[bldg-sim] motor efficiency vs. VFD speed

Jon McHugh mchugh at h-m-g.com
Mon Aug 18 13:52:35 PDT 2003

```Alec,

Another great motors resource is Gil McCoy who works for the Western Area Power Administration (WAPA) Powerline http://www.es.wapa.gov/ta/powerline.cfm.  The WAPA Powerline is a great public service provided by Western and their experts have helped me many times over the past decade on a wide range of energy efficiency topics.

Most fluids books contain equations for estimating pump flow and power at different motor speeds.  The equations are  different for essentially fixed head applications (cooling towers, make-up water) versus systems where head is primarily a function of pipe or duct friction.   The same system can have areas where flow is laminar and other areas where the flow is turbulent.  Since most of the energy in these systems is the energy imparted into the fluid - getting this part right is more critical than the couple of percent variation you might find in motor efficiency.

Jon McHugh, PE, LC

-----Original Message-----
From: Tom Anderson [mailto:hvac at cx-assoc.com]
Sent: Monday, August 18, 2003 8:59 AM
To: bldg-sim at gard.com
Cc: bldg-sim at gard.com
Subject: [bldg-sim] motor efficiency vs. VFD speed

I have had the same question for years and years.  Unfortunately, the only motor part load efficiency curves available are for constant speed motors a various percent loading, where frequency remains constant.

To my knowledge... and this was confirmed by a recent Engineered Systems magazine article (August 2003).  In this article, "How to Calculate Motor Efficiency for Variable Speed Centrifugal Pumps" by Burt Rishel, Burt states there are no part load motor efficiency curves available in the industry.  I agree with Burt recommendation... they should be developed and would be quite useful. Burt is an elder pump guru who has been around since time began, as is considered by many as an authority on pumping systems.

Which is why Burt stated the best one can expect when comparing caclulated part load energy use on a variable speed pump (or fan) to field measured part load results are +/- 10%.

If by chance you should come across such curves, I'd certainly be interested in seeing them.

Thomas E. Anderson

President

Cx Associates, Ltd.

Building Commissioning Specialists

http://www.cx-assoc.com

Jeffersonville, Vermont 05464 USA

hvac at cx-assoc.com

Tel: 802-644-5616 Fax: 802-644-6797

Alec Stevens wrote:

Hello, Does anyone know of published research that relates motor efficiency to percent load as well as drive frequency?We have motor curves for efficiency vs. percent load, but these curves all assume a nominal speed that is being maintained as load decreases.  What happens to your motor efficiency when you are also changing the drive frequency? Thanks for any leads.Sincerely, Alec Stevens, PE
DMI
450 Lexington St, Newton, MA 02466
p: 617-527-1525 x105  f: 617-527-6606
e: astevens at dmiinc.com

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