Variable Frequency Drive System Efficiency – Part 1

Author’s Note;  2020-02-28:  The Functional Testing Guide – a resource I had linked to in the opening paragraph of this post –  no longer exists on the internet due to funding cuts.  However, we have posted the installation files for the guide on our commissioning resources website on the Functional Testing Guide Page.  Once you have installed the guide, you will find the resource in the AHU Reference Guide in the supplemental information associated with Chapter 10 – Fans and Drives.

Another resource that you may find helpful that did not exist at the time this post was written is Appendix 6 of the National Building Control Information Program (NBCIP) Return Fan Capacity Control Reference Guide.  You can find a copy of the guide on the NBCIP Reports page of our commissioning resources website.

In addition, the January/February 2020 issue of Consulting-Specifying Engineer has a pretty good article on Variable Frequency Drives on page 24.  You will also find a link to a pretty good technical guide comparing different types of variable speed drives in a post I did last year that included a section about hydraulic variable speed drives.

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 1^st or 2^nd 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 video segment on YouTube of an experiment you can perform at home to demonstrate induction principles.

• A PowerPoint slide show in a public Google Documents folder that covers the basic theory behind three phase induction motor operation.

• 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

Click here for an index to previous posts