A new breed of products — from self-powered wheelchairs to autonomous aircraft to all-electric plastic injection-molding machines — tells of a trend sweeping the motion industry. Engineers in both component and system development are pulling out the stops trying to maximize torque and force density. Though the going is tough — squeezing more power into less space runs counter to the laws of physics — there's every reason to believe that tomorrow's motors and gears will be stronger as well as smaller, helping motion engineers implement even more aggressive designs.
All motors require a certain amount of mass to generate a certain amount of torque. “To increase torque, it is necessary to increase the content of active materials, such as iron, copper, and permanent magnets,” says John Calico, senior research engineer, Moog Components Group, Blacksburg, Va. Today, engineers around the U.S. are trying to do just that — increase torque — but without making motors larger.
There are two sets of obstacles that researchers face in their quest for higher torque density. One is conventional, based on manufacturing methods, while the other is fundamental, based on the laws of nature.
All motors have internal losses in their electrical and magnetic conducting paths. These current-induced losses generate heat, and if it doesn't escape — through sufficient thermal mass and surface area — it will build up in the motor. “When it comes to torque density, the limiting factor in every electric motor is heat,” says Richard Welch, consulting engineer for Exlar Corp., Chanhassen, Minn. “If a motor exceeds its maximum continuous operating temperature, even for a short time, it can suffer permanent damage.”
Even if the heat doesn't destroy the motor, it will probably degrade the one thing that matters most — torque. “There's a maximum temperature that magnets can withstand,” says Ken Rusiska, MicroMo Electronics Inc., Clearwater, Fla. Beyond that magnets begin to weaken, which reduces motor torque.
Manufacturing methods, too, influence motor size, through slot fills, packing densities, insulation volume, and assembly tolerances. One approach to make motors smaller, in fact, focuses on these things. The idea, according to John Calico, is to reduce the content of inactive materials. “Inactive materials include air, plastic, and insulation. The only air space necessary in a motor is in the airgap,” he explains.
Though there are many manufacturing-related limitations on torque density, some of the biggest seem to come from coil winding. “Traditional winding technology limits the effective use of material,” says Faizul Momen, CMC-Torque Systems, Billerica, Mass. “In other words, it limits the amount of copper that can be inserted inside the slots on laminated single-piece stators.”
Less copper translates to less current, less magnetic flux, and less torque. Actually, it's not so much the current, but current density, that's of interest here. “High slot fills correspond to maximum current per unit of motor circumference,” says Len Wedman, motor design engineer, Thingap Motor Technologies, Ventura, Calif. This, in turn, corresponds to maximum torque density.
Other factors limiting motor size stem from materials and the shapes they normally assume. Consider a typical magnetic core. “Laminated electrical steel forces motor designers to use a 2D magnetic approach,” says Tim Bush, vice president of engineering, Dynetic Systems, Elk River, Minn. “This is somewhat limiting from a packaging standpoint,” he explains. “Sharp corners in laminated steel structures mean that motor interns cannot be routed as efficiently as you would like, leading to higher resistance and lower efficiency. It also limits how much of a magnetic field the motor can support before the associated core losses become too great.”
Despite these barriers, motors are getting smaller and more powerful. Perhaps the most promising new development is the resurgence of a design that first appeared more than 100 years ago. “Without question, the best new way to increase torque density in motors — such as ac induction, brushless dc, and variable reluctance — is cut-core stator technology,” says Richard Welch.
Welch isn't the only one sold on the cut-core design. “Segmented core technology, first introduced in the early 1900's but recently revived, has nearly freed motor designers from the constraints of limited slot fill,” says Faizul Momen. “Using simple and inexpensive winding machines, the technique allows each stator tooth to be wound individually prior to assembly, resulting in higher slot fill and higher torque density. The method's concentrated winding style also makes for shorter end-turns, which reduces copper losses and increases efficiency.”
Another new development — employing powdered metal technology — addresses the limitations imposed by stacked laminations. “Soft magnetic-composite powders are letting engineers design structures with three-dimensional flux paths,” says John Calico. “These novel geometries result in more efficient magnetic circuits, while minimizing winding resistance.”
“Soft magnetic-centered metal is the most promising motor technology today,” adds Tim Bush. “Besides offering 3D magnetic paths, it also allows for a much more economical structure than stamped and stacked laminations. It lets you build in features as well; round corners, barriers for wires, things that make it easier to place wire on the core and minimize excess on the interns.”
