Motor Designs Mitigate High Neodymium Prices
| By: Kristin Lewotsky, Contributing Editor
Alternative design techniques allow motor manufacturers and end-users to produce effective, economical systems.
In the past few decades, motors based on rare-earth-oxide (REO) magnets have dominated the market. The beneficial magnetic characteristics of REOs allowed the production of compact, robust high-torque motors at an economical price point (see figure 1). Over the past several years, however, supply-chain instabilities have caused neodymium, in particular, to skyrocket, driving up the price of rare-earth magnets, and, hence, motors. Although alternative REO sources are under development, it will take several years before new or reopened mines can produce in volume. In the meantime, manufacturers and end-users have two choices: Pay the higher price or look for alternatives.
“When the neodymium first came out, the performance was pretty poor,” says Alan Crapo, Vice President of Engineering, at NovaTorque Inc. “You could get high flux, but the temperature capability and the demag protection were terrible.” With careful development and modifications, the material improved dramatically, providing vendor manufacturers with huge benefits. "It has come a long way," he agrees. "Of course, the real heyday was from the late 1990s to the mid-2000s, when the price dropped around 7% each year. We kept designing more things with neodymium because it just kept getting better and better and cheaper and cheaper. And then in about 2008 or so, everything changed.”
As the price of neodymium oxide rose by more than five times in a bit over a year, the industry found itself facing a volatile market in the midst of a recession, and a whole new set of rules and behaviors. “Our customers are far more in tune with supply chain than ever before," says John Calico, Senior Design Engineer at the Moog Components Group. "Many of them have asked for weekly reports on our entire supply chain. Some magnet vendors have been asking for their money up front, not when you receive the magnets. That's a problem for some of the smaller motor manufacturers, especially at a time that it is hard to borrow money.”
To avoid these challenges, vendors and end-users alike have begun searching for more cost-effective ways to get the motive power they need. Let's take a closer look.
Essentially, we can break down alternative approaches into four classes, listed in order of increasing effect:
Even without the price swings of rare-earth oxides (REOs), the extraction and refining of the materials involves a number of toxic substances that endanger human health and the environment. As a result, work is underway at the research level to develop alternatives.
At the University of Delaware, George Hadjipanayis is investigating Nano composite magnets that promise to incorporate 30% to 50% less neodymium than their conventional counterparts. The group is working to develop an anisotropic exchange-coupled magnet, which consists of a mixture of "soft" (low coercive force) nanoparticles and "hard" (high coercive force) nanoparticles. They have produced the appropriate nano crystals. Over the course of the next year, the team plans to focus on combining the soft and hard nanoparticles into actual magnets.
Alloys such as manganese aluminum, which has an energy product of roughly 6 or 7 MG*Oe, manganese bismuth, or manganese gallium show promise as new magnet materials capable of very-high-temperature operation. At Pacific Northwest National Laboratory (Richland, Washington), materials scientists are investigating composites formed of various manganese-based alloys.
Although iron platinum displays excellent magnetic qualities, the cost of the material makes it impractical. Hadjipanayis points to iron nickel as an intriguing alternative. "It has a phase transformation like iron platinum," he says. "It goes from a face-centered cubic structure to a face-centered center tetragonal structure.” The latter makes the material very hard. Currently, the transformation is sluggish but future developments hope to tackle that problem and make the material more practical.
- Minimize the neodymium content of the magnets
- Avoid the use of rare-earth magnets
- Eliminate magnets altogether, such as with induction motors
Making choices about magnet material requires understanding some of the basic properties. The energy product of a magnet provides a measure of field strength, which is an indication of how effectively the material will allow a motor to convert electricity into torque. The intrinsic coercive force essentially quantifies how much a material resists de-magnetization, which provides an indication of robustness. Ferrite magnets, for example, can achieve energy products of 4.5 MGOe or less and intrinsic coercive forces of around 2800 Oe (see table). In contrast, neodymium-based magnets boast energy products as high as 55 MGOe and intrinsic coercive forces of up to 28,000 Oe. By switching from ferrite magnets to neodymium-based versions, motor manufacturers have been able to dramatically reduce the size of their devices while delivering more robust motors able to tolerate higher temperatures.
Table 1: Approximate characteristics of common magnet types*
* Numbers shown are purely for general comparison. Actual values can change significantly with just a few percent change in composition of various materials.
