ASHRAE Journal - September 2010 - 13

The Authors Respond
We appreciate Mr. Schwedler’s points of agreement with our Emerging Technologies column on VFDs for large chillers and the associated remarks, which are certainly some of the more important points that we were trying to present. We will readily concede that boiling down the potential energy savings from applying VFDs to chillers to a rough percentage based on informal input from one manufacturer is an oversimplification of reality. The point of the column was to provide an overview of the possibilities for applying VFDs in this area, including the potential for significant energy savings, with no pretense of addressing any particular individual building application. Moreover, we think the article is clear that large chiller technology and the many circumstances and ways it is used is highly complex, and that every individual potential application of a chiller plant should be evaluated based on the specifics of that application. Mr. Schwedler’s point is well taken that with the modeling tools that are available, the task of evaluating each potential chiller plant application in detail is not onerous and should be undertaken on all jobs large enough to use large chillers. John Dieckmann, Member ASHRAE, Lexington, Mass. James Brodrick, Ph.D., Member ASHRAE, Washington, D.C.
technical topics

not sure I agree that the information is nowhere to be found. The change in electric motor efficiency with changes in loads is well documented. I use the curve printed on Page 51 of the December 1981 issue of Heating/ Piping/Air Conditioning, though I have seen similar curves published elsewhere. I fit that curve to an equation I use for spreadsheet calculations: Eff’y = 0.352 + 2.27(L) – 3.1(L2) + 1.41(L3) Where, Eff’y is the decimal part-load efficiency of a motor with 93.2% nameplate (full load) efficiency; and L is the motor load as a decimal fraction of motor nameplate horsepower. For motors with other full load efficiencies, I prorate by the ratio of nameplate efficiency to 93.2%. For modern pulse width modulated variable speed drives that use IGBT technology (as most do), I believe the efficiency is pretty constant at 0.95 to 0.97 over the full range of operating load. Multiplying the factors together gives an overall system eff iciency factor. If that infor mation fulf ills Mr. Rishel’s quest for combined motor/ drive efficiency with changes in load, I am glad to have been of service. Whether I answered his question or not, my question for him is: once engineers have workable information on combined motor/drive efficiency, what does he suggest we do differently from what we are doing now? Kenneth M. Elovitz, P.E., Member ASHRAE, Foxboro, Mass.

Wire-to-Shaft Efficiency The Author Responds
The Technical Topics column by James B. Rishel in July revisits a topic that Mr. Rishel has discussed many times: overall electrical efficiency of a drive/motor/ pump system. The key point in July’s column seems to be the dearth of published data for overall system electrical efficiency. I’m I appreciate this response. I am not an electrical engineer, and I have been told that multiplying the two efficiencies together will not provide a true wireto-shaft efficiency due to distortion of the sine wave by the VFD. Also, the efficiency of a variable speed motor is difwww.info.hotims.com/30919-18

September 2010

ASHRAE Journal

ASHRAE Journal - September 2010

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