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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.
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
- 933 Road 101
- Jeffersonville, Vermont 05464 USA
- hvac@xxxxxxxxxxxx
- 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@xxxxxxxxxx
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