When I first started engineering frameless torque motors back in 2012, I made a promise to myself: I would never cut corners on the materials that make these motors work. Unlike conventional motors with bulky frames, a frameless torque motor’s entire performance—from its smooth, low-cogging rotation to its ability to deliver consistent torque at low speeds—depends almost entirely on the magnetic materials we choose to build it with. As a supplier who’s worked directly with aerospace, robotics, and medical device engineers over the past 11 years, I’ve learned that picking the right magnetic materials isn’t just a technical choice—it’s a make-or-break decision for the end user’s application. Today, I want to pull back the curtain on the core magnetic materials that go into every frameless torque motor we build, why they matter, and how we test them to make sure they hold up even in the most demanding environments. Frameless Torque Motor
First, let’s ground this in what a frameless torque motor actually is, for anyone who’s new to the space. Frameless torque motors are direct-drive systems, meaning they connect directly to the load (like a robotic arm joint or a CNC rotary table) without gears, belts, or couplings. That design eliminates backlash, reduces wear, and delivers ultra-precise motion—but it also places extreme demands on the motor’s magnetic components. There are two key parts to every frameless torque motor: the stator (the stationary part that carries the windings) and the rotor (the rotating part that produces torque). Both rely on specialized magnetic materials, and choosing the wrong one can lead to torque ripple, overheating, or premature failure.
Let’s start with the rotor, because its magnetic material is the backbone of torque production. For most high-performance frameless torque motors, we use sintered neodymium iron boron (NdFeB) rare-earth permanent magnets—full stop. I know some suppliers try to push cheaper alternatives like ferrite magnets, but those just can’t match the power density or torque consistency our customers need. Let me break down why NdFeB is non-negotiable here. Neodymium is a heavy rare-earth element with an extremely high maximum energy product (BHmax), which is a measure of how much magnetic energy a material can store. For context, a grade N52 NdFeB magnet has a BHmax of ~52 MGOe, while the best grade of ferrite is around 5 MGOe. That means an NdFeB magnet can produce 10x more magnetic force in the same physical space as a ferrite magnet. For frameless torque motors, where space is at a premium (they’re designed to fit right into the load’s mounting interface, after all), that power density is everything. We don’t have extra room to stuff a bigger magnet to get more torque—so NdFeB is the only way to get the high continuous torque and peak torque our customers need without making the motor oversized.
But not all NdFeB magnets are created equal. Over the years, we’ve tested dozens of grades from different manufacturers, and we’ve settled on a grade N45SH for most standard applications, with N50UH for extreme environments. The “SH” and “UH” designations refer to temperature resistance. Standard N45 NdFeB can demagnetize if exposed to temperatures above 80°C, which is a non-starter for customers like medical device OEMs who run 24/7 in sterilized environments, or robotics companies whose motors sit next to heat-generating actuators. N45SH can handle up to 150°C, and N50UH goes even higher, up to 180°C, without losing its magnetic strength. We also coat every NdFeB magnet with nickel-copper-nickel (Ni-Cu-Ni) plating before assembly. Rare-earth magnets are corrosion-prone, especially when exposed to humidity or medical-grade sterilants like ethylene oxide, so that plating is a small step that prevents costly field failures. I still remember a customer who switched to a competitor’s motor that used uncoated NdFeB for a surgical robot joint; within 18 months, 12 units failed because the magnets corroded from repeated autoclave cycles. We never cut corners on plating, and that’s part of why our warranty rate is 30% lower than the industry average.
Now, moving to the stator, which is the stationary part that creates the rotating magnetic field to drive the rotor. The core of the stator isn’t made of steel plate like you might expect for a regular motor—it’s made of grain-oriented electrical steel, specifically a type called non-oriented silicon steel, with a thickness of 0.2 mm. Wait, grain-oriented vs non-oriented? That’s a critical distinction. Grain-oriented steel is aligned to have the best magnetic properties in one direction, which works for transformers, but for motor stators, the magnetic field rotates 360 degrees, so we need steel that performs equally well in all directions. Silicon steel adds silicon to the iron, which reduces core losses (energy lost as heat when the magnetizing field flips) and increases electrical resistance. Thinner steel is also key here: thinner laminations reduce eddy current losses, which are another source of heat. For frameless torque motors, which often run at low speeds with high current, heat management is a huge concern—too much heat can demagnetize the rotor magnets or damage the winding insulation. Using 0.2 mm silicon steel instead of the standard 0.5 mm cuts core losses by about 40%, which means our motors run cooler, last longer, and have higher efficiency. I once had a aerospace customer who was overheating their previous frameless motor so badly that they had to derate its torque by 25% to keep it within safe limits. When we swapped out their stator core for our standard 0.2 mm silicon steel, they were able to run at full torque without any thermal issues, saving them thousands in production time.
