Variable Frequency Drive System Efficiency – Part 1

I recently was doing some energy savings projections that involved looking at what would happen if I added a Variable Frequency Drive (VFD) to a pump serving a hot water system.  In the course of doing that, I got to wondering if there had been improvements in  VFD efficiency that I should take into account since the last time I researched drive efficiency.

Time flies when you’re having fun, and I realized that the curve at the link, which is a generalized case of the following Safetronic performance curves that I have in my files, was from around 2002-2003.

The efficiency curves allow me to put numbers to something most of us realize, i.e. a VFD is not 100% efficient.  If nothing else, the power consumed by the fans used to cool the electronics represents an efficiency loss.  And then there are the losses associated with the solid state electronics themselves, that in many ways, are the heart of the drive.

VFD or Not VFD;  That is the Question

Before I move on, I want to clarify that I am talking about Variable Frequency Drives, which are typically referred to in the industry as VFDs.  They are a type of Variable Speed Drive (VSD in industry jargon).  It’s important to recognize that there are other VSD technologies out there, like eddy current clutches, hydraulic drives, and even “pulley pinchers”, an older technology that varied speed by changing the pitch diameter of a pulley in the drive train.

Some of these technologies might sound really exotic, but they are more common than you may realize.  For instance, if you have an automatic transmission in your car, you have a form of a hydraulic variable speed drive. And, if you are a wood worker and have a Shopsmith tool, you have a pulley pincher VSD sitting in your shop.

If you are really curious about the “pulley pincher” technology, which you can run into occasionally in an older building, the cutaway in the Shopsmith belt replacement manual will give you a glimpse into how it works.  The bottom line is that all VFDs can be considered VSDs and probably can even be considered AFDs (Adjustable Frequency Drives).  But not every VSD is a VFD or AFD even though AFDs are generally VFDs, which means they are also VSDs.  (Confused yet?)

VFD Efficiency Improvements Since 2003

Surprisingly, there is not as much information out there about VFD efficiency as you might imagine. Generally, it seems to be assumed that applying a VFD will save energy in addition to providing a desirable control function.  And while that is generally true, there are some interesting things that happen at part load that can work against the general case.  More on that later in the next post, but first, I wanted to share a few resources I discovered when I went looking for more current information.

As I suspected, there have been improvements in drive efficiency since 2002/2003, and one resource that summarizes them in more current time frame is the U.S. Department of Energy Industrial Technology Program, which provides a number of very useful resources like software tools, guidelines, and tip sheets on their Best Practices web page.

With respect to our current discussion, Motor Tip Sheet #11 – Adjustable Speed Drive Part-Load Efficiency, dated June 2008, provides more recent Safetronics data on drive efficiency.   The graph below contrasts selected drive efficiency curves from my old data with the newer data in the motor tip sheet.

As you can see, there have been some significant improvements, especially for lower horsepower drives and at the lower load portion of the drive efficiency curve.  This has some important implications, which I will get to in the next post.  First, I wanted to share a couple of other useful resources that I discovered in my search.

Additional VFD Data, Research, and Information

Bear in mind that this does not represent an exhaustive search of the literature, just what I found after poking around for half an hour or so, which answered the questions I had.

Oregon State University Research

Energy efficiency of Variable Speed Drive Systems, published by James A. Rooks, P.E. and Alan K. Wallace, Ph.D.  summarizes and extrapolates information from testing that was done in 1999/2000 by Oregon State University at their Motor Systems Resource Facility.  It then lobbies for additional research on drive system efficiency.

Of particular interest is their exploration of the drive efficiency data published by the Industrial Technologies Program at the time, which was based on one manufacturer (I’m guessing Safetronics).  There effort including comparing the Industrial Technologies Program data to similar data from other manufacturers.  Their conclusion was that the efficiencies were similar from manufacturer to manufacturer, with out regard to the type of VFD.

The bottom line for me was that there conclusion make me reasonably comfortable using the data I have generically on VFDs, especially for Pulse Width Modulation (PWM) type drives.

Cal Poly Research

Electric Motor Efficiency under Variable Frequency Loads, published by Dr. Charles Burt, Dr.Xianshu Piao, Franklin Gaudi, Bryan Bush and Dr. NFN Taufik at Cal Poly reports results from testing conducted to assess the efficiencies of motors when operated at various loads and speeds by a VFD.

