7 myths about energy efficient motors
Despite the obvious cost and energy savings associated with premium efficiency motors, they are not well understood by engineers. Here, we debunk some popular myths
By Ed Cowern, P.E., District Manager
Baldor Motors and Drives
1. Only large motors benefit from energy savings. Although large motors can achieve close to 100% efficiency, keep in mind that the energy-saving potential of small, premium efficient motors is actually greater percentage-wise than the savings on large motors. The reason is that the percentage difference in efficiency between the standard motor and the premium efficiency motor is actually much greater for small motors than it is for large motors (see figure, next page). For example, the difference between a standard motor at 3 hp and the premium efficiency motor could easily be nine or more percentage points. Compare this to a 100-hp motor, where the difference between the two might only be 2%.
2. A motor must be fully loaded to realize the savings available in premium efficiency motors. For optimum efficiency, it is usually advantageous to have motors loaded to more than 50% of rated load. Consequently, it is usually best to resize a motor at the same time it is upgraded to premium efficiency. However, even if this is not done and the motor is oversized, there are still substantial savings to be gained by using a premium efficiency motor. For example, at 25% of rated load, the difference in efficiency between a standard motor and a premium efficiency motor is still substantially better, even at low load levels, than a non-premium efficiency motor. Even without resizing, a substantial efficiency improvement can be made.
3. Premium efficiency motors are too expensive. Generally, premium efficiency motors cost 20 to 30% more, depending on the size and speed of the motor. That's because they use more and better materials. For example, the lamination material is a higher-grade, higher-cost steel. The problem is that many people buy on initial cost rather than considering operating costs.
|To improve the efficiency of a motor, design engineers work on the five categories of losses shown at left. For example, reducing the stator resistance loss involves both magnetic and electric modifications that allow for more copper wire to be inserted in the slots of the stator of the motor. Engineers reduce the second largest loss, rotor resistance, by using special rotor designs with larger areas of aluminum conductor.|
4. Since the savings aren't obvious, there must not be any.Unlike light bulbs that are sold by wattage consumption (input), electric motors are sold by horsepower (output). Thus, subtle differences in efficiency usually appear in the fine print and are often overlooked. For example, it's obvious when you buy a 34W fluorescent light bulb to replace a 40W bulb that some savings are available. It is less obvious when you buy a conventional 5-hp motor versus a 5-hp motor of a premium efficiency design that there will be savings on the electric bill. Also, the vagaries of electric bills and the complications involved in the electric billing process, with demand charges, energy charges, fuel cost adjustments, and occasionally, power factor penalties, create enough confusion so as to obfuscate the savings. But that does not mean they do not exist.
|The process of converting electrical energy to mechanical energy is never perfect, but premium efficiency motors come closer as shown in the figure, which depicts motor efficiency versus motor size for standard and premium motors. Although the difference in percentage points may seem small, particularly when it comes to large motors, energy-and therefore cost-savings can be significant.|
5. It's impossible to calculate a dollar savings on premium efficiency motors. Actually, this calculation can be done quite easily. There are three items needed to conduct an evaluation. First and most important is the average cost per kilowatt hour of electricity. The simplest and most direct way to get this is to take the bottom line cost on a monthly electric bill and divide it by the total kilowatt hours used. This gives a net cost per kilowatt hour, which is generally the best cost to use in evaluating energy-saving equipment. The reason this works is that equipment designed for better efficiency will in general reduce the demand, kilowatt hours, and fuel cost adjustments in equal proportions. Thus, using the average cost per kilowatt hour is the easiest way of making an evaluation. Next would be the size (in hp) of the motor that is operating and, finally, the number of hours per month or year that it operates. With these three items and the efficiency difference between one motor and the other, it's easy to figure the cost savings. The formulas are readily available.
6. The dollar savings of a premium efficiency motor won't be enough to offset the initial investment. The payback period for a premium efficiency motor will depend on the specific application, but motors operating 24 hours a day at or near full load can be expected to pay for themselves in less than two years. The difference between a standard motor's cost and a premium efficiency motor's cost can be paid off in a few months. One thing is certain, however: Regardless of operating details, premium efficiency motors will always save money and those savings will go on as long as the motor is in operation. In many cases this could be 20 to 30 years. Also, as power costs rise, savings will rise in proportion.
7. Premium efficiency motors don't have any advantages beyond energy savings. Because of the superior designs and better materials used, premium efficiency motors tend to run at lower operating temperatures, resulting in longer life for lubricants, bearings, and motor insulation.
