Taming the Power-Hungry Beast
The amount of energy consumed by electrical motors in the U.S. is staggering. A 2006 U.S. DOE report found that within the industrial sector, more than 62 percent of the total electrical energy use is for motor-driven equipment. This industrial sector motor use equates to about 17 percent of the total U.S. electrical energy use. More recent data from the U.S. Energy Information Administration show nearly half of the electricity consumed in the manufacturing sector is used to power motors for fans, pumps, conveyors, compressors and other ancillary equipment. Such motors are ubiquitous in an ethanol plant, from 1,000 HP on down to the smallest.
For more than three decades, federal law has regulated the efficiency of new motors. The Energy Policy and Conservation Act of 1975 required DOE to establish the most stringent standards that are both technologically feasible and economically justifiable and to periodically update these standards as technology and economics evolve. The U.S. Energy Independence and Security Act of 2007, famous in the biofuels world as the legislation in which the second and current version of the Renewable Fuel Standard was passed, updated the EPAct standards starting December 2010 to NEMA-premium, including 201 to 500 horsepower motors. EISA assigns minimum, nominal, full-load efficiency ratings according to motor subtype and size. NEMA is the National Electrical Manufacturers Association, and its standards are adopted by the federal government to establish efficiency regulations.
As of last summer, an updated standard established by DOE in 2014 is broadening the minimum efficiency of a variety of new motors sized at 1 horsepower and up. “When regulations first moved to NEMA-premium, there were exceptions that didn’t necessarily make sense,” says Dale Basso, NEMA vice chairman and WEG Electric Corp.’s low-voltage motors product manager. “There are things in the NEMA-premium requirements today that were not in the regulations in 2010—things like footless motors. It didn’t make sense really why those were exempt. So we were building two designs, one for footless and another for footed. But when the Round 2 changes came into effect in June, they moved that and other stuff into the regulations.”
Round 2 regulations, also called the Integral Horsepower Rule, include motors sized as low as 1 horsepower and incorporate “almost everything except things like special shafts,” Basso says. The new rule means manufacturers cannot produce new motors for sale in the U.S. after June 1, 2016, that do not meet the NEMA-premium standards.
Below the NEMA-premium rating is what’s referred to as high-efficiency. A NEMA-premium rating is two efficiency bands above a high-efficiency rating, according to Basso. The difference between each efficiency band, which is a fixed, nameplate assignment, is 10 percent loss. “You can’t do a 96 percent or 96.1 percent efficient motor,” Basso says. “But 96.2 is an efficiency band, and 95.8 is the next one down.”
DOE’s analyses estimate lifetime savings for electric motors purchased over the 30-year period that begins in the year of compliance with new and amended standards to be 7.0 quadrillion Btu. The annualized energy savings—0.23 quadrillion Btu—is equivalent to 1 percent of total U.S. industrial primary electricity consumption in 2013.
Induction motors are basic creations that turn electrical energy into mechanical movement. Induction motors consist of a stator, which is the stationary component and made of electromagnetic iron or steel layers magnetized with copper winding and 2, 4, 6 or 8 paired poles, says Joe Hanna, marketing development manager for Toshiba International Corp. The stator features a bored center so the rotor—the rotating portion of the motor—can fit inside. The stack length of the motor is determined by how many layers of laminated, stamped steel are in the construction of the stator and rotor, says Patrick Standley, business manager of paper and forest products at Baldor Electric Co. The rotor also is outfitted with electromagnets with paired poles facing toward the stator poles. As the alternating current electricity passes through the copper winding, the stator poles alternate and the rotor poles move to catch up. This occurs several times a second and creates mechanical rotation energy to power almost anything needed.
Toshiba designs its medium-voltage motors differently than the standard design, Hanna says. “We have a form-wound design. It’s not just standard wire. We use multiple wires and we put them all together and wrap with a certain insulation, like a cable. The cost of our form-wound designed motors is higher than the standard induction line.”
