joefiegl:

VFD's are the only way to go.  Liquid rheostats are an old attempt at VFD's at a time when VFD's were non existant, then too expensive for general use.

Is this a new design application?  Tell us a bit about the environment where the equipment will go.  There are several design considerations that you need to think about with VFD's.

BobPE

Thanks for the response.

We are looking at variable speed pumping because we would like to simulate gravity flow, since there are sensitive downstream hydraulic issues that would make flow surges associated with constant speed pumping unattractive.  I knew that liquid rheostats were becoming obsolete, but I need to provide evidence to support our contention that VFD is a better option than liquid rheostats (greater efficiency, etc).  It seems by your response that liquid rheostats were used simply because they were available before newer technology (VFDs) improved.  Are there any examples of how the reliabilty of VFDs has improved since those types of issues (along with cost) used to be one of the main disadvantages of VFDs?

There is a magnetic drive variable speed drive on the market which has less complexity and apparently higher efficency than VFD.  I don't know much about it but have access to the information if you are interested.  The current maximum HP is I believe 1500.  
mac