Efficient motor, inefficient system

Everyone wants to save energy. It's the “green” and “cool” thing to do. And the government may soon pay you to replace inefficient motors with new, energy-efficient NEMA Premium versions.

So, then, you might think that buying a premium-efficient motor is a no-brainer. But you would be overlooking something important: An energy-efficient motor is only one piece of the puzzle when it comes to saving energy. While premium-efficient motors do help reduce energy usage, they are by no means the cure-all to every energy consumption problem.

There are a number of misconceptions that surround energy-efficient motors. So it is interesting to explore some of them to be sure money invested in this area is spent wisely.

For one thing, extra efficiency doesn't always bring savings. Efficiency is a property, much like a color, or a material type; a value that doesn't change much. The Dept. of Energy certifies that a motor meets the “Premium” standard established by NEMA, based on that motor's ability to meet certain efficiency targets. So a NEMA Premium motor from one manufacturer will likely bring basically the same efficiency as a NEMA Premium motor from another manufacturer.

But simply installing a premium-efficient motor doesn't automatically save money, for several reasons. For example, your new motor may only be a few percentage points more efficient than the one it replaces; in cycling or intermittent-duty applications, the recognized savings could be so small as to be outweighed by the higher cost of the new motor.

It's important to look at the entire drivetrain when searching for ways to improve efficiency. It is possible that other parts of your drivetrain may be so inefficient that their effects swamp out the impact of a more efficient motor. And some kinds of energy efficient motors may not be well-suited to saving energy in your type of application, e.g. where there is high cycling. All in all, it's critical to evaluate your entire drivetrain for energy efficiency and remember that energy-efficient motors are just a single part of the efficiency equation.

The SEW-Eurodrive DRP NEMA Premium motor visible in this recycling application is noteworthy in that its internal construction is optimized to handle numerous stop-starts for applications characterized by 1,000 to 2,000 cycles per hour. It is also the only premium-efficient motor with built-in encoder and an integral brake.

A motor is only one component in the drivetrain. (And, truth be told, motors for some time have been comparatively efficient.) Each component in a system will inherently have some inefficiency, and these energy losses multiply to provide an overall system efficiency. Just one component that is relatively inefficient will quickly drag down the rest of the system.

Consider a theoretical example where six components in a drivetrain each have an almost-impossible efficiency of 99%. The product of the six 99%-efficient systems is 94.2%. Thus even in this example with six components of ideal efficiency, you still lose almost 6% of the energy that you started with.

Now, consider a more realistic example having six elements with efficiencies of 98, 96, 85, 75, 90, and 98% respectively. The 85% figure is pertinent because it corresponds to the efficiency found in older pre-Energy Act induction motors. The 75% figure is typical of geared drive trains. There are ohmic losses associated with wiring and other electrical components in the chain, as well as capacitive losses in the cabling. The product of the six efficiencies is 53.5%.

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