[bldg-sim] motor efficiency vs. VFD speed

Tom Anderson hvac at cx-assoc.com
Mon Aug 18 08:59:09 PDT 2003

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
     Cx Associates, Ltd.
     Building Commissioning Specialists
     933 Road 101
     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
> 450 Lexington St, Newton, MA 02466
> p: 617-527-1525 x105  f: 617-527-6606
> e: astevens at dmiinc.com

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