Inrunner vs Outrunner Brushless Motors

Introduction

Brushless motors have become increasingly popular in recent years for applications ranging from small hobby drones to electric vehicles. There are two main types of brushless motors – inrunner and outrunner. Both offer advantages and disadvantages depending on the application. This article will provide a comprehensive overview of inrunner and outrunner brushless motors, their key differences, pros and cons, and typical applications.

Key Takeaways

• Inrunner brushless motors have the rotor inside the stator, while outrunners have the rotor surrounding the stator.
• Inrunners tend to be more compact and have higher torque density. Outrunners can dissipate heat better and are often more powerful.
• Inrunners are commonly used in smaller devices like drones and RC cars. Outrunners work well for applications like e-bikes and scooters.
• Outrunners are simpler to manufacture but inrunners allow more flexibility in motor design.
• Choosing between inrunner vs outrunner depends on factors like size, cooling, torque, and RPM needs.

How Brushless Motors Work

Before comparing inrunner vs outrunner designs, it’s helpful to understand how brushless motors work.

Brushless motors have three main components:

• Stator – the stationary part that contains the windings and magnets.
• Rotor – the rotating part.
• Controller – controls power to the motor coils to rotate the rotor.

In a brushless motor, torque is generated by the interaction between magnetic fields created by permanent magnets on the rotor and alternating current in the stator windings. The controller switches current through the stator coils in sequence to continually rotate the rotor.

Brushless motors provide several advantages over brushed DC motors:

• Increased efficiency and torque density
• Reduced noise
• Increased reliability due to no brushes to wear out
• Ability to achieve very high RPMs

With this basic understanding of brushless motor operation, we can now compare inrunner and outrunner designs.

Inrunner vs Outrunner Design

The key difference between inrunner and outrunner brushless motors lies in the location of the rotor relative to the stator.

Inrunner brushless motors have the rotor inside the stator. The rotor spins around the inner periphery of the stator.

Outrunner brushless motors have the rotor surrounding the stator. The rotor spins around the outer periphery of the stator.

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Inrunner vs outrunner brushless motor designs (image credit: DronesGlobe)

This inside-out difference in rotor/stator placement impacts the motor properties and performance.

Differences Between Inrunners and Outrunners

While inrunner and outrunner brushless motors operate on the same electromagnetic principles, their inverted layouts confer distinct characteristics.

Size

For a given power, inrunners tend to be smaller and more compact than outrunners. This is because in inrunners, the rotor is on the inside, allowing for a reduced overall diameter. Outrunners must house the stator internally, necessitating a larger outer diameter.

Inrunner sizes range from thumbnail-sized motors used in tiny drones up to larger motors used in electric cars. Outrunners also cover a wide range but tend to be used for higher power applications where size is less critical.

Torque

The torque density (torque per unit volume) of inrunner brushless motors is generally higher than that of outrunners. This stems from the inrunner rotor’s greater diameter, which provides higher torque leverage, along with packing the active components (stator and rotor) in a smaller volume.

However, for very high torque applications, outrunners have the advantage. Their exterior rotor allows for a larger diameter, which benefits torque. Very large outrunner designs are used to power electric bikes and scooters.

RPM Capability

Inrunners can achieve extremely high RPMs, over 100,000 RPM in some small designs. Their compact internals and rotor inertia allow rapid acceleration and deceleration.

Outrunners have higher rotational inertia and are limited to lower maximum speeds, typically 15,000 – 50,000 RPM for medium and larger sizes. However, high top speeds are not always critical, and outrunners offer advantages in torque and power in bigger sizes.

Heat Dissipation and Efficiency

Outrunner brushless motors tend to have better heat dissipation properties. Their exterior rotor design allows for more surface area exposure to cooling air. The interior stator also has more access to airflow.

This superior cooling capability translates to higher power output and better efficiency in outrunner motors compared to similarly sized inrunners. Large outrunner designs take particular advantage of this with their huge outer surface area.

Manufacturing and Cost

Inrunners tend to be more complicated and expensive to manufacture than outrunners. The inner rotor requires tighter tolerances and its magnets experience higher centrifugal forces. Outrunner rotors are simpler with their external mounting and readily accessible stator.

