Do the Math: Efficient Motors Alone Don't Add Up

One of our consulting editors debunks some of the myths surrounding the supposed dramatic effect of efficient motors.

08/01/2001


It's a distinct probability that sometime in your career you've read an article on energy conservation and increasing motor efficiency. The whole point of these essays is to provide instruction or influence readers to get into the mindset to conserve resources and decrease pollution—noble causes. But in reality, there is a limit to how much can really be accomplished by designing more efficient motors while ignoring systems as a whole. Minor victory

If one were to examine the tables comparing standard and improved efficiency on say a 10-horsepower motor, the study would reveal that it is possible to reduce losses by utilizing an energy-conservative motor. Such an action, indeed, would typically shift the motor operation from 89 percent efficiency to 92.8 percent.

For the various loss components, the savings reduce a watt here, 26 there and maybe as much as 120 watts somewhere else. And when combined with other reductions, the savings achieve a whopping total of 405 watts.

Here's where most engineers miss the boat. By applying the same effort to reduce motor losses from 11 to 7.2 percent, engineers could easily save as much as 1,700 watts by focusing not only on the motor, but the device the motor powers.

Let's take a closer look. The total energy consumption of the aforementioned 10-hp standard motor is 10.4 kilowatts. Power usage divides into two components: 10.23 kW to drive the mechanical load and 1.17 kW lost to motor inefficiencies. Therefore, the true power consumption of the standard efficiency motor is really only 1,170 watts. If a 10-hp energy-conservative motor is substituted, motor losses can be reduced to 765 watts, but the same 10.23 kW of power is still required to drive the mechanical load.

This reduction is a minor achievement considering the power consumed to drive the load is unchanged because the efficiency of the driven load has not been modified. For example, say the driven load of the motor is a pump with an efficiency of 50 percent. This means that 5.115 kW will be lost in the inefficiencies of the pump.

Here's where the real energy savings opportunities can take place. By reducing the losses of the pump, overall energy losses can drop from 50 to 34 percent—a savings of more than 1,700 watts vs. the more meager 405 watts saved by increasing motor efficiency alone. On history's side

History certainly backs up the argument for a more overall approach. Take centrifugal chillers, for example. Not only do they demonstrate a documented case of true energy conservation, but they also provide a good opportunity to study the benefits of a more integrated approach to engineering design.

A quarter of a century ago, most large chillers operated at around 0.9 kW per ton of cooling capacity. But through a combined effort of improved motor efficiencies, changes in refrigerants and modifications to mechanical design, the same-sized chiller now operates at less than 0.6 kW per ton of capacity—a 33 percent reduction in energy usage.

The combined effort is significant in that if work focused only on increasing motor efficiency, the gain would have only been 5 or 6 percent because large motors already operate in the 94 to 95 percent efficiency range. Big picture

The few engineers who advocate increasing the efficiencies of the driven load vs. the efficiencies of electric motors alone have been muted too long. Designing and manufacturing electric motors even with 100 percent efficiency may result in only nominal savings. Real savings—and conservation—are achieved by utilizing an integrated approach to improving the efficiencies of the entire system.



Motor Efficiency Tips:

Reduce losses to the equipment being powered by the motor

Analyze the system as an integrated whole



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