Resources for the Resourceful: MotorMaster; Optimizing Motor Efficiency

Yesterday, I was using DOE’s MotorMaster software tool (illustrated below) as part of an interactive exercise in a class at the Pacific Energy Center and got to thinking that it might be a good idea to expose some of the tool’s capabilities to a broader audience.

Details on the capabilities of the tool can be found in the descriptive brochure that you can download from the web site, but in general terms, MotorMaster is a database of electric motor performance data representing an significant portion of the actual stock currently available from the various manufacturers.

The tool allows the user perform energy consumption analysis for different motor options, including life cycle cost analysis and/or developing and managing the motor inventory for their facility. These are features are made even more attractive by virtue of the fact that the tool is a free resource.

Over the course of the next few posts, I’ll look at various ways one might use the tool to help them make better decisions when confronted with a motor procurement or replacement issue. While the comparisons are based on the pumps serving the ice storage system in the Pacific Energy Center (PEC), the techniques and thought
processes demonstrated are illustrative of the type of analysis and thinking that can be used by designers, commissioning providers and facilities engineering personnel to find the “best right answer” (a quote I picked up from Tom Stewart) in the myriad of options confronting them when they make equipment selections.

For the current discussion, lets suppose for a moment that the PEC system is just coming out of design and you, as the designer or an owner or commissioning provider performing design review wanted to understand: 

  •  What is the range of efficiency availer for the nominal 1,800 rpm, Totally Enclosed Fan Cooled 5 hp motors that the circulating pumps will require?
  • How much room for improvement is there over the minimum efficiency threshold of 87.5% required by Title 24?
  • What is the cost and payback to improve efficiency over Title 24?
  • Are there other benefits associated with maximizing efficiency that may not be reflected by simply looking at simple payback?

MotorMaster’s “List” function allows the user to view a list of all motors meeting their criteria that are in the current data base. The graphics below illustrate an actual screen
shot of the data from MotorMaster as well as a table I developed to focus in on the key parameters of interest in the context of our discussion.

The list was generating by plugging in the requirements for the pump motor and hitting the search button. Note that there are several motors with efficiencies lower than the current Title 24 requirements and none that match it exactly.

The figure below illustrates the input screen used to enter data for our comparison. The catalog buttons allow the user to select and load data from the catalog screen shown above.  (It compares the motor highlighted in green with the motor highlighted in yellow in my table above.  We’ll talk about the motors highlighted in red and lavender in a subsequent post.

In the screen shot above, I have selected a motor that complies with Title 24 (left) and compare it to the lowest efficiency motor that meets the NEMA standards that define the Premium Efficiency class.

Note that the program has is using efficiencies for both motors that are different from the 100% load efficiency indicated in the summary table above. This is because the design requirement for the pump served will not fully load the motors and motor efficiency varies with load. The graph below shows this variation for a 25 horsepower motor that I happen to have test data for.

While the exact shape of the curve will vary from manufacturer to manufacturer and model to model, there are several general characteristics that, in my experience, seem to apply across the boards, at least for the squirrel cage motors we use in our industry.

  •  The relationship between efficiency and load is not linear.
  • Larger motors tend to be more efficient than smaller motors.
  • Efficiency will not vary much from the 100% load value until the motor is at or below about 50% of full load.
  • Peak efficiency often occurs at some load condition below peak load;  typically between 75% – 85% of full load, as is the case for the 25 hp motor I have illustrated.  

The last two points are particularly important to know because clients are often concerned if they find a motor loaded at less than its nameplate rating, assuming that it is therefore also operating at significantly less than peak efficiency.  In general terms, until the load is less than 50% of the nameplate rating, there is not much of an efficiency penalty associated with part load operation. The part load efficiency ratings published in MotorMaster for many of the motors in the catalog can be handy if
you need to convince a skeptical client.

In any case, returning to the problem at hand, clicking the “Savings” button on the input screen illustrated above generates the simple payback analysis shown below.

A simple payback in the range of 3 years or less is usually attractive to most Owners, and the reality is that the $90 cost premium associated with achieving it will be virtually undetectable in the bid for the project.

But to capture it, as a designer, you will need to specify the higher efficiency level as a requirement and then enforce those requirements rather than simply specifying Title 24 compliance. This can be easily accomplished by simply including a minimum motor efficiency column in the pump schedule.

What will be detectable is the future value of the energy savings achieved by optimizing the pump. The $33 in annual savings invested at 2% adds up to $570 to $1,060 over the
15-25 year service life of the pump .

Bear in mind that this is the value of the savings for optimizing just one 5 horsepower pump if it were invested in some sort of savings account bearing a relatively modest interest rate over its typical life as defined in ASHRAE and other sources.  Imagine what the accumulated savings might be if you performed a similar optimization on all of the HVAC motors on a project! And MotorMaster is just to the tool to help
you to it.

Incidentally, commissioning providers and facilities engineers performing design and/or shop drawing review for LEED™ projects or their own projects should target optimizations of this type of this type. Modifying the documents to reflect them as design requirements can achieve significant savings for virtually no additional cost in many cases. And making sure that the intent of the contract documents is reflected in the equipment provided for the project is critical to ensuring success. Small differences between the design and reality can add up to missing the desired efficiency target!

In subsequent posts, I’ll look at other ways to use MotorMaster as a commissioning and operations tool when evaluating existing motors in the field for repair or replacement.

David Sellers
Senior Engineer – Facility Dynamics Engineering

Click here for an index to previous posts

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