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Do high efficiency motors always save energy?
February 11, 2008

This question comes from Lynn Wheat, senior project engineer at AOC, a global supplier of resins, gel coats, colorants, and additives for composites and cast polymers, who earns an “Engineer, and Proud of It” pocket protector for sending it in.

Lynn, who was responding to a sidebar in Peter Welander’s January 2008 article entitled “Energy as a process variable,” went on to discuss AOC’s recent review of high-efficiency (HE), or premium, motors: “We found that simply replacing an existing motor with a premium efficiency motor was not always an energy savings.”

Control Engineering has covered HE motors since before the Energy Policy Act of 1992, which defined and popularized the category. (Visit the Control Engineering website and search on “high efficiency motors.”) Electric motors sporting high efficiency didn’t appear out of nothing in 1992. That was just when they popped up on the U.S. government radar. Every motor ever made has (had) an efficiency associated with it. Sometimes they’ve been high, and sometimes not.

So, what do we really mean by an HE motor? There are two official types, and their classification depends on the ratings claimed on their nameplates. The earliest HE classification—EPAct—came out of the original 1992 Policy Act, which gave the U.S. Department of Energy (DOE) authority to set minimum efficiency standards for certain classes of electric motors. Essentially, DOE based its standards on the best in class of what was available at the time. Later, in 2001, the National Electrical Manufacturers Association (NEMA) created a designation called “NEMA Premium” for more advanced motors then available, which exceeded the EPAct standards.

It is important to recognize that electric motors are none-too-shabby in the efficiency department to begin with. The Copper Development Association (CDA) Website contains a most interesting white paper discussing HE motors, their history, and comparisons of the two types versus standard motors.

Depending on motor size, the paper lists average efficiencies for standard motors between 84% and 91%. Compared to most mechanical systems, that is nothing less than spectacular! Brayton cycle gas turbines, for example, run at well under 50% efficiency. Most gasoline-fueled internal combustion engines have a mechanical efficiency of about 20%. Nuclear power plants waste two thirds of the energy in the fuel they use.

When you’re dealing with a device whose energy efficiency is 85-90% to begin with, there isn’t much room for improvement. The CDA paper shows EPAct motors running 3-4 percentage points better than standard motors (88-94%), and NEMA Premium motors adding another 1-3 percentage points onto that (91-95%). The biggest gains come with the smallest motors.

While it is possible to argue that any improvement is an improvement, you can’t just read numbers off a nameplate and expect gains. Motor efficiency is not a single number. It’s a moving target that depends on the motor construction (the only thing a manufacturer can control), and how it’s used. The CDA numbers, for example, are measured at 75% output. What might the numbers be like at 50% output, or 90% output? What if your application, like most electric motor applications, presents a highly variable load?

For example, a very important consideration is actual motor speed. Both AOC’s analysis and the CDA paper make the point that part of the HE motor’s energy savings come from reduced rotor slip. Rotor slip compares the motor’s output speed with that of the stator’s magnetic field. Reducing rotor slip raises the actual output speed as well as the efficiency. That increases the net energy usage in many types of loads, such as fans and pumps. So, you might increase efficiency without reducing power usage… just use more energy at the higher efficiency!

Simply swapping an HE motor with the same output and speed ratings for you’re old standard motor may or may not lower your electric bill. It might just as easily raise it, or leave it unchanged. What you really need to do is redesign the application with the HE motor in mind, and efficient operation as the goal. That strategy really will net overall cost-of-ownership returns, as well as energy savings.

Posted by Charlie Masi on February 11, 2008 | Comments (1)