Variable Frequency Drive System Efficiency – Part 3

Well, I am finally getting back to finishing my series on Variable Speed Drives after another long hiatus.   The series started with a post that looked at drive and motor efficiency vs. load characteristics, including improvements that have over the years in drive efficiency.   Then, I did a post that looked at why folks involved with HVAC systems want to vary the speed  of motors in the first place and what some of the benefits are.

In this post, I’ll take a look at what happens when you combine a drive with a motor to serve a fan system.  In general terms, what I am about to present is based a little bit on published papers, a little bit on my field experience and a little bit on “playing” with the data I discussed previously in a spreadsheet.  In fact, that is where all of the images come from.

In many ways, the graph below is the bottom line on this particular post.

The Goes Intas vs. the Goes Outta’s

(Footnote 1)

In the graph above, the “fuzzy” red line is the air brake horse power expressed as kW.  In other words, it is the energy put into the air stream by the fan as calculated by the fan power equation.  Here is a the fan power equation for those who are not familiar with it. 

The pump power equation is very similar and all of the concepts I discuss in this post apply to pumps as well as fans.

You can find these equations and others like them in a number of resources including the ASHRAE Handbooks and pocket guides and Arthur Bell’s HVAC Equations, Data, and Rules of Thumb , which I discuss in a previous post.

In the context of this post and the graph above, the point is that the “fuzzy” red line represents the actual useful work that shows up in the air stream as air in motion at an elevated static pressure.  Note that in the graph, I have converted brake horse power to kW so that the fuzzy line can be compared directly with the solid line. 

Note also that this is the power and energy relationship for a constant volume, fixed fan system as the fan speed is varied while making no other changes  to the system and that the fan is not controlled for a fixed pressure at some point in the system.  Most of the systems we deal with in HVAC actually do control for a fixed duct pressure at some point in the system and that does make a bit of a difference in the curves I am talking about.  I will address that a bit further into the post but for now, I wanted to make a few points before adding that layer of complexity.

The Goes Inta’s Go Back Up While the Goes Outa’s Keep Going Down

Returning to the opening illustration, which I have reproduced below for convenience, the solid red line in the graph is the energy that went into the system to produce the result depicted by the fuzzy line.   It takes into account the losses in the motor, the losses in the belts, and the losses in the variable speed drive. 

This is where things get interesting.  As you might expect, because the motor and drive system is not 100% efficient, the energy input to the system is greater than the result that shows up as mass in motion.   But, as you might not expect, there is an inflection point in the “energy in” curve. 

In other words, at some point, even though the energy that ends up in the system as air in motion at an elevated static pressure is going down, the energy that must be put into the system to do that starts back up again.   I first became interested in this when I read an article that appeared in the ASHRAE journal back in December of 1999 by Michel A. Bernier and Bernard Bourret which was titled Pumping Energy And Variable Frequency Drives

Some Context with Regard to the Data

Before moving forward, I want to point out that while much of the data I tend to present on the blog is actual measured field data, the data that was used to develop the graphs is not.  Rather it’s calculated data based on:

  • The fan power equation for the air horsepower; i.e the energy that shows up as mass in motion in the system;
  • Published data from motor and drive manufacturers for the efficiency of the motor and drives;
  • Published data from Twin City Blower based on their fan selection software;
  • Industry rules of thumb for belt efficiency in a well adjusted belt drive.

In other words, the graphs are based on calculations performed using data measured by others, fundamental physical relationships, and accepted assumptions.  In addition, I believe I have observed the inflection point via informal testing int he field where I was experimenting but did not have time or the equipment necessary to capture the results.  The bottom line is that I am fairly confident that this is a real world phenomenon, not a theoretical phenomenon.

The Significance

The reason I think this is important is that the data says that there comes a point when using a drive to slow a fan or pump down starts to have the opposite result from what you intend, at  least in terms of energy consumption.   And,  while that may not be a big deal in a system that only sees minor variations in load, it could be significant for a large system or central plant that sees a significant load variation.

For example, if you examine the load profile I used to illustrate the 2nd post in this series, you will notice that the chilled water plant in question had hours at 50 tons and hours a 1,500 tons in addition to hours at many load conditions in between.  Incidentally, the data used to generate the load profiles was actual logged real time data from the system in question.

The phenomenon we are discussing could also be an issue for a system where a VFD had been installed and then adjusted to make a significant, fixed reduction in the operating capacity.

The Variables and Their Impact on the Inflection Point

In general terms, the inflection point is the result of the combined effect of the non-linear reduction in drive and motor efficiency that occurs as the output power requirement drops off.  The shape of  the curve and the location of the inflection point seem to depend on a number of factors.  The curve we have been talking about is based on:

  • A fairly current technology VFD that has efficiency curves like the 2008 Safetronics curves I discussed in the first post in this series.
  •  A fairly new, relatively high efficiency motor.
  •  A fixed, wide open system with no form of fixed pressure control in the discharge duct.

In the following paragraphs, I will look at what happens as you vary some of  these parameters.

Controlling for a Fixed Static Pressure

The following figure contrasts the curves we have been looking at for a fixed system that has no fixed discharge static pressure that it is controlling to, with the curves for the same combination of motor, drive, and fan operating at different fixed static pressures at some point in the system.   Operating at a fixed static pressure at some point in the system is a fairly typical way for a VAV system to be operated so these curves could be thought of as representing that situation with various set points for the constant pressure control loop.

