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.
Figure 1: Comparison of motor efficiency standards
As horsepower increases, so does motor efficiency. The standards implemented in 2012 have mandated a 5% increase in efficiency by now mandating NEMA’s Premium efficiency category. Though this may seem small, energy savings will add up quickly for motors that operate frequently.
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).
Figure 2: EISA minimum full-load efficiency standards
Replace NEMA design A and B three-phase low-voltage induction motors with units that meet premium-efficiency motor standards; the new standards mean that new units designed for service at 600 volts or less are rated from 1 to 500 hp. The standards cover products such as floor-bolted motors with speeds of 1,200, 1,800, 3,600 rpm that have open drip-proof, explosion-proof, and totally enclosed fan-cooled enclosures.