Soft start or VFD for high starting torque loads

There are a number of 'soft start' options suitable for high starting torque loads; star-delta, RVAT, and solid state starter (not the same thing as variable frequency drive - VFD). These are usually not recommended for ordinary centrifugal pumps, which don't have very high starting torque, but might be recommended for a vertical turbine with a long shaft.

Soft starting is also helpful when you have an emergency backup generator. Sometimes, the torque (and thus current) limiting from one of these can make the difference between a larger sized generator set and a smaller one, and the soft start equipment pays for itself. This is the kind of thing that an electrical engineer, cognizant of the mechanical factors, needs to analyze, in order to help the mechanical engineer determine the best configuration.

The number of starts & time between starts too should be considered when selecting starters. That's yet another issue. Larger motors may have limits on how many full-speed starts can be performed in a particular time frame. There are several ways to deal with this. One is to have a PLC refuse to start if the number of starts in the time frame has been exceeded. Using a soft start or a VFD can extend the number allotted. On very large motors, temperature sensors can be embedded in the windings, and you can simply refuse start if the winding temperature is too high.

This is usually somewhat independent of the mechanical characteristics, but high starting torque will decrease the number of starts or increase the time frame; i.e. it will change the heating curve. Again, you have to know the mechanical characteristics to know exactly what the rule is (or use temperature sensors, and then you need a rule).

Normally, this doesn't apply to motors smaller than 100 kW, in fact, it's not always good to choose a soft starter over a VFD.