Hypoid gear set combines high reduction and compactness
Offers 15:1 reduction ratio for motion control applications
By Charles J. Murray, Senior Regional Editor
Chicago, IL-By employing a hypoid gear, engineers have created a right angle gearbox that offers high reduction, high efficiency, and compactness.
|Dyna Series hypoid gearbox|
|Gam Gear's Dyna Series achieves a 15:1 reduction ratio using a hypoid gear.|
Known as the Dyna Series, the new design offers a 15:1 reduction ratio. That's reportedly the highest hypoid gear reduction ratio ever available for motion control applications. Typically, hypoid gears which are so named for their rolling hyperboloids of revolution offer about 3:1.
To accomplish these benefits, engineers developed a High Reduction Hypoid computer program, which calculated the gear tooth profiles. Because hypoid gear teeth are cone-shaped, and the mathematics used in the calculations are complex, engineers used the computer program to perform the iterative, high-reduction calculations that would have otherwise been too time consuming.
The result, however, is that the Dyna Series dramatically reduces the size of the gearbox, while offering improved design flexibility for users. By achieving 15:1 in the single-stage hypoid gear set, Gam Gear eliminated the need for a bulky planetary-and-bevel combination. Typically, a 3:1 bevel gear and 5:1 planetary set enables gearbox designers to achieve a 15:1 ratio. But the path from the input to output is longer. What's more, the planetary-bevel combination is not conducive to multiple output shafts or hollow bore shafts. In contrast, the hypoid design can be used in applications that call for such configurations.
|By employing a hypoid gear, instead of a planetary-bevel combination, a right angle gear box can take up about 30% less space|
In the past, users of gearboxes often achieved high reduction in smaller spaces by employing worm gear drives. Worm gears, however, offer lower efficiencies at start up. Typically, a 15:1 worm gear achieves efficiencies ranging from 70-75%. In contrast, the Dyna Series offers 93% at 15:1, making it comparable with the planetary-bevel combination.
The new hypoid design is also back-driveable, meaning that it can be back-driven by a servo motor as a means of storing energy for the next forward motion. Back-driveability is important for such applications as converting, metal forming, and material handling. In contrast, worm gears cannot be back-driven because they are self-locking.
Additional details...Contact Craig Van Den Avont, Gam Gear, 7333 W. Wilson Ave.; Tel: (708) 887-5000.
Mounting system combines pneumatics and electrics
Enables users to cut costs on two-axis systems by replacing one electric ball screw with a pneumatic slide
By Charles J. Murray, Senior Regional Editor
Avon, OH-Usually, when engineers create a two-axis motion system, they use the same power medium on both axes. If, for example, they employ an electric ball screw on the X-axis, then they use an electric ball screw on the Y-axis, as well.
Thanks to a new mounting technique, however, engineers can now easily mix pneumatic components with electrically-powered ball screws. In the process, they can dramatically cut the design time and costs of two-axis systems.
|The NuMate mounting technique allows creation of a two-axis system using an electric ball screw gantry and a pneumatic linear slide.|
Known as NuMate, the mounting technique is an important step forward for designers who don't need the precision of a ball screw on both axes. In cases where a pneumatic actuator can adequately serve on one of the two axes, designers can eliminate as much as 50 to 70% from the cost of that axis.
Up to now, mixing and matching of pneumatics and electrics has not been common among design engineers. "The mind set has always been, 'If I need one electrical axis, then both axes should be electrically-driven ,'" notes Jeff Schneid, president of Numation and inventor of the concept. "Even if the other motion axis was a bang-bang axis moving from point A to point B, engineers used a ball screw on both axes."
To solve that problem, Numation developed a series of linear slides and ball screw gantries with integrated, counter-bored and tapped mounting holes. The holes, which are included on four faces of the company's SH-Series linear slides, align with holes on its ball screw gantry. As a result, the slide's tooling plate connects to the ball screw's movable carriage.
Because the NuMate Mounting System is made for such connections, users need not add custom-designed mounting plates. In the past, the need for mounting plates sometimes discouraged engineers from connecting disparate power mediums.
The pre-drilled holes also prevent end users from drilling their own holes a process that can result in damage. Very often, users drill holes into bearing chambers, shafting, and pneumatic ports, thus rendering their product useless.
Schneid, who modeled the mounting techniques after those of a child's Erector Set, applied the same principles to varying pneumatic systems, as well. The NuMate Mounting System can be used to mix and match pneumatic linear slides, pneumatic and rodless gantry slides, and grippers.
In marrying pneumatics to pneumatics, Schneid says, NuMate's advantages are the same as with pneumatics-to-electrics: It eliminates the need for a custom adaptor plate and prevents users from drilling holes in the wrong locations.
Ultimately, however, the concept's greatest cost savings may stem from its ability to combine pneumatics with electrics. Typically, Schneid says, a pneumatic gantry costs roughly $600-$700. In contrast, a ball screw, along with motor, drive, controller, and power supply, will usually cost more than $1,500. "If you don't need precision motion on both axes, there's no reason to go with two ball screws," Schneid says. "If you use pneumatics for one axis, there's a huge cost savings."
