Hey guys, it’s Jake from your go-to brushless motor supplier – the one who’s actually been messing with these motors long before they showed up in every RC car and drone on the market today. Let’s cut to the chase: if you’re here, you’re probably deep in a project where you need to make sure your new brushless motor isn’t gonna quit when the load gets real. That’s where stall torque comes in, right? But here’s the thing I see so many new builders (and even some seasoned ones) mess up: they Google a quick formula, grab a multimeter, and call it a day – and that’s how you end up with a motor that’s way weaker than you thought, or worse, fried it before you even got it installed. Brushless Motor
I’ve lost count of how many times a customer hits me up panicking because their “10 Nm stall torque motor” delivered way less on their test bench. Most of the time, it’s not that the motor is garbage – it’s that they measured stall torque wrong. So today, I’m walking you through exactly how to do this right, with stuff most people already have lying around (or can grab from your local hardware store for 20 bucks), and zero fancy lab equipment required. As a supplier, this stuff matters to me because if you spec our motor wrong, you’ll be back buying a replacement in two weeks. No one wants that – not you, not me.
First, let’s make sure we’re on the same page about what stall torque actually is, because if you mix this up with running torque, you’re already off. Stall torque is the maximum twisting force a brushless motor can output when it’s locked up – like, the shaft can’t rotate at all. If you’ve ever held a drill bit against a piece of wood and felt the drill strain before it finally cuts through? That’s stall torque in action. It’s not the constant torque the motor makes while spinning – that’s rated torque, which is lower and way more useful for everyday operation. Stall is the “make or break” number; if your application needs to lift 20 kg, you don’t want to guess at this.
Now, before we dive into the steps, let’s address the elephant in the room: why does this matter so much, especially if you’re working with our motors? Here’s the truth: brushless motors aren’t like their brushed cousins. Their stall torque depends on two big things: how much current you feed them (overcurrent = higher stall torque, obviously), and how well you control that current (bad controllers = wonky numbers). I’ve had customers try to use a cheap off-the-shelf ESC for a 20 Nm motor, and wonder why their stall torque was half what it should be – the ESC was limiting current to save itself. So when you measure, you’ve gotta make sure your power supply is capable of delivering the peak current the motor is rated for. For our motors, we list the stall current on every spec sheet – use that number, don’t wing it.
Alright, let’s get to the actual test. I’m gonna make this as simple as possible, no jargon that’ll make your eyes glaze over. All you need is: your brushless motor, its matching ESC (we sell these with our motors, by the way – just saying), a power supply that can hit the motor’s stall current (look for something with at least 2x the stall current rating, to be safe), a way to measure force (a luggage scale works, trust me), a long metal rod or a metal pipe that fits snugly on your motor’s output shaft, and a way to lock the motor’s shaft from spinning – a vise clamp or a C-clamp pair works great, just be gentle so you don’t bend the shaft.
Wait, hold up – safety first, always. I can’t stress this enough: when you lock a motor’s shaft and crank up the power, that current is gonna surge. Your power supply might hum loud, the ESC might get warm, and the motor’s gonna vibrate like crazy. Don’t hold anything metal near the shaft, don’t stand directly in front of the rod, and never leave this test unattended for more than 10 seconds max. Overcurrent for too long can not only fry the motor’s windings (which is a costly repair) but also cause the shaft to shear or the ESC to short. I’ve seen a rod go flying once because a customer left it on for 30 seconds – we still talk about that. So safety: read the spec, don’t exceed time, stay clear.
Okay, test time. Step 1: Prep the motor. First, mount your motor securely – if you don’t have a motor mount, you can clamp it in a vise (but use soft jaws or a piece of rubber between the vise and motor housing so you don’t scratch the finish or crack the end bell). Make sure it’s level, no wobble. Then, slide that metal rod onto the output shaft – we’re gonna use the distance from the shaft’s center to where we pull the scale, that’s the radius for torque calculation. So if your rod is 50 cm long (measured from center of shaft to where the scale hooks), that’s a radius of 0.5 meters. Easy.