The way manufacturers such as Baldor make motors more efficient revolves around material quality, contents and stack length, Standley says. “It’s about the ability to cool. When a motor is rated at 96.2 percent efficient, it means 96.2 percent of the electricity consumed is converted to mechanical energy, while the other 3.8 percent is lost as excess heat or friction. Therefore, the ability of a motor to keep cool means less loss to heat and greater efficiency.”
When motors wear out, Standley says many industrial facilities have what’s called throw-away horsepower, meaning motors less than a designated horsepower are replaced while those that are higher are rewound, or otherwise repaired. “In rewinding a motor, you can lose efficiency in the process,” he says. “And if a motor is rewound several times, it’s not going to be close to the original efficiency.” While many of these larger motors in the field have a 20-plus year lifespan, certain wear items such as bearings and seals have shorter lifespans. “If you choose to repair, you need to replace the bearings and seals,” Standley says. “And they need to be greased in day-to-day operations. Lube,” or the lack of, “is the biggest failure point.”
When choosing a motor for a particular application, it’s important to look at the situation from all different angles, Hanna says. “One is the power line and what’s available for the motor to start,” he says. “The second is amps draw. We say, based on the application, we expect the motor to draw 200 amps. Then we look at voltage, load type, starting conditions, load inertia and how you are going to start the motor in order to select the right motor and design for that application. But it always depends on the voltage and the power line coming in.”
Load capability is a relationship of speed and torque. “Horsepower equals torque times speed divided by a constant,” Standley says.
The power factor, a function of load, is important, too, Basso says. Power factor is the ratio of the actual electrical power dissipated by an AC circuit to the product of the root mean square values of current and voltage. The difference between the two essentially represents wasted power. “When it goes down, the load goes down,” Basso says. “If you have a bad power factor, it looks like a bigger number than it is, so you size the equipment to match the kilovolt amps supplied to them. So you’re not just looking at kilowatts.” A good power factor, or the proper utilization of the electricity supplied through proper planning and equipment sizing, will benefit the plant. “You have a choice when buying a motor,” Basso says. “If you know you’ll only be running at 75 percent load, don’t worry about oversizing it.” What’s more important is the power factor, he says. “If you’re trying to size it close to the horsepower, then you need to look at load and size,” Basso explains. “Everyone has their own philosophy on how to operate, but if you size it right at the horsepower needed, there’s the risk of shutting down.” He says if a larger motor than the load requires is bought and installed, it costs more but, on the plus side, it will stay cooler and will have a longer life. And less load on an oversized motor shouldn’t affect the efficiency.
Though NEMA-premium regulations went into full force last summer, manufacturers were already providing motors that surpass these standards. “WEG makes some models that are two more efficiency bands above NEMA-premium,” Basso says. “We call it SuperPremium.” The downside, he says, is that it’s a Design A. Different motors with the same nominal horsepower may have different start current, torque curves, speeds and other variables. NEMA classifies motor designs into A, B, C and D. With WEG’s Design A SuperPremium, Basso says the purchaser should review the starter and cable sizing before installing so it doesn’t present problems because it draws more current. “A plant may already have Design A, but they have to know,” he says. “You can’t go blindly into it or you may trip the breakers since it’s got a high starting torque.” Also, direct-drive permanent magnet motors that eliminate the rotor and its inherent losses are advances that will help move motor technology forward into even further efficiencies, Basso adds. And unlike conventional induction motor technology, which usually requires larger sizes for increased efficiencies, direct-drive permanent magnet motors can gain efficiencies while decreasing in size.
Standley says efficiency in conventional motor technology is close to being tapped out. “At 95-plus percent efficiency, how much more can you drive out?” he asks. “New technologies in construction material or ways to remove friction forces are constantly being reviewed.”
“The motor industry is not glitzy, it’s not high-tech,” Standley says. “But we’re constantly looking for ways to improve.”
Author: Ron Kotrba
Senior Editor, BBI International