter extends the starting-time (and increases starting energy) of the motor.