The basic material for neodymium-containing magnets is neodymium iron boron (NdFeB), but the actual composition varies widely. To improve performance, magnet manufacturers dope the NdFeB with a few percent of REOs such as dysprosium or praseodymium. Although adding those materials can adjust the operating temperature range of the magnets from 150° C to more than 200° C, adding the exotic materials further increases costs. One of the simplest ways that OEMs and system designers can control costs is to carefully review their application and specify a motor with only the performance that they need.
Most motors are designed to run hot. Frequently, however, end-users become concerned when the housing reaches a high temperature and tend to either oversize the motors or choose magnets that provide an excessively high operating temperature range. Especially in the case of NdFeB-based motors, OEMs can wind up paying for high-end motors when a lower-performance design will work just fine. "There's no reason why the end-user should not utilize a motor at its full rating," says Calico. "You're really trying to not design your magnet to be any bigger or more powerful, or operate at a temperature any higher than what is actually required.”
Minimizing neodymium content
If loosening specifications doesn't help, the next step is to minimize the neodymium content of the motor design (see sidebar). Good motor design typically yields a rotor diameter between 40% and 65% of the stator diameter. Depending on the ratio of rotor diameter to stator diameter, a permanent-magnet motor can be designed to be either more copper intensive or more magnet intensive. A motor with 40% ratio of rotor to stator diameter would be termed a “copper motor,” while a 65% ratio of rotor to stator diameter would be considered a “magnet motor.” If magnet costs dominate the cost budget, then you would opt for a copper motor for the lowest cost design.
Adjusting a design from magnet intensive to copper intensive is less about altering the power density than redistributing the active materials within a given volume. Properly refined, the design will yield the motor of roughly the same size and weight, but with lower cost.
If tuning the design cannot realize the desired results, the third approach is to eliminate neodymium from the magnet altogether. Past a certain threshold, samarium cobalt magnets present a high-performance option. In previous decades, NdFeB magnets were significantly cheaper than samarium cobalt versions. As neodymium prices soared in the summer of 2011, however, samarium cobalt magnets actually offered a more economical alternative. That only holds for price extremes, however. Given the rapid fluctuations in the price of neodymium, designing a samarium cobalt motor into an OEM product may be risky.
Calico, for one, no longer sees samarium cobalt as a useful substitution for NdFeB. "I think we'll see the pressure taken off of trying to replace neodymium with samarium cobalt, simply because samarium cobalt in the foreseeable future is going to remain a more expensive material than neodymium," he says. "OEMs should only use it for those high temperature applications where they need a motor that can operate above 150o C.”
Aluminum nickel cobalt (AlNiCo) or ferrite magnets provide more economical options, but the trade-off is performance. In a direct replacement of NdFeB magnets with ferrite, the motor would have to be roughly 50% larger to generate the same amount of power. Making the material competitive with rare-Earth counterparts requires new designs.
Conventional motors based on radial or axial gap designs can focus flux in, at best, two dimensions. One way to enhance the performance of a ferrite permanent-magnet motor is to focus flux in three dimensions, an approach taken by NovaTorque. The company's design starts with a stacked, laminated field pole inserted into a bobbin-wound copper winding (see figure 2). Six of the field pole/winding combinations are assembled in a ring formation to form the stator (see video). The ends of the field poles match up to the shape of a conical rotor, minimizing the air gap (see figure 3). Because of this three-dimensional design, the motor achieves flux densities and efficiencies comparable to those of motors with rare-earth magnets, says Crapo.
Of course, there are always trade-offs in engineering. Although the motors tend to be smaller than conventional ferrite designs, they still feature higher mass and inertia than an equivalent NdFeB-magnet motor. As a result, they tend to be good solutions for variable speed applications like fans or pumps. They can operate closed loop as servomotors, but would not be a good fit for high-speed, low-inertia applications like packaging.
A final approach to avoiding the price premiums of rare-earth magnets is the obvious one—eliminate magnets altogether. An induction motor, for example, features much lower parts cost than a unit incorporating NdFeB magnets. They consume more power, however. Depending on the application, the cost of operating the motor over the lifetime of the product may exceed the savings introduced by the elimination of the magnets.
Switched reluctance motors offer another alternative. With no windings on the rotor, they can present even lower materials costs. Their controls can be complex, however, requiring a microprocessor for basic operations. The motors can also suffer from high torque ripple.
Although price fluctuations in neodymium and other REO will continue to challenge the industry for the foreseeable future, designers have a range of mitigation techniques at their disposal. By balancing application requirements like power, torque, duty cycle, product lifetime, and product lifecycle with the savings afforded by the various mitigation options, designers can determine the approach that provides best savings over time.