Selecting the right equipment to perform the right task during design is a tedious job. A very good example is the variable speed drive (VSD) versus Soft Starter(SS).

On low capacity drives (<25kW), there is no significant cost difference between the two equipment. In high capacity drive application (rating above 1MW) and moving into the medium voltage (6.6kV - 11kV), there will be significant difference in the total cost of ownership between the two equipment.

Here are list of pros and cons for the VSDs and SSs.

Variable Speed Drives:

  • Pros
    • Variable speed
    • Variable torque
    • Low Starting current
  • Cons
    • High Initial cost
    • Issues of harmonics
    • Large footprint
    • Requires ventilation to extract heat generated.
    • Special cables

Soft starters

  • Pros
    • Lower initial cost
    • Smaller foot print
    • No issues of harmonics
    • Higer efficiency
  • Cons
    • Higher Starting Current at 2-3X full load current
    • Low starting torque - if increased torque is required, increased starting current is necessary
    • Bypass contactor required.

Other considerations that need to be considered is the capacity of the network supply.

The selection of drive is basically driven by the type of application. Water pump stations may require variable flow requiring the installation of variable speed drives to cope with the the usage and considering the effects of water hammer which cause cavitation reducing the life of pumps.