The various ways of starting your electric motor show technological advancement. They are: Direct-On-Line (DOL), Star-Delta and Solid-State electronic devices.
1. A Soft Starter limits the surge of the inrush current thereby resulting in a smooth and gradual start-up while avoiding the complexity of Star-Delta wiring.
2. A VFD however goes an extra step by allowing the frequency of the motor to be varied and can get feedback on its state but without taking any action as a result of the feedback. A soft starter however only soft starts and stops the motor.
3. With a Servo Drive, physical properties like speed, torque and position of a motor are what can be varied. The feedback system in a servo drive compares the real and desired variables and automatically adjusts the value until the desired value is achieved.
As a PLC programmer, knowing how each of these drives work and their peculiarities is not first of all to prefer one over the other but to increase expertise on the scenarios where each drive can be used in the industry.