If you’ve ever held a drone mid-flight after a long mapping session, a small electric scooter that’s just finished a steep hill climb, or a model airplane that’s completed a 15-minute aerobatic routine, you’ve likely felt it: that gentle (or sometimes surprisingly warm) hum coming from the outrunner brushless motor powering it. As a supplier of these motors, I get dozens of questions every week from engineers, product designers, and hobbyists asking the same core question: What is the actual heat dissipation of an outrunner brushless motor, and why does it matter? Outrunner Brushless Motor
I don’t blame them. Heat is one of the most overlooked yet critical factors in any application that uses brushless outrunners—miss the mark on heat management, and you’ll see reduced performance, shortened component life, or even a motor failing mid-use. Over my 12 years in this business, I’ve worked with clients who swore their 500W motor could handle 10 minutes of continuous use at full throttle, only to have it seize up halfway through a build test because they skipped a basic understanding of its heat output. Let’s break this down, no overly complex jargon, just the real-world facts I pass along to every customer that reaches out.
First, let’s get on the same page about what an outrunner brushless motor is, because that’s foundational to how it dissipates heat. Unlike inrunner motors, where the permanent magnets are inside the stator (the stationary part with wire windings), outrunners have the magnets mounted on the outer rotor casing, which spins around the stator. That outer spinning design is why they’re called outrunners, and it’s the biggest driver of their heat behavior. The stator’s copper windings are where most of the heat in any electric motor comes from—copper loss, to be exact. When electricity flows through those windings, the natural resistance of copper converts some of that electrical energy into heat, not motion. That’s not a flaw; it’s physics. All motors waste a small amount of energy as heat, but outrunners have a unique quirk here: their outer rotor spins around the stator, which creates natural airflow as long as the motor’s moving.
Now, to answer the core question: What is the heat dissipation of an outrunner brushless motor? The short, practical answer is that it’s not a single number—it’s dependent on three key factors: the motor’s size, its load, and how it’s cooled. But let’s give that some concrete numbers to make it real, because that’s what engineers need. For a common mid-sized outrunner used in entry-level drones (say, a 2205 size motor rated for 1500W peak and 500W continuous), the heat output at full continuous load is roughly 30-40 watts. That’s enough to raise the motor’s internal operating temperature (the windings, not the outer casing) to about 120-130°C, as long as there’s proper airflow. If you stall that same motor—meaning the rotor can’t spin, so all electrical energy goes to heat instead of motion—its heat output jumps to almost full rated power, so 1500W in this case, and it will hit a critical temperature (around 155°C, the point where winding insulation starts to degrade) in less than 2 minutes.
For larger outrunners, like the ones used in electric scooters or light electric vehicles, let’s take a 3000W continuous rated motor. Its heat dissipation at full continuous load is around 200-250 watts. That makes sense: bigger motors have larger windings, so more surface area to move heat, but their higher power means more total energy is being wasted as heat. At full throttle on a 5% incline, that motor will run at around 110-120°C if it’s using natural cooling, or 100-110°C if you add a small external heat sink. The key difference here is that outrunners, because their rotor spins around the stator, don’t need separate cooling fans in most small to mid-sized applications—their own rotation creates enough air flow over the stator and outer casing to dissipate most of the heat generated during normal use.
But here’s the part most hobbyists and even some new engineers get wrong: heat dissipation isn’t just about how much heat a motor generates—it’s about how well it can get rid of that heat. That’s where my team and I spend a lot of our time with clients, because a motor that works great on a drone’s stationary test bench might overheat on a delivery e-bike that’s stuck in stop-and-go city traffic. Let’s explain the two main cooling methods for outrunners, because they directly impact heat dissipation:
Natural Convection Cooling: This is the default for most small to mid-sized outrunners. The spinning rotor creates a low-pressure zone around the stator, pulling air through the gaps between the windings and over the motor casing. For drones, this works because the drone is always moving, so there’s constant airflow. For model airplanes, same thing—when the plane’s in flight, airflow is non-stop. The problem with natural convection is that it stops when the motor stops. That’s why if you land a drone after a 20-minute flight, you can usually touch the motor casing lightly, but if you leave the propeller spinning after landing to test takeoff, the motor will start to warm up quickly because the airflow from the prop is slower than when the drone is moving.
Forced Cooling: This is where we add an external fan or a heat sink with fins to push air over the motor’s surface, which drastically improves heat dissipation. For larger motors in electric scooters, for example, the motor is often mounted in the wheel hub, so there’s limited airflow around it when the scooter is moving at low speeds. Adding a small 12V fan that’s powered by the scooter’s battery can cut the motor’s operating temperature by 15-20°C, which extends its lifespan by years. I can’t tell you how many scooter brands we’ve worked with that came to us with motors failing after 6 months of use, only to find they skipped forced cooling for urban stop-and-go routes.
