Some of the largest opportunities to save energy and reduce operating costs in buildings and industrial facilities come from optimizing electric motor systems. About half of all electricity consumed in the US flows through motors, 90% of which are alternating-current (AC) induction motors. The US Department of Energy (DOE) estimates that, on average, the manufacturing sector could reduce industrial electric motor energy use 11% to 18% by using proven efficiency technologies and practices. In a single year, a fully loaded motor operating continuously can consume energy worth about 10 times its initial cost. That’s why even small improvements in efficiency can pay back quickly. The key is to choose the right-sized, energy-efficient motor and to integrate it into an optimized drivepower system.
What are the options?
The AC induction motor is the dominant motor technology in use today, representing more than 90% of installed motor capacity. Induction motors are available in single-phase and polyphase configurations, in sizes ranging from less than one to more than ten thousand horsepower (hp). They may run at fixed speeds—most commonly 900, 1,200, 1,800, or 3,600 rpm—or be equipped with an adjustable-speed drive. The most widely used AC motors by far have a squirrel-cage configuration—so named because of the shape of the rotor bar structure. Wound-rotor models, in which coils of wire turn the rotor, are also available. Although they are expensive, they offer greater control of the motor’s performance characteristics and are most often used for special torque, acceleration, and adjustable-speed applications.
As of 2012, under the Energy Independence and Security Act (EISA), all general purpose motors between up to 200 hp sold in both the US and Canada must meet the specification designated by the National Electrical Manufacturers Association (NEMA) Premium standard. These standards also require that NEMA Design B motors with power ratings between 201 and 500 hp have a full-load efficiency that meets or exceeds the energy-efficient motor standards. At these higher power levels, the energy-efficient designation still represents relatively high efficiencies, though you can choose motors from the premium category as well. Figure 1 shows a comparison of current NEMA premium and NEMA energy efficient categories to prior efficiency standards.
As of 2015, the DOE implemented new efficiency standards for fractional-hp polyphase and single-phase motors. If you are considering replacing your motors or doing retrofit work, motors manufactured after 2015 will be more efficient. For both polyphase and single-phase motors, the standards cover units operating at 1,200, 1,800, and 3,600 rpm. For single phase motor types that are capacitor-start/induction run and capacitor-start/capacitor run, the standards cover open units rated between 0.25 and 3.00 hp.
In retrofit situations, users have the choice of repairing old standard-efficiency motors or replacing them. It’s common practice among energy-conscious companies to replace all failed, moderate-duty induction motors up to about 125 hp with new premium-efficiency models rather than repairing and rewinding the failed motor. If not done carefully, rewinding can decrease motor efficiency by up to 2%.
To make sure that expected energy savings materialize, you should consider choosing a motor that has an efficiency band marking on its nameplate that is one or two bandwidths of efficiency levels above the minimum full-load efficiency standard for premium-efficiency motors (Figure 2).
How to make the best choice
Determine cost-effectiveness AC motors are available in a range of efficiencies. Although the economics will vary by application, replacing an old standard-efficiency motor with a newly installed, premium-efficiency motor under typical operation will often pay for its price in reduced energy bills within a year or two. A quick calculation to determine motor savings is outlined below.
Consider downsizing when a motor is operating at less than 40% of its rated output. The following circumstances are opportunities for choosing premium-efficiency motors:
- When purchasing a new motor where lower-energy-efficient units can still be sold
- Instead of rewinding failed standard-efficiency or energy-efficient motors
- To replace an operable-but-inefficient motor for greater energy savings and reliability
It’s best to opt for premium-efficiency motors:
- In all new motor installations
- When major modifications are made to existing facilities or processes
- For all new purchases of products that contain electric motors
- When purchasing spare motors
- As an alternative to rewinding failed standard-efficiency or energy-efficient motors
- To replace oversized and underloaded motors
- As part of an energy management plan or preventive maintenance program
- When utility conservation programs, rebates, or incentives make retrofits cost-effective
Think systematically The full potential of an efficient motor can best be captured if it’s integrated into an optimized drivepower system. This may be difficult to do in retrofit applications, but it’s important when designing new systems, when all components can be right-sized from the start. Properly optimized motor systems often use less than half the energy of systems designed according to standard rules of thumb. To create an efficient drivepower system, select efficient, properly sized models of the equipment that the motor will drive, such as pumps and fans. (The DOE also offers a free Pumping System Assessment Tool that can help industrial users assess the efficiency of their pumping system operations.) For more information on sizing fans, see the guide on Fans. Check to see that pressure drops in coils, heat exchangers, or other auxiliary devices are optimized for good life-cycle economics. Use efficient, properly aligned belts, cogged belts, or direct-drive connections between the motor and the equipment to minimize power loss through friction. Select the right controls to regulate motor and equipment operation.
Buy the right size of motor Motors operate at their highest efficiency between about 60% and 100% of their full-rated load, dropping off sharply in efficiency below 50% loading. About one-third of motors in the field are so oversized that they operate below 50% of rated load most of the time. Motors only operate at their peak efficiency if they are sized correctly for the load they drive. In addition to operating inefficiently, oversized motors carry a higher first cost than right-sized units. They can also contribute to reduced power factor, which increases load on the building’s electrical system and can result in utility fees for low power factor.
Watch your speed When replacing an old motor with a new premium-efficiency model in fan and pump applications, make sure the new motor’s full-load speed is the same as or slower than that of the old motor (making certain, of course, that it meets the minimum speed necessary for the application). The energy required by many fan and pump applications varies significantly based on its rotational speed—increasing the speed by only 10% can increase energy use by more than 33%. Therefore, putting in a premium-efficiency motor that rotates faster than the old standard-efficiency one may negate predicted energy savings. It may be necessary to adjust fan sheaves or pump impeller diameters to achieve the correct motor speed.