Note the following:

  • The curves all pass through the same end point.  That is because it is the design operating point for the system and all other things being equal, the power in and out at that condition will be the same irrespective of the fixed pressure that is selected to control the fan speed.
  • The power out curves all go through “0”.  That is because the power out is based on the fan power equation and, if the flow rate in the numerator of that equation goes to zero, the result will be zero.  Bear in mind that the fan will still be using energy at this point because it is not 100% efficient and there are still losses in the bearings, via leakage from the discharge to the inlet, etc.  But, since no air is moving, no useful work is done.
  • Increasing the the fixed static pressure control point tends to shift the inflection point up and to the left.  This is a mixed blessing.  On the one had, it means that the minimum possible energy consumption for the system will increase as the set point increases.  On the other hand, it means that the system will need to see a larger turn down before the point will come where a reduction in the actual flow delivered causes the power required to deliver it to start increasing again.

 Using an Older, Less Efficient Motor

Using an older, less efficient motor has an effect on the shape of the curve that you might intuitively expect.  Note that in this graph, the only change from the original graph is a change in the motor.

As you can see, the inflection point shifts to the right a bit; i.e. you reach it sooner as you reduce flow.  But more significantly, the minimum achievable energy consumption for the system shifts up significantly.

Using an Older (2003 Vintage) Variable Frequency Drive

The result of using an older VFD is less intuitive.  Note that in this graph, the only change from our original graph is a change in the VFD.

Because the older drive efficiency starts to drop off at a higher load condition than the newer motor efficiency does, it starts to impact the power in curve significantly before the impact of the motor efficiency kicks  in.  In fact, if you study the drive efficiency curve relative to the motor efficiency curve, at the point where the drive efficiency starts to fall off rapidly, the motor efficiency curve is still flat at the peak efficiency point that most motors achieve in the 65-85% of full load range.

As a result, the power in curve has what is often  termed “a double whipsy doodle” or more correctly, “a double whoopty doo” in it (just seeing if you were still paying attention).  In practical terms, what I think this means is that if you are dealing with a system that has an older drive technology installed on it, there is probably a measurable payback associated with upgrading to a newer technology drive.  

But, that also means that if you did it, you would need to understand the specific efficiency vs. load characteristics for the drive you are working with in addition to the load profile of the system.  For systems with out much turn down, the savings will not be as significant as they would be fore systems with a significant turn down, espeically if a lot of the hours are spent at low load conditions.

If you did move forward, you would need to carefully specify the efficiency vs. load characteristics that you desired for the new drive since it will vary from manufacturer to manufacturer and technology to technology.   Otherwise, you could be in for an unpleasant surprise when the Measurement and Verification folks showed up and reality does not match your expectations.

It is also interesting to note that despite the “wiggle” (the actual technical term that applies) in the curve there is still only one inflection point.  In other words, the instantaneous rate at which the power input to the system drops off changes but the trend is till  down until a point around 10,000 cfm, where it starts back up again.

Using an Older (2003 Vintage) Variable Frequency Drive and an Older, Less Efficient Motor

Combining an older, less efficient motor with an older drive generally does what you probably anticipate it will do based on our discussion to this point.

But there is one subtle but important difference between this curve and the preceding one.  Specifically, this curve has two inflection points.  In other words, as the flow rate in the system drops off, the power required drops, then starts to rise, and then drops off again, and finally, rises a second time.  This all happens while the requirement for power (i.e. the air horsepower) steadily drops.

The other interesting thing to notice is that the calculations say that it would actually be possible for the system to use more energy at virtually no load than it does at full load.  Mathematically, this is simply the compounding of several very small (low) efficiency numbers.  In reality, the published data for the older drives stops at about 25% rated load, so my projections beyond that point are based on projecting the trend line for the published curves.  That means  they may or may not be right, but the trend probably is right.

The Bottom Line

For me, the bottom line on all of this is that while it is generally true that installing a VFD on a fan or pumping system will save energy, there are a lot of variables that come in to play if you want to optimize performance and efficiency.  If you pay attention to the details and really look at what happens in a specific application, you may discover an unrecognized opportunity.  And, if you don’t pay attention to the details, you may end up “shooting yourself in the foot” wasting energy that you thought you had saved.

David Sellers

Senior Engineer – Facility Dynamics Engineering

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  1. I would be remiss if I didn’t cite the inspiration for this subtitle;  it is Dr. Albert Black, who would often explain conservation of energy by saying The Goes Intas Gotta Equal the Goes Outtas.  Al is a mentor and friend from my college days and may be the fundamental reason I am working in this industry as it was he who suggested to Chuck McClure that he hire an Aircraft Maintenance Engineer (me) as a HVAC field technician.  Al is brilliant and has a wonderful way of taking complex subjects and making them easy to understand (all though I would have to say I followed his conservation of energy explanation more readily than the one he gave me about the Newton-Raphson iteration).
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2 Responses to Variable Frequency Drive System Efficiency – Part 3

  1. Dave Moser says:

    Great post, David! Thanks for taking the time to put that together.

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