The Cal Poly research suggests and/or demonstrates that:

  • VFDs can improve motor power factor;
  • That motor efficiency is generally unaffected by the use of a VFD;  the inherent motor efficiency is probably more important than variations introduced by the drive as is the degradation in motor efficiency that is generally seen at low loads, with or without a VFD (more on that in a minute);
  • For variable speed drives that are applied in conditioned areas, the losses from the drive become an air conditioning load that will require power that would otherwise not be consumed if the VFD was not there.

Motor Efficiency

The Cal Poly research conclusion about the efficiency of a motor when served by a VFD is important to recognize since, at least in our industry, we are usually applying the drive to serve a motor.

Having said that, a motor efficiency curve is generally like the drive efficiency curves we have been looking at;  its not constant with load.  In fact none of the motor performance factors are constant with load, as can be seen from this graph that I developed from the performance curves I obtained for a 5 hp motor via MotorBoss.

MotorBoss is a great resource;  in a matter of a few key clicks, with about a 15 to 30 minute wait, you can have a complete set of motor performance curves and submittal data for any motor in the U.S. Motor inventory.  And, its free.

If you don’t have a U.S. Motor, then MotorMaster, a software resource from the Industrial Technologies Program is the way to go.  It contains a fairly current data base of most of the motors in production currently, and frequently includes efficiency and power factor at conditions other than part load.  Here is an example for a low efficiency 30 hp motor I picked to contrast with a higher efficiency model.

In any case, when you start looking at data for different motors, the general trend is for things to improve as motor size increases, as can be seen from these curves I made from Gould test data I had in my files.

One of the more interesting things about motor efficiency is that it generally increases as a motor unloads before it starts to drop.   Most three phase induction motors will actually be more efficient at about 70% – 80% load vs. full load.

Motor efficiency is not a one size fits all proposition.  For instance, if you ask MotorMaster about a what’s available in a 10 hp, NEMA Design B, 1800 rpm, 460 volt, 3 phase, open drip proof motor you get 29 options with an efficiency range of 89.5% to 93% and while best efficiency is most expensive, you can buy 92% efficiency for about 10% less than 90.2% efficiency, at least in the list pricing contained in the data base.

Where I was headed with all of this was to say that its not out of the question that a 10 hp motor operating at 7.5 hp will be a bit more efficient than a 7.5 hp motor operating at full load.  Just something to think about before you suggest someone buy a new, more efficient motor because their 10 hp pump motor is now only running at 7.5 hp since you trimmed the impeller.

If all of this has tweaked your interest in induction motors, you may be interested in the early release materials from a project I am working on with my friends at the Pacific Energy Center.

Our goal is to create “on demand” content to support the technical classes we teach there.  It will be available for public access at no cost on the internet sometime in the 1st or 2nd quarter of 2011.

While the actual web based content is not yet finished, some of the video and PowerPoint files that will be behind it are.  So, we’ve posted them on YouTube and Google Documents for folks to use while we finish our development project.  The video is narrated but the slides currently are not.  Ultimately, they will be and if you monitor the PEC’s web site, you eventually will be able to access them in that form from that location.

The following content is available

  • A PowerPoint slide show in a public Google Documents folder that covers induction motor characteristics, including how variables like efficiency, speed, power factor, kW, and amperage vary with load, a discussion of torque vs. speed curves, and a discussion of how the moment of inertia of a load interacts with the motor torque characteristic to accelerate (or not) the load.

You can view the slides on the Google Documents web site, but you can also download the presentation and view it on your laptop.  The advantage to the latter is that you can run it as a PowerPoint show, which will allow the animations that are built in the slides to work.  This may make some of the points easier to understand vs. viewing the un-animated slides.  If you don’t have PowerPoint, you can download a free PowerPoint viewer from the Microsoft web site.

Well, I need to go help Kathy with the Christmas tree, so I’ll close this for now.  In the next post, I’ll look at what happens when you look at the over-all efficiency of a motor, a drive, and the load they serve.

David Sellers
Senior Engineer – Facility Dynamics Engineering

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10 Responses to Variable Frequency Drive System Efficiency – Part 1

  1. Terrific piece. We’d like permission to reprint in our industry magazine, DrivesMag. Of course, credits and links provided. Can you help us with image files?