Another key factor that affects heat dissipation is the motor’s winding design. At our factory, we offer two common winding options for outrunners: Litz wire and solid copper wire. Litz wire is made of multiple thin strands of insulated copper twisted together, and it reduces what’s called “skin effect” in high-speed motors. Skin effect is when alternating current (AC) (which brushless motors use) flows only on the outer surface of the wire, increasing resistance and generating more heat. Litz wire cuts that heat generation by up to 25% in high-RPM applications, so an outrunner with Litz wire will dissipate less heat for the same power output than one with solid copper wire. That’s a big selling point for us for drones and electric planes that run at high RPM, because it means longer flight times and less heat-related wear.
Now, let’s talk about real-world performance, because numbers on a spec sheet don’t mean anything if they don’t hold up in use. Last year, we worked with a client that was building a line of heavy-lift delivery drones. They initially chose a standard 500W outrunner for their 2kg payload drone, and the motor was overheating after 8 minutes of flight, with the winding temperature hitting 145°C. That’s above the 130°C threshold that we recommend for continuous use to avoid insulation degradation. We walked them through adjusting their propeller size (a larger prop reduces the load on the motor, lowering current draw and heat) and recommended a winding design optimized for lower current in hover mode. After making those changes, the motor’s heat dissipation dropped by 12 watts, and they were able to fly for 22 minutes non-stop without the temperature going above 118°C. The client ended up reordering motors for all 500 of their drones, because that one change cut their component failure rate by 90%.
Another example: the electric scooter line we mentioned earlier. A startup came to us last year, saying their scooters had motors seizing up after 3 months of use in rainy, hilly Seattle. Their original setup was a 3000W outrunner with natural cooling, but in stop-and-go traffic, the motor would run at 135°C for hours, and the rain (which caused minor water buildup in the stator gaps) reduced the airflow even more. We recommended adding a small waterproof fan, and switching to Litz wire windings to reduce heat generation. That small adjustment fixed their overheating issue, and their motor lifespan doubled. They’re now our largest scooter client in North America, ordering 10,000 motors a month.
Now, let’s address a common myth I hear all the time: “Higher power motors run hotter, so I don’t need a 1000W motor if my application only needs 500W.” That’s partially true, but not entirely. A 500W motor running at full load will generate the same amount of heat as a 1000W motor running at half load? No, that’s not right. The heat generated by a motor is proportional to the square of the current flowing through the windings, not just the power. So a 500W motor pulling 10A will generate roughly the same heat as a 1000W motor pulling 7A, because lower current equals less heat. That’s why matching the motor’s size to your application is so important—too big a motor for a small load will run at a low current, generate less heat, and actually be more efficient than an undersized motor that’s forced to pull high current to do the same work.
So, what should you take away from all this? The heat dissipation of an outrunner brushless motor is not a fixed value—it’s a function of its design, your application’s load, and the cooling system you use. As a supplier, our job is to give you not just a motor that meets your power needs, but one that’s optimized for its intended use to minimize heat-related issues. Whether you’re building a drone, an electric scooter, a model airplane, or an industrial pump, we test every motor in our facility under real-world load conditions to give you accurate heat dissipation data, not just spec sheet numbers.
If you’re designing a new product, or having issues with overheating, I’d love to help you figure out the right setup. We don’t do one-size-fits-all solutions, and we work closely with every client to adjust winding designs, cooling options, and motor size to fit their specific needs. Over the years, I’ve learned that the best partnerships come from listening to what our clients actually need, not just selling them the most powerful motor we have.
Reach out to us to discuss your application, and we’ll provide you with detailed heat dissipation testing data, custom winding options, and recommendations to make sure your motor runs efficiently and reliably for years.
Automotive Brushless Motor References
- Chan, C. C. (2010). Thepast, present, and future of power electronics and motor drives for electric, hybrid, and fuel cell vehicles. Proceedings of the IEEE, 98(3), 462-477.
- Hemingway, C., & Emadi, A. (2014). Hot topic: A review of thermal management of electric motors in vehicular applications. IEEE Transactions on Transportation Electrification, 1(1), 14-25.
- Gieras, J. F. (2002). Permanent magnet motor technology: Design and applications (2nd ed.). CRC Press.
- Howey, D. A., Holmes, A. S., & Markos, A. (2010). Thermal analysis of permanent magnet brushless machines. IEEE Transactions on Energy Conversion, 25(3), 677-685.
- Bose, B. K. (2006). Power electronics and motor drives: Advances and trends. Academic Press.
Shenzhen HengDrive Technologies Co., Ltd.
Shenzhen HengDrive Technologies Co., Ltd. is one of the most professional outrunner brushless motor manufacturers and suppliers in China, specialized in providing high quality customized service. We warmly welcome you to buy the newest outrunner brushless motor in stock here from our factory.
Address: Building A & F, FuNing Hi-Tech Park, XinTian Road, FuHai Street, BaoAn District, ShenZhen, GuangDong Province, China.
E-mail: Marketing001@hengdrive.com
WebSite: https://www.hengdrivemotor.com/
