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What to Look for in Purchasing High-Efficiency Motors

Key Points
  • High-efficiency motors use less energy and can save substantially on operating costs.
  • Carefully assess your existing motors before replacing them with a higher efficiency model.
  • A motor efficiency improvement program can help to reduce long-term operating costs and improve overall system performance.

Source: U.S. Department of Energy
Inspecting a high efficiency motor
Motors consume about half of the energy used in commercial and industrial facilities. High-efficiency motors use less energy and can provide substantial savings in operating costs. Many facility managers, however, are reluctant to invest in new equipment and continue to repair their old, reliable, standard motor systems. A careful assessment of your current motor systems and an understanding of the potential benefits of high-efficiency motors can help take some of the fear out of the process of purchasing and installing new equipment.

What Is a High-Efficiency Motor?

Motor efficiency is the ratio of mechanical power output to the electrical power input, usually expressed as a percentage. High-efficiency motors use less energy to perform the same amount of work as standard models. Key design improvements and more accurate manufacturing tolerances are largely responsible for the increase in performance. Key design factors include:

  • Lengthening of the core and using lower-electrical-loss steel
  • Thinner stator laminations and more copper in the windings to reduce electrical losses
  • Improved bearings and smaller, more aerodynamic, cooling fans can also increase efficiency

High-efficiency motors are generally up to 8% more efficient than standard models. To qualify as high efficiency, a motor must meet or exceed the efficiency levels specified by the National Electrical Manufacturing Association's (NEMA) MG-1 standard for motors and generators. The following table provides  minimum efficiency values specified in the NEMA standards for common horsepower (hp) ratings, enclosure types, and speed combinations.

Horsepower

Open Efficiency (%)

Enclosed Efficiency (%)

 

 3,600

 1,800

1,200

 3,600

 1,800

1,200

 2

 84

 84

 85.5

 84

 84

 86.5

 5

 85.5

 87.5

 87.5

 87.5

 87.5

 87.5

 10

 88.5

 89.5

 90.2

 89.5

 89.5

 89.5

 20

 90.2

 91

 91

 90.2

 91

 90.2

 50

 92.4

 93

 93

 92.4

 93

 93

 100

 93

 94.1

 94.1

 93.6

 94.5

 94.1

 200

 94.5

 95

 94.5

 95

 95

 95

Note: Open motors allow free air movement, while enclosed motors restrict air infiltration.


Benefits of High-Efficiency Motors

High-efficiency motors may cost more, but many models can provide a return on investment within two years. For example, an energy-efficient, full-load, 50-hp motor with 75% load factor can save approximately 4,000 kilowatt-hours (kWh) per year over a standard model. At $.06 per kWh, the annual savings would be $240.

The improved design and construction of high-efficiency motors can lead to other benefits as well, including:

  • Longer insulation and bearing lives
  • Lower heat output and less vibration
  • Extended winding life
  • Increased tolerance of overload conditions
  • Higher tolerance for increased voltage rates or phase imbalance
  • Lower failure rates and extended manufacturer warranties

In certain applications—such as a pump or fan—high-efficiency motors may actually result in increased energy consumption because of their tendency to have less slip, or because they run faster. This is because the energy consumption is proportional to rotational speed in loads such as these. In these cases, the rotational speed may need to be lowered with a variable speed drive, gears, or pulleys.

When To Purchase High-Efficiency Models

Despite the benefits of high-efficiency models, motors are expensive and replacement of working motors may not be justified unless motors are oversized or you are making major modifications. Motors that fail can usually be restored. Many companies choose this less expensive option over replacement, and in some situations, rewinding is appropriate. Most motors that are half load (4,000 hours) or less, for example, can be rewound if the iron core specifications are still acceptable. Motors that are 50 hp or more should be rewound a maximum of three times. Replacement with a high efficiency model is typically more cost effective at this point. No motor should be rewound if the stator core is defective, or if the cost of the repair exceeds 60% of the cost of a new energy-efficient motor.

The purchase of high-efficiency motors is highly recommended for all new installations, and to replace failed standard models that operate at 4,000 hours per year or more. Make sure to specify high-efficiency models when purchasing equipment packages and use high-efficiency motors as part of any preventive maintenance program.

Starting a Motor Efficiency Improvement Program

A motor efficiency improvement program is a great way to coordinate efforts between maintenance and operations, and take some of the guesswork out of purchasing decisions. Start by performing a survey of your motor system, gathering information from nameplates, and obtaining operating measurements. These measurements include the following: voltage, amperage, power factor, and operating speed. Initially, focus on motors that are oversized and or have exceeded operating lifetime specifications.

Conduct a motor replacement analysis and categorize your motor systems into the following groups:

  • Immediate replacement—full-load motors which are inefficient or are not reliable due to age or disrepair.
  • Replacement at time of failure—part-load motors currently in good working order. While replacing these motors would offer an extended payback, they do not justify the cost of immediate replacement.

Like any successful improvement effort, a motor efficiency program requires follow-through and analysis. Establish a procedure to periodically check your motor system and track changes in energy use after motor upgrades.

 

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