Step 2: Hook up the power. Connect the ESC to the motor (correctly – red to positive, black to negative, no crosswiring, that’s a quick way to kill it), then connect the power supply to the ESC. For now, keep the power supply at low voltage, ESC in “manual mode” (no telemetry, just basic throttle control – most ESCs have a switch or setting for this), and throttle at zero. We’re gonna build up slowly.
Step 3: Lock the shaft and measure force. This is the part everyone gets wrong. Take your C-clamp or vise and lock the shaft completely – wiggle it to make sure it can’t rotate even a little bit. Once it’s locked, hook the luggage scale to the end of the metal rod. Hold the scale parallel to the floor (that’s important – if you pull up or down at an angle, your force number will be off), then slowly increase the throttle on the ESC. Watch the scale – when it stops moving and the motor is straining but not spinning, that’s your peak force reading. Jot that number down in Newtons or pounds, whatever your scale uses. Pro tip: do this 3 times, average them out – there’s a little variability every test.
Step 4: Calculate the torque. Torque is just force times radius, right? So if your rod is 0.5 meters, and your average force was 40 Newtons, that’s 40 N * 0.5 m = 20 Nm of stall torque. If your scale is in pounds, convert that to Newtons (1 pound = ~4.45 Newtons) first, then multiply by radius in feet or meters. Wait, why meters? Because most of our motors use metric torque ratings, but if you’re working with imperial, just adjust the units. Don’t mix meters and inches, that’s a classic math fail.
Now, let’s talk about common mistakes I see every single test. First, not locking the shaft tight enough. If there’s even a tiny bit of play, the motor will rotate before it hits peak torque, and your number will be way too low. Second, pulling at an angle. If you yank up at 30 degrees, the scale will show less force than you’re actually applying, because some of the force is going sideways, not tangential to the rod. Third, using a power supply that can’t hit the stall current. For example, if your motor’s stall current is 50 A, and your power supply only does 30 A, you’ll never get the full stall torque – you’re basically testing a half-powered motor. Fourth, running the test too long. I mentioned this earlier, but I’ll say it again: max 10 seconds. Brushless motors are designed to run at high speeds, not locked at full current. Doing a 30-second test is a great way to get a 10% lower torque number because the windings heat up and increase resistance (which drops torque) – that’s called “thermal derating”, super important. Heat is the enemy of these motors, so keep tests short.
Wait, what if you don’t have a scale? I know not everyone has a luggage scale lying around. Another method we use at the shop is the string and weight method. Instead of a rod, tie a string around the shaft, wrap a few times, then hang weights on the end. The weight you need to get the shaft to stop rotating is your force – same calculation. Just make sure the string is straight down, no slack. That’s actually a super accurate method for small motors, and way cheaper than a scale. I’ve used this for 10 years, no issues.
Also, let’s clarify something about ESC settings. A lot of ESCs have current limiting built in, which is good for normal use, but bad for testing stall torque. So when you’re doing this test, turn off any current limit, thermal limit, or boost settings. You want the ESC to push as much current as possible, same as if the motor was actually stalled under load. If you leave those settings on, you’ll see a lower number, and that’s not the real stall torque. For our ESCs, we have a “test mode” that disables those limits – just hold the button for 5 seconds after powering on, and you’re good.
Now, as your brushless motor supplier, let’s add a bit of context here. Every motor we ship has a spec sheet with published stall torque, but that number is at 25°C ambient temperature and peak rated current. If you’re testing in a hot workshop (like a garage in summer, 35°C+), your measured torque will be a tiny bit lower, because the motor’s windings are warmer, resistance is higher, current is a touch lower. That’s normal, so don’t panic if your test number is off by 5-10%. Also, if you’re using a higher voltage than the motor’s rated voltage, you’ll get higher stall torque – but we don’t recommend that, it’s a fire hazard. Stick to the rated voltage, that’s how we measure it in our lab.