However, inrunner design complexity allows more options for optimizing performance. Different magnet types, exotic rotor designs, and advanced control algorithms can squeeze out every bit of torque and efficiency – critical in applications like electric vehicles.

Vibration and Noise

Outrunner brushless motors typically run more smoothly and quietly than similar inrunner designs. The lower rotational speeds, looser mechanical tolerances, and external mounting of outrunner rotors result in less vibration and noise.

Water Resistance

Outrunners offer better water resistance, especially when seals are added between the rotor and stator. Water can more easily penetrate the small internal gap in inrunners. Outrunners block water entry into the stator chamber.

This wet environment capability makes sealed outrunners a favored choice for applications like jet drives and underwater drones.

Typical Applications

The different characteristics of inrunner and outrunner brushless motors lend them to certain applications.

Inrunner Applications

• Small drones – require high RPMs and torque in a compact package.
• RC cars/trucks – high acceleration and speed.
• Robotics – precise control and high torque density.
• Electric vehicles – optimized torque, power, and efficiency.
• Computer fans – compact size and quiet operation.

Outrunner Applications

• Large drones – stability and power for lifting heavy payloads.
• Electric bicycles – high torque for pedal assists.
• Electric scooters/motorcycles – powerful propulsion.
• Submersible ROVs and boats – waterproof and high-torque.
• High power fans – move large volumes of air.

Inrunner vs Outrunner Comparison Chart

Pros and Cons of Inrunner vs Outrunner

Inrunner Pros

• Higher torque density
• Extremely high max RPM capability
• Very compact size for same power
• Can be highly optimized with advanced control and design

Inrunner Cons

• Poorer heat dissipation
• Lower efficiency
• More complicated manufacturing
• Costly in smaller sizes

Outrunner Pros

• Superior heat dissipation and higher efficiency
• Simpler and cheaper to manufacture
• Lower noise and vibration
• Better water resistance when sealed

Outrunner Cons

• Larger size for same power
• Lower max RPM capability
• Lower torque density

Conclusion

Inrunner and outrunner brushless motors offer distinct advantages that make each better suited for certain applications.

Inrunners shine where small size, high RPMs, rapid response, and torque density are critical – applications like small drones, robots, computer fans and performance electric vehicles.

Outrunners are the better choice where high cooling, efficiency, smooth operation and high overall power are needed – uses like large drones, electric bikes, underwater vehicles and ventilation fans.

By understanding the key differences in inrunner vs outrunner performance, size, cooling, cost, noise and more, engineers can select the optimal design for their particular application. With their many benefits, brushless motors will continue growing in popularity across a wide range of industries.

FAQ

What is an inrunner motor?

An inrunner brushless motor has its rotor inside the stator. The rotor spins around the inner periphery of the stator.

What is an outrunner motor?

An outrunner brushless motor has its rotor surrounding the stator, with the rotor spinning around the outer periphery of the stator.

Are inrunners better than outrunners?

There is no definitively “better” option. Inrunners offer higher torque density, compact size, very high max RPMs and precision control. Outrunners provide superior cooling, efficiency, lower cost, and can generate high power and torque in larger sizes.

What is the main difference between inrunner and outrunner?

The key difference is rotor placement – inside vs outside the stator. This impacts size, torque density, cooling, RPM capability, cost, and ideal applications.

Why are outrunners bigger than inrunners?

For the same power, outrunners need a larger diameter to house the stator internally. Inrunners can have a smaller overall size with the rotor inside.

Do inrunners have more torque?

In general, yes – the smaller form factor of inrunners allows higher torque density. But very large outrunner sizes can produce very high torque.

Which is better for drones – inrunner or outrunner?

Smaller drones usually benefit from compact inrunners, while larger drones favor the power and cooling of outrunners. Multicopters can use a combination.

Are inrunner motors waterproof?

Inrunners offer poor water resistance due to the internal gap between rotor and stator. Sealed outrunners are a better choice for wet environments.

Why do outrunners have lower RPMs?

The external rotor of outrunners has higher rotational inertia. This limits acceleration and max RPMs compared to inrunners.