    • Thanks for the kind words. Reprinting it would be fine. All of the images are screen shots from the spreadsheet I used to make the graphs except for the MotorMaster detail, which ia screen shot of the output for the selection I made. I could just send you the spreadsheet for that stuff; not sure what the best way is for you to do the MotorMaster data, but it may be fasted for someone there to make the same selection and capture it that way.

      Let me know what you think; Thanks for visiting the blog.

      David Sellers
      Facility Dynamics Engineering
      Northwest Satellite Office
      8560 North Buchanan Avenue
      Portland, Oregon 97203

      • Hi David,

        No need to send the spreadsheet. We’ll post with your native images. Also, thanks for V2. We’d like to run that next week, with permission. This article will be featured later today, and will go as an email supplement to our email subscribers either today or tomorrow. Nice work!

        -DMag Ed Team

  2. Note: it would be helpful to have your text in file format. Would you be able to email to

    Thanks again

  3. Gareth Jones says:

    Great post and raises as many questions as it answers. In Europe there is allot of work going on with regards to rating the efficiency of VSDs, this is not as easy as it sounds. Here is an extract of a comment from one of our E&D engineers:

    “Developing this standard will be a difficult technical challenge. Measuring the efficiency of a VSD with sufficient accuracy and repeatability is quite difficult. The efficiency is so high (e.g 97% – 98%) that the simple idea of measuring the input and output powers and taking their ratio demands instruments and transducers with accuracies approaching the limits of what is available. This is before we have addressed the more general questions about which operating conditions should be covered and how the results should be weighted. There is also a good argument to be made that it is the power loss which should be the headline figure – it is the loss which you pay the power company for and which burns the extra carbon!

    The danger with a simple efficiency rating for the VSD is that it ignores the vital system-level view. The loss in the VSD is in the region of 2-3% of rated throughput. The loss in the motor is between about 20% and 5% depending on the rating (larger motors are more efficient). Because of the PWM (pulsed) output voltage, the VSD always causes some increase in losses in the motor. A well designed and correctly applied VSD minimises the total loss between the power terminals and the motor shaft. An inferior VSD using a low switching frequency or an inefficient control algorithm might well offer lower losses within itself, but still cause higher overall losses because of its effect on the motor.”

    Another aspect of all this is the affinity laws ( dictate that in fan and pump applications where the speed of the motor can be varied instead of using bypass /damper systems, the potential for saving energy is enormous and massively outweighs the losses of the VSD, particulalry at low speed when the lossess are greatest. In fan and pumping applications, if you reduce the speed by 50%, you reduce torque and energy consumption to around 25%.

    • Hi Gareth,

      Thanks for your comments. I was hoping that the post would generate some discussion and feedback about what was going on out there with regard to drive efficiency and application. Some of the things you mention are where I am headed in the third part of the post, which I am still working on but hope to finish this week.

      For me, the comment that The danger with a simple efficiency rating for the VSD is that it ignores the vital system-level view is really the heart of the matter and not only for VFDs/VSDs. Things in general and building systems in particular are highly interactive, and in some ways, insidious. It’s really easy to “shoot yourself in the foot” and apply a technology that you think will do some good only to have the interactions with everything else waste more than is saved.

      We see that sort of thing all of the time when we are out commissioning things. In fact, I think its one of the big benefits of commissioning, especially if we take what we learn out in the field back to the drawing boards.

      In any case, thanks for your feedback and information. I hope you are having a nice holiday season.

  4. Dave Moser says:

    Hey David, I ran across the following (old) publication from the DOE on older VFD efficiencies. See page 7: The efficiency values seem to match up with your 2002/03 Safetronics data pretty well.

    • Hi Dave,

      Yes, in hindsight, I think its the same data. I came by it from a different source, but at one point, heard that the DOE data was Saftronics data.

      Take care,

  5. Dave Roberts says:

    He Mr Sellers,
    Very good article. I also wanted to see if I could include a copy of your second graph in a report I am developing? The purpose is to show how VFD drive efficiencies have, in general, improved over time.
    Thank you,

    • Hi Dave,

      Thanks for asking; that’s fine with me. The data is from DOE and I believe has its roots in information published by Saftronics. So, its important to realize its the trend for their drives under the mode that they were tested in and may be a general indicator of a trend. But it may not be the exact case for a specific drive by a specific manufacture; i.e. tor an exact answer you need to look at the details of the drive you are working with. I’ve been doing some more research and talking with a few people and once I know a little more I will be donig another post. Meanwhile, feel free to use the graph if it is helpful to you.



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