Motor Controls: VFD, Soft Starter or Servo Drive

VFDs (Variable Frequency Drive) refer only to the power stage of the system that can provide output of any magnitude and frequency in its range. This is an open loop system where by changing the frequency you can control the output speed of a motor. Without feedback, the accuracy of the regulation may not be precise. However for applications like changing a speed of fan or a waste-water pump where the small variation in speed is acceptable. VFDs have huge cost saving potential.

Soft starter - an electronic component which helps in reducing the inrush current during acceleration of a motor from zero speed to rated speed. Once the motor reaches its rated speed, it doesn't play much role.

Servo Drive - This is a closed loop system, which monitors current, voltage and speed to ensure that the commanded speed / torque is achieved. Essentially it is a control system which automatically change commands to the VFD such that the commanded speed / torque is achieved. Servo drives are required where high precision is required, e.g a CNC machine or a 3D printer.