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What is the difference between a VSD and a Soft Start Starter ?

The difference is the speed that the motor can run, when at full speed. With a soft start starter, the motor is reduced voltage started, and then when the motor is at full speed, or a timing circuit has timed out, a "running by-pass contactor" pulls in and the motor continues to run at full base speed. With a VSD (Variable Speed Drive) or VFD (Variable Frequency Drive) (these are the same thing, just different names for the same unit), 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 anytime, the correct solution is a VSD / VFD.
The cost for a soft start starter is much less than a VSD or VFD.