As a DC motor supplier who’s spent the last 12 years answering the same question at least three times a week, I can promise you this: choosing the right DC motor frame size isn’t just about pulling a number from a spec sheet. I’ve seen it go wrong—too small a frame leads to overheating and a motor that dies before its time, too big and you’re wasting money on unnecessary space, weight, and energy. Last year alone, I helped a robotics integrator fix a last-minute mess where they’d ordered a NEMA 23 frame instead of NEMA 17 for their collaborative arm, and it cost them two weeks of production delays. That’s the kind of mistake we’re here to avoid, so let’s break this down like we do with every customer who walks through our door. DC Motor
First, let’s get one thing straight: what even is a DC motor frame size, beyond a random set of numbers? If you’re new to this, you might think it’s just the overall length or width of the motor, but it’s so much more. Frame size is a standardized designation that covers two critical things: the physical mounting dimensions that let you bolt the motor to your machine correctly, and the space available for internal components like the stator, rotor, and windings that deliver torque and power. The most common standards we work with for our DC motors are NEMA (National Electrical Manufacturers Association) for North America, IEC (International Electrotechnical Commission) for global markets, and sometimes custom frame sizes for specialized applications. For example, NEMA 17 frames are 1.7 inches square (the mounting bolt circle is exactly 1.7 inches, which is where the name comes from) and are ubiquitous in 3D printers, small robotics, and medical devices. NEMA 23 is 2.3 inches square, with a larger shaft and more space for windings, so it handles heavier loads. IEC frames, on the other hand, use millimeters—something like IEC 63 or IEC 80, where the number refers to the diameter of the stator housing in millimeters. That’s the first big check: know which standard your machine uses, or we can help you cross-reference if you’re mixing standards.
Now, the first step in choosing the right frame size is not even looking at frame numbers—it’s defining your application’s core requirements. I tell every customer to start with three non-negotiables: torque, speed, and physical space. Let’s start with torque, because that’s the work the motor has to do. Torque is the rotational force that moves your load—whether that’s turning a conveyor belt, spinning a surgical tool, or moving a robotic arm. A motor with too little torque for your application will stall, overheat, and burn out winding insulation fast. When torque goes up, you need more internal space in the motor to fit larger windings or stronger permanent magnets, which means a larger frame size. For example, if you’re building an automated packaging conveyor that needs to move 50-pound boxes, you’re not going to use a NEMA 17—you’ll need a NEMA 34 or an IEC 100 frame motor to deliver the high starting torque required to get that load moving. Conversely, a small portable pump for a blood analyzer needs only a few ounce-inches of torque, so a NEMA 17 or IEC 63 frame is more than enough.
Next, speed. Speed is how fast the motor can rotate, usually measured in RPM (revolutions per minute). Most DC motors have a rated speed, and if you need a certain speed, that affects frame size too, but it’s a little more nuanced than torque. Higher speed motors often need smaller shafts and more precise bearings, but they can be smaller in frame size than high-torque, low-speed motors. For example, a 10,000 RPM DC motor for a CNC spindle might fit in a NEMA 23 frame, while a 100 RPM motor for a conveyor that needs the same torque as the spindle will need a NEMA 34 frame because the lower speed means you need more torque per RPM, which requires larger internal components. It’s also worth noting that if you plan to use a gearbox with your motor (which almost everyone does for industrial applications), that changes the torque and speed, so you have to account for gear reduction when calculating what torque you actually need at the load, not just the motor shaft. We have a free calculator on our website for this, by the way—just plug in your load weight, gear ratio, and required speed, and it’ll give you a baseline torque number to work from.
The second big factor is physical space, and this is where so many customers get tripped up. You might have the perfect torque and speed requirements, but your machine only has a 2-inch by 2-inch space for the motor. There’s no point in ordering a NEMA 25 frame motor that’s 2.5 inches square if it won’t fit in the housing you’ve already designed. I had a customer last quarter who built a custom test rig for aerospace sensors, and he’d designed the motor mount for a NEMA 17 frame, but he tried to save money by ordering a smaller NEMA 14 motor because it was cheaper. The problem? The NEMA 14 had a smaller shaft, so the pulley he used for his test rig didn’t fit. He ended up having to re-machine three custom parts, costing him three times what he saved on the cheaper motor. Always measure the exact space available: overall length (from the mounting flange to the end of the shaft), width and height of the mounting surface, and even the diameter of the shaft opening in your machine. If you’re working with a custom design, it’s always better to send us a CAD file of your motor mount—our engineering team will check if a standard frame will fit, or if we can adjust a custom frame to match your specs without extra lead time.
