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By Carrie Ellis
Wednesday, January 09, 2008

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More For Less

By Carrie Ellis, Assistant Editor

The growing trend toward energy efficiency has motors absorbing less energy, while maximizing output.

With the growing emphasis on going green, design engineers are bound to be under pressure to creatively seek out how to improve the energy efficiency in their designs

One trend that has swept the design engineering field is aimed at using the most application-specific energy-efficient materials.
With the growing emphasis on going green, design engineers are bound to be under pressure to creatively seek out how to improve the energy efficiency in their designs, while simultaneously cutting energy costs for the end-user. According to the Department of Energy (DOE), "Over half of all electrical energy consumed in the United States is used by electric motors. Improving the efficiency of electric motors and the equipment they drive can save energy and reduce operating costs.

For example, even at the relatively low energy rate of $0.04/kWh, a typical 20-HP continuously running motor uses almost $6,000 worth of electricity annually, about six times its initial purchase price." One way some design engineers are improving efficiency is by carefully selecting motors and drives. Experts say the use of energy-efficient motors can improve efficiency from three to eight percent.

As always, motor choice must depend on the designated application. "The cost effectiveness of an energy-efficient motor in a specific situation depends on several factors, including motor price, efficiency rating, annual hours of use, energy rates" and more, according to the DOE. There are three factors design engineers must consider before choosing a certain motor for their design:

  • Motor size. According to the DOE, "Motors should be sized to operate with a load factor between 65 and 100 percent" at full-load rating.
  • Operating speed. The operating speeds of motors at full-load RPM should be displayed on motor nameplates.
  • Inrush current. Design engineers should try to circumvent overloading circuits.

What's The Difference?
According to John Malinowski, Baldor product manager, "Brush-type DC motors are relatively efficient, but when operated from an SCR [silicon-controlled rectifier] control, the form factor is very poor. Plus, the brush and commutator maintenance is expensive. A premium efficient AC induction motor may be four to six percent more efficient than a DC motor."

In brushless DC motors, "There is a huge increase in efficiency and power density when compared to an AC induction motor." Malinowski continues, "For example a 400-HP AC motor may operate at 95.8 percent, a premium at 96.2 percent, but an interior permanent magnet [IPM] motor could be as high as 98.3 percent. At 6.5 cents per kWh, annual electric savings for the premium motor over the standard would be $2,500. The IPM motor would have an additional $4,500 of savings. Stepper motors are only available in small low-torque ratings and are used for incremental motion," in contrast.

Electronic variable speed drives control the speed and torque of an AC electric motor by varying the frequency and voltage of the electricity supplied to the motor

Because they are constructed with improved manufacturing techniques and superior materials, energy-efficient motors usually have higher service factors, longer insulation and bearing lives, lower waste heat output and less vibration, all of which increase reliability," - Department of Energy.
Applications Engineering Manager Bob Parente says one energy-efficient feature that Intelligent Motion Systems has implemented into their stepper motors is "programmable run and hold currents [that] can result in consuming less power by taking advantage of the stepper's high torque at low (including zero) speed performance."

Materials Selection
One trend that has swept the design engineering field is the trend toward aiming for the most application-specific energy-efficient materials. "High efficiency is obtained by careful design of the motor parts that cause losses (i.e. brush commutations, bearings, laminated iron stack)," says Urs Kafader, technical director at maxon motors, and some of those decisions are relative to what kinds of materials are used in the design. He continues, "One of the ways [maxon motors] has strived to improve efficiency was to have a special material for laminated iron stack."

"Lower loss steel and more copper improve efficiency," Malinowski says. "Baldor is always looking for improved materials that can help improve efficiency or allow use of less active material in the motor to offset always-increasing steel, aluminum and copper prices. Motors with better efficiency allow the end-user a lower lifecycle cost. The motor price is only about two percent of the lifecycle cost; around 97 percent is electricity. Improved materials may help offset price increases."

More Miniaturization
Making the natural transition from materials selection to miniaturized motors, Malinowski says, "Premium motors are made using electrical lamination steel with lower losses than our EPAct motors. These lams also have room for increased copper windings. Because of the lower losses, a smaller cooling fan is needed, which requires less power. Motor efficiencies may be increased by up to four percent on smaller motors and one to one-and-a-half percent on larger motors. For motors operating continuously, the savings may pay for the upgrade in less than two years."

