“If you run through an optimization routine, you’re going to have certain limits,” said Jim Calico, senior design engineer at the Moog Components Group. “One would be the ratio of the stator outer diameter to the rotor outer diameter. For a given application, there’s going to be an optimal performance ratio for that, but there’s going to be a range within that optimal size. I'm saying push the design to a minimum rotor outer diameter.”
As always, there are tradeoffs. Going with a copper-centric design might preserve the power density but will add to the size, not to mention inertia. How far to push the tradeoff, if at all, will depend on the application.
Another approach is to adjust the air gap between the rotor and the stator. According to Hanselman, the magnetic performance of a brushless permanent-magnet motor is directly related to the ratio of the radial magnet length to the radial air gap length. “If manufacturing allows one to decrease the air gap length by a factor of α, then the magnet length can also be decreased by the same factor of α, and the motor performance will remain unchanged."
Of course, leveraging the technique depends on the supplier’s ability to maintain tight manufacturing tolerances, which can add cost and complicate assembly. It’s also important to keep in mind that the effect is subtle. “It's not substantial,” said Joseph Profeta, director of product management for control systems at Aerotech Inc. “It's more like a 5 percent, 10 percent change.” Depending on magnet pricing and budget, it may still be in effect worth pursuing.
Increasing the number of magnet poles on the rotor can also yield a magnet-sparing design. “If done carefully, this will increase the torque creation ability of the motor while keeping the amount of magnet material constant,” Hanselman said. “Therefore, one might be able to shrink the motor size and still meet the required performance specs.”
For a greenfield design, the sky's the limit. In the case of users searching for a more economical solution for a product, it's important to keep in mind the form factor of the original. Depending on the manufacturing tolerances, a diameter change of a few millimeters may make the difference between a motor that fits into a machine and one that does not. A lower-price motor might be appealing, but not if a redesign ultimately raises the costs for the project as a whole. Once again, there is no right answer -- just a set of tradeoffs.