When a motor is rising to full speed, there is an inrush of current that can be harmful to connected devices and can wear out machinery quickly over time. Two options to reduce the inrush are Soft Starters and Variable Frequency Drives (VFDs).
The biggest benefits of soft starters are the price and size. Generally, soft starters are much cheaper and smaller than VFDs. Their uses, however, are much more limited. Soft starters are strictly used in the startup of the motor. They limit the inrush to protect the AC motor by increasing the speed gradually until the full speed is reached. From there, bypass contactors take over and reduce motor heating. Because soft starters reduce torque in order to reduce voltage and current, they are needed when speed and torque need to be controlled during the startup. They are also well utilized when pumps in the system are able to remove pressure surges in the piping.
Variable frequency drives serve roughly the same purposes as soft starters plus many more. Not only can they be used in startup, but during the cycle and to stop the cycle as well. Because they are able to adjust frequency and voltages, they can more efficiently use energy. When less power is needed, motors can decelerate. This makes VFDs more energy efficient than soft starters, subsequently saving money in the long run. Pumps, fans, and conveyors are all adjustable along with communications to other motors and I/O modules. Additionally, software can be used with VFDs to give them functionality aspects of PLCs. Applications that require total control of speeds throughout cycles and machinery that calls for smooth motion (ex: Elevators) best utilize VFDs.
Soft starters are a good alternative for basic needs, limited space, or low initial cost, but VFDs are superior concerning energy efficiency and customizable speed control.