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With a soft start starter, the motor speedis reduced when voltage is started,and then when the motor is at full speed, a running by-pass contractor pulls in and the motor continues to run at full base speed.
With a VSD or VFD the motor will soft start, and you can vary the speed of the motor by varying the output frequency from the VSD or VFD. So, if you don’t need to vary the speed of the motor, once the motor is up to speed, then the correct solution is a soft start starter for that motor. If the process requires the motor speed to vary at any time, the correct solution is a VSD / VFD. The cost for a soft start starter is much less than a VSD or VFD.
Variable speed drives (VSD) allow the motor speed to be controlled on a continuous basis and like soft starters have integrated motor protection features.
A motor soft starter is a device used with AC electrical motors to temporarily reduce the load and torque in the power train and electric current surge of the motor during start-up.
The main difference between VSD and soft starter is that the soft starter doesn't provide variable speed control. This means that soft start is the most appropriate choice for applications where motor protection, a controlled stop, and/or the quality of the electrical supply are important factors but where speed control is not required.
