In processes where variable speed is not an absolute necessity, but where users are concerned about energy savings and the impacts on electrical and mechanical systems of full-voltage starting, the choice between a variable speed drive (VSD) and a reduced voltage (i.e. “soft”) starter can be a difficult one. There are a number of factors which should be considered when making this choice. Several of these factors are described below, together with recommendations for the starting/control method to be used.

  • Overall system design efficiency: consider two piping systems: one sized to utilize pumps designed to operate at their “best efficiency point” (BEP); and one designed and built with excess capacity available, perhaps for future expansion. In the former instance, regulating process flow by controlling motor speed will typically not lead to enhanced process efficiency, and choosing a  VSD which will only serve to operate its motor at base (rated) speed will not gain you much in terms of energy use, either. However, a system built with excess capacity will typically suffer in efficiency when that capacity is not needed, so regulating process output by reducing pump speed may actually enhance efficiency, and can certainly reduce operating costs. So all other things being equal, the first system would be best served by a soft-starter, while the second one would benefit more through the use of a VSD. Note that both of these approaches would still provide a reduced voltage start, thereby decreasing the electrical and mechanical stresses which are placed on driven components when starting at full voltage.
  • Energy savings: whether a soft-starter or VSD is used, there is nominal reduction in overall energy consumption if the system is sized such that full speed is necessary most or all of the time. This small reduction can have a larger impact at locations subject to demand metering, because demand peaks can be reduced no matter which reduced voltage starting method is used. But in systems which can operate effectively at lower speeds, significant energy savings can result. In these cases, by using a VSD the user might not only get a  reduction in demand charges, but also the extra savings which follow from operating a driven load at less than full speed. The often-noted guideline is that, for centrifugal pump or fan systems, power is reduced by the cube of the speed, so that a 15% reduction in speed can result in 39% reduction in power consumption (.853 = .61). So in that case a VSD tends to be a better choice than a soft-starter.
  • Economics: any savings in energy consumption resulting from VSD use usually must be offset by the additional capital needed for VSD purchase and installation. VSD’s are by definition more complex components than soft-starters and require more training to operate. Depending on rating and supply voltage, they can cost anywhere from two to eight times that of a soft-starter. Often, however, the day-in, day-out savings resulting from reduced speed operation can reduce the ROI to one year or less. Again, the process dictates the speed reduction allowed, and thus governs the choice of reduced starting method.

This brief overview touches on only a few important factors figuring into the choice of a starting method. If you want to learn more, please visit our web site at www.joliettech.com, or contact us at info@joliettech.com or by phone at (866) 492-9888. And please feel free to share your thoughts in this blog’s Comments section. Have a great week, and please stop back by next time.

Jay Baima - Author

Regards,

Jay Baima
Joliet Technologies