Makita XRU09 18V X2 LXT Brushless String Trimmer Power Tool Technology

What is an Outer Rotor Brushless Motor? Comparing Inner and Outer Rotor Designs

New technology is yielding more powerful cordless outdoor power equipment and tools. So, what is an outer rotor brushless motor and how does it compare to inner rotor designs?

I spoke with Carlos Quintana, Senior Product Manager of Makita’s Cordless Tools division to get an idea of the difference along with the pros and cons of both designs.


General Design Differences

The main difference between inner rotor and outer rotor brushless motors lies in the basic design. All motors, brushed or brushless, use magnets to turn. The role of the stator is to provide a charge that will repel or attract the magnets and make the motor spin. The rotor contains those magnets and does the spinning.

On an inner rotor design, the stator is on the outside and the rotor is on the inside. On an outer rotor brushless motor, they flip.

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Design Consequences


Let’s say you have one of each type of motor with identical power outputs and overall diameters. How to do they look compared to each other?

They need the same magnetic surface area. Since the inner rotor design has the magnets against the inside, they’re on a smaller diameter – they will need to be longer.

The opposite is true on an outer rotor brushless motor. Attaching to a wider diameter means the overall motor is shorter. It’s also lighter weight.

What is an Outer Rotor Brushless Motor? Comparing Inner and Outer Rotor Designs


Inner rotor motors also have inherent wobble – they ride in and out slightly within the stator and lose efficiency. There are ways to deal with that. For example, Makita adds ridges to their rotors that help stabilize it and bring the efficiency to 99%. But not all motor manufacturers do that.

Outer rotor brushless motors don’t have to deal with that movement and are inherently more efficient by design.

Balancing the Benefits

So you end up with two choices when you go with an outer rotor design. One option is to shrink the length and weight of the motor to get the same power with a bump in efficiency.

Alternatively, you can keep the weight and get more power while maintaining the efficiency gains. Of course, you can also find a balance between the two and shave some of the weight while increasing the power on a less substantial level.


Tool Applications

The majority of brushless tools you use contain an inner rotor design. Makita has four in their arsenal currently: 80mm, 54mm, 44mm, and 38mm.

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Makita’s 18V X2 string trimmers and chainsaws get a fifth design – an 80mm outer rotor brushless motor. These larger tools have room to house the motor’s diameter while finding relief in the overall footprint since they have a narrower length.

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That profile allows Makita to put the motors in those tools with a direct drive so they’re getting the benefit of the most efficient power transfer.

Looking Forward to More Outer Rotor Brushless Motor Tools

As manufacturers look to drive more traditionally corded tools to cordless, you can expect to see more outer rotor brushless motors coming with them. Lithium-ion still hasn’t reached the pinnacle of what it’s capable of and outer rotor designs will help generate more power and efficiency as we push the limits of what battery power can do.

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