New technologies are yielding more powerful cordless outdoor power equipment and tools. A new type of motor integrated into power tools is the outer rotor brushless motor design. Also known as external rotor motors, we've seen them used in everything from cordless circular saws to battery-powered chainsaws, cord trimmers, and lawn mowers. So, what is an outer rotor brushless motor and how are inner rotor and outer rotor motors different?
We spoke to Carlos Quintana, senior product manager for Makita's cordless tools division, to find out the difference and the pros and cons of both designs.
Be sure to check out our general article on Makita brushless motors.
Table of contents
- General Design Differences for Outer Rotor Brushless Motors
- Inrunner and Outrunner motor footprint
- Outer rotor brushless motor efficiency
- Balance the benefits of each design
- Inrunner and Outrunner Applications for Power Tools
- Looking forward to more outer rotor brushless motor tools
General Design Differences for Outer Rotor Brushless Motors
The main difference between inner rotor and outer rotor brushless motors is the basic design. All brushed or brushless motors use magnets to turn. The role of the stator is to provide the charge that repels or attracts the magnets and makes the motor spin. The rotor contains these magnets and spins them.
In an inner rotor design, the stator is on the outside and the rotor is on the inside. On outer rotor brushless motors, they flip over.
Let's say you have one motor of each type with the same power output and overall diameter. How do they look compared to each other?
Comparing the inner rotor with the outer rotor, you will find that both have the same magnetic surface area. Since the inner rotor design has the magnets resting inside, they are smaller in diameter – they need to be longer.
The opposite is true for outer rotor brushless motors. Connecting to a wider diameter means you can shorten the overall length of the motor. Plus, your weight will naturally decrease.
Outer rotor brushless motor efficiency
Inner rotor motors also have inherent wobble. They move slightly in and out of the stator and lose efficiency. However, there are some ways around this problem. For example, Makita added ridges to help stabilize its rotors and increase efficiency to 99%. But not all motor manufacturers do this.
Outer rotor brushless motors don't have to deal with this movement and are inherently more efficient by design.
Balance the benefits of each design
So when you go with an outer rotor design you end up with two options. One option is to shrink the length and weight of the motor to get the same power while increasing efficiency.
Or, you can maintain your weight and gain more power by increasing your efficiency. Of course, you can also find a balance between the two, shedding some weight while adding power on a less substantial level. We do see this with battery powered lawnmowers that could benefit from the weight savings of a front brushless motor.
Comparing outer rotor (external rotor) brushless motors to inner rotor (inner rotor) designs, we can see that each typically leads in different categories.
| Outer rotor brushless | Inner rotor brushless | |
|---|---|---|
| outer diameter | wider | Narrower |
| overall length | Thinner | thicker |
| torque | higher* | reduce* |
| Efficiency (including speed and power usage) | reduce | higher |
| cooling | smaller | bigger |
| typical tool use | Large tools (lawn mowers) | Smaller tools (drills) |
Most brushless tools use an inner rotor design to reduce overall size (circumference). Makita uses at least four brushless inner rotor designs: 80mm, 54mm, 44mm and 38mm.
The Makita 18V X2 string trimmer and its chainsaw is a fifth design with an 80mm outer rotor brushless motor. These larger tools have room to accommodate the larger diameter of the motor while also lightening the overall footprint due to their narrower length.
This profile allows Makita to install motors in direct drive tools so that they receive the most efficient power transfer.
You can expect to see more external rotor brushless motors as manufacturers look to drive more traditional corded tools into cordless tools. Lithium-ion batteries have still not reached the peak of their capabilities. The outer rotor design helps generate more power and efficiency as we push the limits of what battery power can achieve.