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.