Lamination technology, however, is not standing still. Laminations are getting thinner and more efficient. “Thinner laminations and new lamination materials are improving torque density by reducing eddy currents and hysteresis losses,” reports Ken Rusiska. “Lamination materials are now available down to 0.005 in. Motor designers also have access to amorphous metals that can reduce losses in high-frequency lamination stacks. These materials, in addition, minimize magnetic saturation, especially in momentary situations when motors are pushed very hard,” Rusiska explains.
New ways to keep windings cool also contribute to higher torque density. One is thermally conductive epoxy. “Thermally conductive epoxy is being used to pot stator windings, replacing the varnish normally used to impregnate them,” says Welch. This improves heat transfer between the winding and the motor's housing, where it can be more easily dissipated. According to Welch, thermally conductive epoxy lowers winding-to-ambient thermal resistance by as much as 40% over varnish.
Another promising development is the advent of materials that let motors run hotter and harder. “Insulation systems rated to 200°C are becoming common,” says Bush. Magnets, likewise, are improving. “We're seeing neodymium magnets with temperature capabilities exceeding 180°C,” says Rusiska.
As for the strength of today's magnets, they are now at the point where they produce more flux than ordinary electrical steels can handle, according to Bush. As magnets get stronger, it may precipitate a switch to more exotic materials with higher permeability.
Magnets getting stronger are almost a certainty. “Stronger permanent magnets are on the horizon thanks to nano-technology,” says Calico. “Improvements in magnetizing systems also point to stronger magnets,” says Rusiska, “while the ability to make sintered magnet rings (for multi-pole systems) promises to raise torque density even further.”
It may seem that the quest for higher torque density ends once a motor is designed and built, but nothing could be further from the truth. Torque density is ultimately a system variable, the responsibility for which rests heavily on motion system designers.
The main application issue, not surprisingly, is cooling. “It's all about removing heat,” says Richard Welch.
“For maximum torque density, proper heat sinking and unimpeded air flow are mandatory,” adds Ken Rusiska. “The last thing you want to do is mount the motor so that it's thermally insulated; embedding in the plastic handle of a hand tool, for example.” Tim Bush agrees. “If the motor's in an area with little heat sinking or air flow, you're not going to get as much power out of it as you would if the motor was properly cooled.”
Experts say most problems can be avoided if motion engineers address cooling issues early in the design process. Perspective can play a big role as well. System designers should think of the motor's external environment as an extension of its internal environment. Thermal paths may begin inside the motor, but they don't end there.
“To maximize torque density, motion system designers need to optimize heat transfer efficiency from the motor to its surrounding ambient environment,” says Welch. He also suggests a few tricks on how it might be done: “Attach the motor to a heat sink; use forced fluid or air flow; paint the motor black to maximize heat radiation; lower the ambient temperature.”
Designers also need to think about how to best drive the motor. “Drive techniques can have an enormous influence on thermal issues,” says Rusiska, “Torque in a dc motor, for example, is proportional to the average current, but thermal dissipation in the windings is a function of the RMS current.”
As torque density increases, product innovation will continue at its impressive pace. Many of those innovations are in development right now. Take, for example, a company that builds mobile land-based robots. They're leveraging torque density to make their robots faster and stronger — by a factor of four.
“These particular robots used to get around on permanent-magnet brushed motors and gears,” explains Tim Bush. “But we replaced that with a compact drive, incorporating a brushless motor, gears, encoder, and brake in the exact envelope where the previous motor resided. The robots are now four times more powerful, having twice the speed and twice the torque. Drive efficiency has also improved, increasing from 50% to an overall efficiency of 86%.”
More incredible still is the case of some new linear actuators. Based on cut-core motors manufactured with thermally conductive epoxy, the actuators rival hydraulics in terms of force density. “Linear actuators incorporating cut-core stator design with integrated roller screws can output as much continuous force per unit volume as a 3,000-psi hydraulic actuator,” says Richard Welch. They also offer much greater precision and control.
According to Welch, the actuators are being used or tested in many applications previously limited to fluid power. These include flight simulators, aircraft control, large watercraft, food processing equipment, machine tools, medical devices, tanks, missile launchers, and even animation equipment for the movie and entertainment industry.
