If you’ve ever worked with precision motion control, chances are you’ve bumped into frameless torque motors—those sleek, motor-in-hand, stator-and-rotor-only units that don’t come with a housing, flange, or any extraneous parts. As a supplier who’s been troubleshooting these things for 8+ years, the question I get asked most isn’t “what’s a frameless torque motor?” It’s “does size actually matter here?” Frameless Torque Motor
Spoiler: Yes, it does. But not in the way you might think.
Let’s start with what a frameless torque motor actually does, for anyone who isn’t deep in the weeds. Unlike a conventional servo motor that pairs a rotor, stator, encoder, and housing into one neat package, these are the core parts—mounted directly to your load and your machine frame, no extra gaps, no adapter plates that add play. They’re called torque motors because they deliver zero-backlash torque right at the load, no gearboxes to dilute that power. They’re used in everything from robotic arms that assemble phone screens to CNC machines that cut aluminum to satellite positioning systems that have to stay aligned for 15 years straight.
Now, size here isn’t just “how big is the motor” — it’s two separate, linked things: frame size (the outer diameter of the stator/rotor stack) and stack length (how long the core of the motor is, along the shaft axis). Most people mix these up, and that’s where mistakes start.
Let’s break down performance metrics that actually matter, and how size shifts each one.
First up is torque output. If you want more torque, you have two choices with frameless torque motors: go wider (bigger frame size) or go longer (bigger stack length). Simple physics here: torque in a motor is the product of force and radius. A wider stator means you’ve got more distance from the center of the shaft to the point where magnetic force is applied—so a 100mm diameter motor will naturally make more torque than a 50mm diameter motor, even if their stacks are the same length. Longer stack length means more copper windings and more magnets, so more magnetic force acting along the shaft. It’s like having more horses pulling a wagon, same path, just more pulling power.
But here’s the catch: torque density. That’s how much torque you get per unit of motor size and weight. If you take a tiny 50mm frame motor and stretch its stack to twice the length, you’ll get more torque, but your torque density drops. You’re adding a lot of length for only a little more torque, and the motor might get too long to fit in your machine’s design envelope. On the flip side, if you take a 200mm frame motor and only need 10% more torque, stretching the stack might be overkill—you’re carrying extra bulk and weight you don’t need. I had a customer a few months back who tried to cram a 200mm frame motor into a robot joint that only had a 120mm diameter opening. He thought “bigger = more torque” but didn’t account for the fact his machine couldn’t physically accept the width. We swapped him to a 150mm frame motor with a longer stack, and he got 15% more torque without modifying his robot. That’s the sweet spot.
Next, speed. Speed here isn’t just how fast it spins—it’s the maximum continuous speed the motor can handle without overheating or losing torque. How does size play into that? Bigger frame motors have larger rotors, so their maximum RPM is actually lower. The centrifugal force on the magnets and windings increases as the square of the RPM times the radius. A 200mm diameter motor spinning at 3,000 RPM has way more force pulling on its magnets than a 50mm diameter motor spinning at the same speed. So if you need a motor that spins fast, you don’t go big frame—you can go a smaller frame with a shorter stack, because shorter stacks run cooler too? Wait, no—let’s clarify heat. Heat is a big killer of motor performance.
Heat management ties directly to size. The windings in a motor generate resistance, and resistance makes heat. The bigger the motor’s surface area (more surface to dissipate heat), the easier it is to get rid of that heat. A large frame motor has more outer surface area than a small frame one, so it can handle more current (which makes more torque) without overheating. Small frame motors have less surface area, so they heat up faster. That’s why small frameless motors are rarely used for continuous high-torque applications—they can’t get rid of the heat fast enough, even if they’re running at low speeds. I worked on a medical device project a while back where the design team tried to use a tiny 60mm frame motor for a surgical robot joint that needed 2 Nm of continuous torque. It worked for 10 seconds, then overheated and shut down. We swapped it for an 85mm frame motor, same stack length, and it ran 8 hours straight with no issues. That’s heat in action.