Evaluate the cost-effectiveness of VFDs Variable-frequency drives (VFDs) are electronic or mechanical devices that allow a motor designed for single-speed operation to drive a load at variable speeds. By varying a motor’s load speed so that it closely corresponds to the load requirements, VFDs can reduce energy consumption, and in some cases, energy savings can exceed 50%. Variable-frequency drives—electronic VFDs that vary the voltage and frequency of the power provided to the motor—can also improve power factor and provide performance benefits such as soft-starting and overspeed capability. VFDs require a small amount of power to operate, so motors with an VFD consume more power at full load than single-speed motors. However, it takes very little time operating at part load to make up this difference. VFDs can be cost-effective in cases with average loadings as high as 90%, but an analysis should be performed for each individual case based on the time spent at part-load conditions and efficiency with and without the VFD.
Account for the motor’s impact on power factor Power factor is an indicator of how much of a power system’s capacity is available for productive work. Low power factor is undesirable because it increases the load on a building’s electrical system, and utilities sometimes charge customers a penalty for facilities with low power factor. Because power factor is lower when a motor is lightly loaded, be sure to choose the right-sized motor. You can also specify a motor with a high power factor, but such models sometimes have lower efficiency. The ultimate selection depends, in part, on whether a facility is subject to power factor penalty charges. A facility with a significant number of induction motors and a low power factor can solve the problem with premium-efficiency motors that are properly sized. If new motors are not an option, other power factor–correction methods are available, including static capacitor banks, rotary condensers, and static and dynamic volt-ampere reactive devices.
Consider the economics of repairing versus replacing your motor A comprehensive economic analysis of whether to repair or replace a motor should consider:
- First cost, life-cycle cost, cost of saved energy, or at least a simple payback analysis of the new motor
- The difference in nameplate efficiency between the failed motor and a potential replacement
- Actual versus nameplate efficiency of the existing motor (prior repairs may have degraded actual efficiency)
- Duty factor
- Electricity price and demand charges
- Motor capacity versus peak loading (oversized motors operate at low efficiency)
- Expected lifetime of the repaired motor versus that of a replacement
New premium-efficiency motors typically cost about two to three times as much as a repair job for motors up to 200 hp. The cost-effectiveness of repairs tends to improve at larger motor sizes because labor requirements increase more slowly with motor size than materials requirements for new motors do. Although a new premium-efficiency motor costs more than a repair, it typically pays back quickly in reduced energy costs.
Motor repair tends to be preferable to replacement when:
- The original motor is a premium-efficiency unit, and replacing it will achieve little gain in efficiency
- The application has a low duty factor (it only runs a few hours per year)
- The original motor is a specialty design and a high-efficiency replacement is hard to get
- The repair shop can guarantee and verify that its repair will not degrade efficiency
Develop a basic motor management planThe simplest approach is to lay out basic decision-making rules that would apply to all motors in your facility. For instance, you might decide to replace all failed motors below a given horsepower threshold with the highest-efficiency models available. You might also specify that you would repair all motors above this threshold unless the cost would exceed 50% of the price of a replacement motor. In either case, you would want to work with your local motor distributors to ensure that they will have appropriate energy-efficient replacements available. Because a simple motor management plan does not consider the specifics of each motor in your facility, it may result in less-than-optimal decisions in some cases. However, it does have the advantage of being easy to develop and easy to implement.
Focus on critical motors Another approach to motor planning is to focus solely on the largest motors or those that are most critical to your operations. After collecting detailed information on each of these motors, you would decide ahead of time whether to repair or replace each of these motors should they fail. In some cases, you may find that it makes economic sense to replace one or more of these motors with a higher-efficiency model immediately, without waiting for failure to occur.
The survey of critical motors should collect the following information:
- Motor horsepower
- Design and code letter—for example, type A, B, C, D, or E—to define inrush current and torque, respectively
- Type of enclosure—for example, totally enclosed fan-cooled (TEFC) or open drip-proof (ODP)
- Frame size and special mounting features—for example, C-face
- Full-load efficiency
- Full-load speed
- Where the motor is located
- Motor application
- When the motor was put in service
- When the motor was last repaired
- The name of the shop that last repaired the motor
- How many times the motor has been repaired or rewound, and why
- Motor loading and operating hours
Create comprehensive motor inventories This is the most demanding type of motor management plan because it requires recording nameplate and operating data for every motor in a facility.
What’s on the horizon?
AC induction motors have long been the industry workhouse, benefiting from incremental efficiency gains over the years. As efficiency standards continue to tighten, “super premium-efficiency” motors may become mandatory. AC induction motors are near their technological limit and cannot reach these new standards without adding high-cost materials. Therefore, industry personnel will be looking to different motor types such as permanent magnet motors or induction motors to make further efficiency gains. Permanent magnet motors are becoming more and more suited for higher hp operations, more efficient at variable speeds, and better able to provide continuous torque especially at very low speeds, making these optimal for energy-demanding applications in the future.
Who are the manufacturers?
The US motor industry is relatively fragmented—roughly 20 manufacturers make most of the integral-horsepower induction motors sold in the US, and none of them dominate the market. Many of the selected manufacturers listed here also sell variable-speed motors, which can help to match the motor to variable loads.
- A.O. Smith
- Brook Crompton
- Leeson Electric Corp. (part of Regal Beloit)
- Marathon Motors (part of Regal Beloit)
- Sterling Electric Inc.
- Toshiba International Corp.
- US Motors
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