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Things to consider when you need to optimize torque density


As Printed in MOTION SYSTEM DESIGN MARCH 2006


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.

Going small

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, MN, “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, FL. 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 face, 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, MA. “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 slots fills correspond to maximum current per unit of motor circumference,” says Len Wedman, motor design engineer, Thingap Motor Technologies, Ventura, CA. 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, MN. “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.”

What’s New

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 designer from the constraints of limited slot fill,” says Faizul Momen. “Using simple and inexpensive winding machines, the technique allows such 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.”

“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.”

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.

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 conducted 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 magnetiziting 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.”

Cool it

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 motors in an area with little heat sinking or air flow, you’re not going to get as much power out of is 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.”

Seeing results

As torque density increases, product innovation will continue as 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.


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