Improvements big and small
Specialized shapes in belts, gears, and other systems can address specific design issues, freeing systems to push through more torque and force. Take brakes, for example. On large magnetic brakes and clutches, coils can be level wound, with the copper wire arranged in perfect layers. “This way, the coil has the same turns in less space, for more torque,” explains Jeff Pedu, Placid Industries, Lake Placid, N.Y.
Likewise, in gearboxes, replacing involute gearing with cycloidals and Novikovs lowers stresses and increases loadability for higher torque density. One caveat, according to Gerhard G. Antony, Neugart USA, Bethel Park, Pa., is that these special tooth shapes are more difficult and expensive to machine, and some of them are unsuitable for high speeds.
Materials can also boost the torque output of a system. One of the most engineered materials available is case hardened steel. “Case hardened steel has greater load capacity than bronze and resists wear and backlash change,” notes Russell Beach, Nissei Corp., Greenville, S.C. Chromium, nickel, and molybdenum in this alloy increase strength for increased load carrying.
Titanium, though frequently mentioned as the next super material, has about the same strength and durability as case hardened steel. “Titanium offers no advantages volume-wise,” says Gerhard Antony, Neugart USA. “However, because it's about two times lighter than steel, it does offer more torque destiny per unit weight,” he adds. Still, the high cost of titanium often limits its application to special designs.
Teeth with more bite
In gearboxes, increased tooth contact makes for more torque density. Not surprisingly, planetary sets — which engage multiple coplanar gears at once — are almost synonymous with torque density. Also called epicyclics, they're growing in use, largely because they're 40 to 60% more compact than traditional spur sets.
In standard gears, the answer is increasing tooth size. “Increasing torque capability per gearing stage equates to larger teeth and wider gear faces,” says Mike Niemela, Bison Gear and Engineering Corp., St. Charles, Ill. “But as the gear teeth grow in size, the number of teeth that will fit on a gear decreases, necessitating increased gear center distances to meet the gear ratio requirements,” he adds. “We recently worked on an application where increased torque was needed; our normal gearmotors delivered 350 lb-in. of torque from a specific envelope,” says Niemela. “But increasing the center distance in the final output stage and changing from spur to helical gearing increased output power to 900 lb-in. at 6 rpm.”
Right angle sets — such as gears with angled or curved teeth that require mating pairs be perpendicular — rival planetaries in torque capabilities. “Right-angle gearing includes worm, bevel, and hypoid types,” explains Tom Provencher, Mijno Precision Gearing, Park Ridge, Ill. “In double-enveloping gearing the hourglass shape of the worm and its throated gearwheel bring many teeth into contact for more load sharing and higher torque density.”
High torque — but for how long?
It's easy to crank a lot of torque out of a small system if the design only has to last a short time. “A gearbox can be rated for a very high torque if it need only last a couple of minutes, but will be rated much lower if it needs to last thousands of hours,” says Gerhard Antony, Neugart USA. Likewise, compact systems can transmit exceptionally high force — if it is intermittent.
Target life expectancy is part of a fully defined torque or force rating. Volume and weight can be determined impartially; however, torque allows a number of interpretations. Catalogs often list acceleration torques, peak torques, 50% duty cycle torques, and so on. That's why designers should be certain torque values used to compare torque-density ratings are analogous.
“Torque density is an excellent measure allowing objective, impartial comparison, assuming variables are determined by objective criteria,” says Antony. “For transparency reasons, the value used for torque density calculations should be the rated torque at continuous duty, for an exactly defined life.”
Minimizing the interface between a motor and its application is one way to increase torque or force density. And the best way to accomplish this, according to John Calico, senior research engineer, Moog Components Group, Blacksburg, Va., is to remove the boundaries between the motor and application.
Instead of building a motor and attaching it to a system, build motor functions into the various elements of the machine, Calico suggests. For example, assemble the rotor directly onto the shaft it's supposed to drive. And build the stator mounting into surrounding machine parts. “The space you save by eliminating the housing can be filled with active motor lamination material,” says Calico.
The approach works best, Calico explains, if the motor is able to drive the load without intermediary components, such as belts, gears, and pulleys. In addition to freeing up volume for more torque-producing materials, this eliminates inherent inefficiencies, making more torque available for the application.