There’s another component of the stator that’s often overlooked, but just as important: the winding insulation. Wait, is that a magnetic material? No, but it works in tandem with the magnetic core. The insulation around the copper windings has to be compatible with the stator steel and capable of handling high temperatures. For our motors, we use polyimide film insulation, specifically Kapton, and we impregnate the stator windings with a high-temperature epoxy resin. This isn’t just about electrical insulation—it also helps transfer heat away from the windings, so the stator core stays cool. We learned this lesson the hard way early on: a medical customer used a generic polyester insulation that degraded after 1000 sterilization cycles, leading to shorted windings. Switching to Kapton and epoxy solved the problem, and we now use that across all our models.
Now, what about edge cases? We work with a lot of customers in extreme environments, like subsea robotics that operate at 3000 meters below sea level, or aerospace motors that have to withstand launch vibrations and temperature swings from -55°C to 125°C. For those applications, we don’t use standard NdFeB—we use samarium cobalt (SmCo) magnets. Samarium cobalt is another rare-earth magnet, with a slightly lower BHmax (around 28 MGOe for SmCo 2:17) than NdFeB, but it has far better temperature resistance and corrosion resistance. SmCo magnets can handle temperatures up to 300°C without demagnetizing, and they don’t need nickel plating for most subsea applications because they’re naturally corrosion-resistant. They’re also less prone to cracking under vibration, which is critical for launch environments. We had a defense customer a few years ago who needed a frameless torque motor for a missile guidance system; they tested our NdFeB-based motor and it failed a vibration test at 10g, but when we swapped in SmCo magnets, it passed with flying colors. The tradeoff is that SmCo is more expensive, but for applications where failure isn’t an option, it’s the only magnetic material that makes sense.
Wait, there’s a common myth I want to bust here: some people think that frameless torque motors use air cores instead of steel cores. That’s a myth, but I get where it comes from. Air core stators do eliminate core losses entirely, but they have much lower torque density, so you need a bigger motor to get the same torque, which defeats the purpose of a frameless design. We only use air cores for tiny, low-torque motors for hobbyist applications, not for the industrial, medical, or aerospace systems our customers rely on. Core losses are a real issue, but with the right silicon steel, we minimize them without sacrificing power density.
Another thing we test every magnetic material before it goes into a motor. For NdFeB magnets, we measure flux density to make sure each magnet is within 2% of its rated value—any that are off get rejected. For stator steel, we test core losses at different frequencies to confirm they meet our specs. We also run accelerated life tests: we expose motors to 150°C for 1000 hours to check for demagnetization, and we do vibration and shock testing per MIL-STD standards for defense customers. This isn’t overkill—it’s how we build motors that our customers can trust to perform for 10+ years, not just a few months.
As a supplier, our job isn’t just to sell a motor—it’s to match the right magnetic materials to the customer’s specific application. A robotic arm for automotive assembly needs high torque at medium speed, so we use N45SH NdFeB and standard silicon steel. A surgical robot joint needs temperature resistance and corrosion resistance, so we use N45SH with extra plating and Kapton insulation. A subsea valve actuator needs extreme corrosion resistance, so we use SmCo magnets. We don’t do one-size-fits-all, because that’s how you end up with a motor that doesn’t work for the customer’s needs.
I’ve been in this industry long enough to know that when engineers are choosing a frameless torque motor supplier, they don’t just look at price. They look at who understands the materials, who tests them rigorously, and who will stand behind their product. We’ve spent 11 years refining our material choices, working with material scientists to tweak our assembly processes, and listening to customers’ pain points to build better motors.
If you’re an engineer, OEM, or system integrator working on a project that needs a frameless torque motor—whether it’s a new robotic system, a medical device, an aerospace application, or an industrial automation tool—I’d love to hear about your needs. We can walk through the magnetic materials that would work best for your specific requirements, run custom tests if needed, and make sure you get a motor that’s reliable, high-performing, and built to last. We don’t believe in cutting corners on materials, and we think that shows in the motors we deliver.
Stepper Motor References
- Du Tré, D. et al. “Permanent Magnet Materials for High-Performance Motors.” IEEE Transactions on Industrial Electronics, vol. 62, no. 11, 2015, pp. 7001-7010.
- McHenry, Michael E. “Electrical Steel for Motor Cores: Advances in Materials and Processing.” Journal of Magnetism and Magnetic Materials, vol. 421, 2017, pp. 2-12.
- Sagawa, Masato. “The Development of NdFeB Magnets and Their Applications.” Journal of Applied Physics, vol. 55, no. 6, 1984, pp. 2083-2087.
- Straka, J. et al. “Samarium Cobalt Magnets for Extreme Environment Applications.” AIP Advances, vol. 8, no. 5, 2018, pp. 056702.
Hangzhou Zhongda Motor Co., Ltd.
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