Then there’s environmental and operational considerations, which are often overlooked but can make or break your frame size choice. If your motor is going to be used in a harsh environment—like a food processing plant with washdowns, or a outdoor agricultural robot with dust and moisture—you might need a larger frame to accommodate thicker insulation, better seals, or larger heat sinks for heat dissipation. DC motors generate heat when they run, and larger frames have more surface area to dissipate that heat, which is critical for continuous operation. If your motor is going to run 24/7, like in a warehouse conveyor system, a smaller frame might overheat even if it has enough torque on paper, because it can’t get rid of the heat fast enough. We had a customer in the logistics industry go with a NEMA 23 motor for their 24/7 conveyor, and within six months, 12 motors had failed due to overheating. Swapping them for NEMA 34 frames solved the problem immediately, because the larger frame’s heat dissipation kept the winding temperatures within safe limits even during long runs. Another environmental factor: vibration. If your application has high vibration, like heavy machinery or construction equipment, larger frames are more rigid, so they resist flex better than smaller, lighter frames, which can prevent premature bearing failure.
Wait, let’s talk about shaft size and mounting style too—those are part of the frame size package, and they matter a lot for integration. Frame size isn’t just about the housing dimensions; it includes the shaft diameter, keyway size, mounting flange type, and even the location of the electrical connections. For example, two motors could have the same NEMA 23 frame size, but one has a round shaft and the other has a D-shaped shaft, and if your pulley or gearbox requires a keyed shaft, the round shaft one won’t work. That’s why when you’re checking frame sizes, you have to make sure all the mating dimensions match your existing hardware, or that your new hardware is compatible with the motor’s specifications. Some customers assume all NEMA 23 motors are the same, but that’s not true—different manufacturers might have slight variations in flange mounting holes or shaft lengths, which can lead to fit issues. We pride ourselves on machining every frame to exact industry standards, so our NEMA 23 frames have a 0.25-inch diameter shaft, a 0.06-inch keyway, and mounting holes that match ANSI B5.54 specs, so you can replace any brand’s NEMA 23 motor with ours without any machining.
Another common mistake is ignoring efficiency, which ties back to frame size and operating costs. A larger motor frame might have a higher initial cost, but if it’s more efficient, it will save you money on electricity over time. For example, a small frame motor that runs at 70% efficiency will use more power to deliver the same torque as a larger frame motor that runs at 90% efficiency. For a machine that runs 8 hours a day, that adds up to hundreds or thousands of dollars in extra energy costs every year. We always walk customers through this trade-off—sometimes the slightly more expensive, larger frame motor is a better long-term investment, especially for industrial applications where uptime is critical. We had a manufacturing customer who saved $1,200 a year on energy by switching from a 75% efficient small frame motor to a 90% efficient larger frame motor, and he also cut his maintenance costs because the larger motor ran cooler and had fewer failures.
So let’s recap the step-by-step process we use with every customer, because that’s the part you can take with you when you’re choosing your DC motor frame size. First, define your application requirements: calculate the exact torque needed at the load (not just the motor shaft, including gear ratios if used), the required speed, and the duty cycle (how long the motor runs at a time). Second, measure the physical space available: confirm the mounting flange dimensions, shaft opening size, and overall length of the motor. Third, consider the environment: will the motor be in a harsh, high-vibration, or continuous-use setting? Fourth, check compatibility with existing hardware: shaft size, keyway, flange type, and electrical connection location. Fifth, factor in long-term costs: energy efficiency and maintenance, not just the initial purchase price.
If you’re still unsure, that’s what we’re here for. We don’t just sell motors—we’ve been solving integration problems for small startup robotics companies and large industrial manufacturers for over a decade, and we’ve seen every mistake you can make when choosing a frame size. Last month, a startup robotics team reached out because their custom frame motor had a shaft that was too long, and their end effector wouldn’t fit. Our engineering team worked with them to adjust the frame’s length without changing the torque or speed specs, and we got a custom prototype to them in three days, so they didn’t miss their launch deadline. We’ve done everything from cross-referencing NEMA and IEC frames to designing custom frames for medical devices that need to fit in tiny handheld housings, so no matter what your application is, we can help you get the frame size right the first time.
The worst thing you can do is guess at frame size and hope for the best. I’ve seen customers lose weeks of production, waste thousands of dollars on incorrect parts, and even damage their entire machine because they ordered a motor that was the wrong size. Take the time to do the calculations, measure your space, and account for all the factors we talked about—torque, speed, space, environment, and compatibility. If you’re stuck, send us your specs, measurements, or even a CAD file, and our team will help you pick the perfect frame size motor for your needs. We’ll never push you to get a frame that’s too big, and we’ll tell you if a smaller frame will work, because our goal is for your motor to run smoothly, last a long time, and fit exactly where you need it.
Don’t let a wrong frame size slow you down. Reach out to us today to discuss your DC motor requirements, and we’ll walk you through every step of choosing the right frame size for your application.
AC Gear Motor References
NEMA Standards Publication No. MG1-2021, Motors and Generators, National Electrical Manufacturers Association.
International Electrotechnical Commission (IEC) Standard 60034-1, Rotating electrical machines – Part 1: Rating and performance.
“DC Motor Selection Guide”, Motion Control Association, 2022.
“Thermal Management of Electric Motors”, IEEE Transactions on Industry Applications, Vol. 58, No. 3, May-June 2022.
Hangzhou ANG Drive Co., Ltd.
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Address: No.185, Jincheng Rd., Hangzhou 311202, China
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