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How to Avoid Overestimating Variable Speed Drive Savings
Abstract
This paper addresses eight factors that can cause
incorrect and often excessive savings estimates for
pump and fan variable speed drive applications. To
avoid overestimating savings:
1. Identify system elements that affect head
pressure independently of flow rate.
2. Identify system elements that change head
pressure in proportion to less than the square of
flow rate.
3. Account for dynamic system elements,
especially when in systems with minimum static
pressure controls.
4. Consider changes in fan efficiency.
5. Account for decreases in motor efficiency at part
load, particularly for smaller motors below about
35 percent load.
6. Recognize that existing part load controls may be
more efficient than expected.
7. Account for drive losses.
8. Measure full flow power, rather than assuming it
is the same as motor nameplate or design power.
For many pump and fan systems, none of the eight
factors will apply, or their effects will be negligible.
However, analysts should consider their applicability
when estimating savings for a particular system. This
paper provides tools for accounting for the factors.
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Citation
Maxwell, J. B. (2005). How to Avoid Overestimating Variable Speed Drive Savings. Energy Systems Laboratory (http://esl.tamu.edu); Texas A&M University (http://www.tamu.edu). Available electronically from https : / /hdl .handle .net /1969 .1 /5621.