Additional details...Contact Jeff Schneid, Numation, 1324 Chester Industrial Pkwy., Avon, OH; Tel: (440) 934-3200.
The 287 Series skeleton universal motors are available in 120 or 240V ac. Units measure 2.87 inches in diameter and can use either a sleeve/sleeve or ball/sleeve bearing arrangement. Options include multi-speed designs, thermal protection, grounding provisions, and various terminal styles. Applications include kitchen appliances, floor care products, and yard and garden tools.
GS Electric, 1700 Ritner Hwy., Carlisle, PA 17013; FAX: (717) 243-6519.
New linear actuator consists of a NEMA size 23 high-torque stepping motor and lead screw interface. The actuator measures 1.8 inches long and delivers up to 180 lb of linear force. Hollow-shaft motor design accommodates leadscrews and ballscrews up to 3/8-inch diameter. Special leadscrews can be machined in choice of material and length. Applications include aircraft systems, substitutes for hydraulic and pneumatic actuators, valve actuation, and dispensing systems.
Eastern Air Devices, 1 Progress Dr., Dover, NH 03820; FAX: (603) 742-9080.
Brushless rotary servo motor
MFM rotary servo motors deliver continuous torque of up to 13 Nm (114 inch-lb) and peak torque up to 38.5 Nm (340 inch-lb) in a compact format. Available in frame sizes from 40 to 115 mm, or in NEMA frame sizes 17, 23, 34, and 42, with steel disc encoders. Optional resolver and/or brake is available. Rotary servos are also available as brushless, two-wire motors to directly replace existing brush-type motors.
Bayside Motion Group,27 Seaview Blvd., Port Washington, NY 11050; FAX: (516) 484-5496.
A-max, 22-mm (0.87 inch) diameter motor features a rhombic, moving-coil design that provides for long life and low electrical noise, says the manufacturer. Length is 31.9 mm (1.26 inches) and the motor weighs 1.9 oz. Ambient temperature range is -30 to 85C. Maximum continuous torque is up to 7.5 mN-m (1 oz-inch). Matching gearheads are available with ratios ranging from 4.4:1 to 4,592:1 capable of delivering 1 Nm (141.6 oz-inch) continuous torque.
Maxon Precision Motors, 838 Mitten Rd., Port Burlingame, CA 94010; FAX: (650) 697-2887.
Brushless dc motor
Custom brushless dc servomotors are available in torque ratings from 60 to 760 oz-inches and with motor diameters of 2.30 to 3.43 inches. High-energy, rare-earth magnets result in low-inertia rotors that allow rapid acceleration and deceleration, says the manufacturer. The motor is mechanically equivalent to NEMA Size 23 and 34 step motors.
Kollmorgen, 201 Rock Rd., Radford, VA 24141; FAX: (540)731-0847.
Series IP65 and CE-rated linear motors have a patented tubular design that provides optimum magnetic flux usage and high continuous forces in a small cross-section. According to the manufacturer, the motors provide high duty cycles while offering good heat dissipation without the need for complex cooling arrangements. Applications include scanning systems, pick-and-place, printing, materials handling, biomedical equipment, and tool selection/positioning.
Industrial Devices Corp., 64 Digital Dr., Novato, CA 94949; FAX: (415) 883-2094.
New, 1.8 degrees stepper motor is microstepped to 25,000 steps per revolution and weighs 0.2 lb. Size 11 motors feature a 1.1-inch diameter. Target application is mechanical manipulators used in neurosurgery applications. A pin and locator dowel construction allows for easy assembly, even by technicians wearing rubber gloves, says the manufacturer. Motors can be sterilized with alcohol.
Empire Magnetics, 5780B La Bath Ave., Rohnert Park, CA 94928; FAX: (707) 584-3418.
Brushless dc motor
Elcom(R) II slotless, brushless dc motors and gearmotors are available in three frame sizes (motor Series 3400, 4400, and 5400 and gearmotor Series GM3400, GM4400, and GM5400) with several length and performance options available. Motors feature a modular design to meet application-specific torque, noise, output-shaft, and cost requirements.
Pittman, 343 Godshall Dr., Harleysville, PA 19438; FAX: (215) 256-1338.
The Series 42MO48C-R permanent-magnet stepper motor's round gearbox configuration features a smaller footprint than standard, pear-shaped gearboxes. The motor measures 42 mm in diameter 3 24 mm long and the gearbox measures 54 mm in diameter 3 15 mm long and weighs 11 oz. Applications include computer peripherals, office automation, HVAC, gaming, instrumentation, and medical/lab equipment.
Thomson Industries, 2 Channel Dr., Port Washington, NY 11050; FAX: (516) 883-9039.