Another big one: inertia matching. Inertia is the resistance to motion—so your load’s inertia has to match the motor’s inertia for it to move smoothly. Frameless torque motors shine here because you mount the rotor directly to the load, so there’s no extra inertia from a gearbox or coupling. But the motor’s own inertia is tied to its size. Small frame motors have low inertia—they spin fast, but can’t move heavy loads. Big frame motors have high inertia—they can move heavy loads, but they take longer to accelerate if the load is light. If your load is super light, you don’t want a big motor with high inertia—it’ll feel sluggish, like driving a truck to the grocery store. Conversely, if your load is heavy, a tiny motor with low inertia will stall out because it can’t overcome the load’s mass. That’s why inertia matching is all about size, not just power. I had a CNC customer last year who was using a 180mm frame motor for a table that only weighed 5kg. The motor’s high inertia meant each move took an extra 200ms, slowing down his whole production line. We swapped him to a 120mm frame motor, same torque output, and his cycle time dropped by 18%. He couldn’t believe it was the same torque—he thought bigger = better, but inertia made all the difference.
Wait, what about precision? That’s a big one for these motors, right? Zero backlash, super accurate positioning. Does size affect that? Let’s see. The bigger the frame, the stiffer the motor. A larger stator and rotor have more structural rigidity, so they don’t twist or flex under load. That means better position accuracy, especially when you’re applying high torque. A small frame motor will flex more, even a little, under high load, which can throw off positioning by tiny amounts—enough to matter in applications like semiconductor wafer handling, where you need 0.001mm accuracy. But if you don’t need ultra-high precision, a smaller motor is fine, and might even be more responsive. So size here is a trade-off between stiffness and application requirements.
Let’s talk about cost too, because no one works in a vacuum. Bigger motors cost more, plain and simple. More copper, more magnets, more material, more machining time. But a lot of customers oversize their motors just to be safe, which adds unnecessary cost. I see this all the time: someone designs a machine, picks a motor that’s 2x the torque they need, and then complains their machine is too heavy, too expensive, and uses too much power. The right approach is to match size to your specific needs: what’s your required torque, speed, load inertia, machine envelope, and precision? That’s how you get the best performance without wasting money.
I should also mention something people rarely talk about: customization. Since frameless torque motors are designed to integrate with your machine, size is super flexible. We can tweak stack length, magnet grade, winding design, even the shape of the stator to fit into weird spaces. For example, a customer in the packaging industry needed a motor that fit into a curved corner of a wrapping machine, so we built a slightly oval-shaped rotor that was the right frame size to deliver their exact torque, no extra bulk. That’s the beauty of frameless designs—size isn’t a one-size-fits-all.
Let’s wrap this up with a real quick example to tie it all together. Suppose you’re building a robotic arm for warehouse picking. You need: 5 Nm of continuous torque, maximum speed of 100 RPM, load inertia of 0.05 kg·m², and the joint can only be 120mm in diameter. Let’s compare sizes:
- 100mm frame: If we pick a 100mm frame with a 20mm stack length, it delivers ~4.5 Nm—close, but not enough. If we bump stack to 25mm, we get ~5.8 Nm, but heat might be an issue for continuous use.
- 120mm frame: A 120mm frame with a 18mm stack length delivers ~5.2 Nm, fits the diameter, has more surface area for heat dissipation, and lower inertia than a stretched 100mm frame. That’s the sweet spot. If we’d gone to a 150mm frame, we’d get more torque than needed, higher inertia, and a bigger, more expensive motor.
So what’s the takeaway? Size doesn’t make a motor “better” or “worse”—it makes it better suited for a specific job. Too small, and you’ll lack torque, overheat, and struggle with heavy loads. Too big, and you’ll have sluggish acceleration, extra cost, and wasted space. Frame size and stack length work together, not in isolation, to deliver the exact performance you need.
As someone who’s been in this game for years, I’ll tell you this: the worst mistake you can make is guessing at motor size. Too many customers just order the biggest one they can fit, and end up paying for features they don’t need. The right way is to map out your exact requirements first, then match the motor size to those specs.
If you’re working on a project and you’re not sure if your frameless torque motor is the right size, or if you’re looking to upgrade performance without changing your machine design, reach out. We don’t do one-size-fits-all, and we’re happy to walk through your requirements to figure out exactly what size motor will get you the performance you need, without the extra bulk or cost. No sales pitch, just real talk from someone who’s seen hundreds of these projects go right and wrong because of motor size.
Stepper Motor References
- Bose, B. K. (2017). Modern Power Electronics and AC Drives. Pearson Education.
- Frameless Torque Motor Design Guidelines. Motion Control Association, 2021.
- Miller, T. J. E. (2008). Permanent Magnet Synchronous and Brushless DC Motors. CRC Press.
- Precision Motion Control for Industrial Automation. International Society of Automation, 2020.
- Thermal Management in Electric Machines. IEEE Transactions on Energy Conversion, Vol. 35, No. 2, 2020.
Hangzhou Zhongda Motor Co., Ltd.
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