Let’s run through a quick example to make this concrete. Suppose you just got our 500W brushless motor for a robotic arm, and the spec says 12 Nm stall torque, 48 V, 25 A stall current. Here’s how you test it: get a vise, clamp the motor so it can’t move, slide a 0.4-meter rod on the shaft, lock the shaft tight with a C-clamp, hook your scale to the rod, crank the ESC to full throttle (no limits), pull until the rod stops moving at full thrust. If your scale reads 300 N, that’s 300 N * 0.4 m = 12 Nm – perfect, matches the spec. If it reads 270 N, that’s 10.8 Nm, which is normal if you’re a bit angled or the rod is a tiny bit loose. If it reads way less than that, check if your power supply is delivering 25 A, or if you accidentally left the ESC in current limit.
Another thing: what if your motor has a gearbox? Oh right, gear motors are super common for robotics and automation, and they change stall torque completely. Wait, no – actually, the gearbox multiplies stall torque. So if you have a 12 Nm motor with a 5:1 gear ratio, the output stall torque is 60 Nm, minus a tiny bit for gear friction. How do you test that? Same method, just lock the output shaft of the gearbox, not the motor shaft. If you lock the motor shaft, you’ll get the motor’s stall torque, not the gearbox’s output. That’s another mistake I see all the time – people test the motor, not the assembly. So make sure you’re testing the exact component you’re gonna use in your project.
Now, let’s talk about when you might not need to measure it. If you’re using our motor in a off-the-shelf build, like a standard drone or RC car, our published stall torque is more than enough. The test is for custom projects: robotic arms, conveyor belts, material handling equipment, anything where the load is high and failure is costly. For those, testing is non-negotiable. I’ve had a customer build a small conveyor system and skip the test, then when they loaded it up, the motor stalled, burned out, and they lost a week of production. Don’t be that guy – 10 minutes of testing saves you thousands in repairs.
Quick recap to make sure you don’t forget anything: 1. Lock the correct shaft (motor or gear output). 2. Use tangential force, pull at 90 degrees to the rod. 3. Short test (max 10 seconds). 4. Match power supply current to stall rating, disable ESC limits. 5. Average multiple tests to avoid error.
If you’re still confused, or if you need help picking the right motor for your project, hit us up. As a brushless motor supplier, we don’t just sell parts – we help you get the right ones for your job. Whether you need a motor for a small DIY project or a heavy-duty industrial application, our team can walk you through specs, answer your questions, and even point you to the right test equipment if you need it. No sales pressure, just real advice from people who’ve been doing this for years.
Before I sign off, let’s talk about one last thing: why our motors are worth testing, and why our approach is different. A lot of cheap motors out there fudge their specs – their “15 Nm stall torque” is actually measured at 20 seconds, or with a low current limit, so it’s not real. All our spec sheets are from lab tests done exactly like I showed you here, 10-second tests, 25°C, full stall current. So when you measure our motor, you should get a number really close to what we publish. If you don’t, shoot us a pic of your test setup and the numbers, and we’ll help you figure out what went wrong – maybe a loose connection, a bad ESC, or something simple.
At the end of the day, measuring stall torque isn’t rocket science, but it’s not guesswork either. Do it right, and you’ll have a motor that works as hard as you need it to, no surprises. If you’re working on a project and need to nail this down, or if you just have questions about brushless motor specs, reach out. We’re here to help you build something that lasts, not just sell you a motor and ghost.
Brushless Motor REFERENCES
- Bose, B. H. (2006). Modern Power Electronics and AC Drives. Prentice Hall.
- Electric Motor Tech. (2022). "Stall Torque Measurement Methods for Brushless DC Motors". Industrial Motor Review.
- Aerospace Applied Electromechanical Systems. (2019). Test Standards for Brushless Motor Performance. SAE International.
Zibo Auric Mechanical and Electrical Technology Co., Ltd.
As one of the leading brushless motor manufacturers and suppliers in China, we warmly welcome you to buy advanced brushless motor for sale here from our factory. All customized motors are with high quality and competitive price.
Address: B419, High-tech Entrepreneurship Park, High-tech Zone, Zibo City
E-mail: cui@auricmotor.com
WebSite: https://www.auricmotor.com/