"Smaller motors typically require lower current. Lower current results in lower power dissipation. Applications that require precise positioning and low torque requirements, such as in the medical, semiconductor, packaging and inspection equipment" fields bode well with miniaturized motors. Parente also says, "Intelligent Motion Systems has introduced the NEMA 14 motion control (our most complex) version, which is a 1.4" square motor. Previous versions have been the NEMA 17 and 23 (1.7" and 2.3"). Smaller envelopes or a smaller footprint may take up less factory floor space, so the efficiency can go beyond the device itself and have an impact on the size of the department or building that it's in."

Malinowski agrees, "I think advances in power density combined with efficiency improvements will be what is seen. When we talk about smaller motors, we're not talking about micro-size, but motors that may be two to three NEMA frame sizes smaller than the norm. True miniature motors might be seen in the servo or linear motor products. These provide more accuracy in placement and faster response. Baldor has several applications in medical equipment already using these technologies."

Improved Heat Dissipation
Application-specific expectations in motor efficiency
What impact does heat dissipation of a motor have on its energy efficiency?Frankly, Malinowski says, "Heating in the motor adds to its losses. Baldor built several motor families with exposed laminations so the winding heat doesn't need to transfer to lams and then a separate frame. By eliminating the frame, the Diameter2 x Length can be improved, which improves power density for the package."

Parente claims, "Hotter motors, for a given amount of work, are less efficient." Parente's company has "reduced harmonic content in the motor current and uses lower current, higher performance motors" to improve heat dissipation.

The Driver Behind It All
Malinowski explains, "An adjustable speed drive does not improve the efficiency of the motor; it improves the efficiency of the system it drives. This needs to be split into the two basic types of loads-constant torque and variable torque. Constant torque loads, such as conveyors, require the same torque to operate regardless of motor speed. As speed is decreased, the current required remains relatively constant, and provides little or no energy savings. But savings can be realized through improvements in process flow.

"Variable torque loads, such as centrifugal pumps and fans, fall into applications covered by the affinity law. On these applications, as speed is decreased, the load to the motor is decreased by the cube. This means that there are substantial energy savings as the motor speed is adjusted, instead of using a valve or damper to control flow. Payback for a drive and premium motor may be less than one year if there is a need for reduced flow."

Electronic variable speed drives control the speed and torque of an AC electric motor by varying the frequency and voltage of the electricity supplied to the motor. These types of drives can have some advantages, including:

  • No friction loss-there are no moving parts.
  • Instant and accurate control of equipment speed-one variable speed drive can manage several motors.
  • Gradual startups and slowdowns, which reduce motor stress and inrush current.
  • A small profile that makes them easy to install.
  • Energy savings to approximately 20 percent, although savings vary widely.

All variable speed drives with power switching devices generate harmonics, and according to Parente, "Decreasing the harmonics in the motor current impacts motor temperature, and thus efficiency. The drive design team needs to pay attention to many of the [aforementioned] items when the drive is being designed." Parente also thinks drivers have the capability to "better overall efficiency and reliability, lower heat, lessen down time and lower operating cost."

Peripheral Benefits
"Because they are constructed with improved manufacturing techniques and superior materials, energy-efficient motors usually have higher service factors, longer insulation and bearing lives, lower waste heat output and less vibration, all of which increase reliability," according to the DOE.Paul Webster, CNC product manager of GE Fanuc Intelligent Platforms, thinks, "Direct energy reduction efforts can result in many related benefits for end-users, including:

  • Power source regeneration.
  • Power loss reduction through switching devices.
  • Spindle motor temperature rise reduction by maximum efficiency control.
  • Iron loss reduction of motor by higher frequency of pulse-width modulation."

In order to actualize the energy efficiency of a motor, "It is important to run the motor at a high speed near its maximum efficiency," reminds Kafader, being one of the most critical considerations a design engineer must encounter when choosing a motor and/or drive for their specific design. However, it's getting easier to keep up with the growing trend toward energy efficiency: motors are absorbing less energy, while maximizing output when paired up with the right application.

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