What is an Outrunner Motor?

Typically an outrunner motor is a Permanent Magnet Synchronous Motor (PMSM) torque motor. Therefore, the motor consists of a stator with coils and a rotor with permanent magnets. With conventional motors, the magnets are attached to a shaft that rotates on the inside of the stator coils. This is called an inrunner motor. With an outrunner motor the magnets are attached to a ring or sleeve on the outside of the stator coils. As a result, the motor has a rotating ring or sleeve, instead of a rotating shaft.

Outrunner and inrunner torque motors

There are two types of direct drive frameless torque motors: the outrunner and the inrunner torque motors. In case of an inrunner motor, the rotor is located on the inside of the stator.  In case of the outrunner motor, the rotor is located on the outside of the stator. 

Outrunner motors produce more torque for the same build volume compared to inrunner motors. Magnetic Innovations is specialised in the outrunner typology. But both type of torque motors, each with their own advantages, still function in a similar way.

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Differences between an outrunner motor vs inrunner motor

An advantage of an outrunner motor compared to an inrunner motor, is that the air gap surface is substantially larger. In other words, the surface area through which the electromagnetic field lines pass from rotor to stator, is much larger. This way more electromechanical force is generated.

Additionally the torque arm is longer for an outrunner motor, as the force is generated further from the centre of rotation. Consequently a larger air gap surface area and a longer torque arm both lead to higher torque. Hence, outrunner motors can achieve much higher torque levels than inrunner motors with the same build volume. 

To compensate for the lower torque, inrunner motors are often equipped with transmissions or gearboxes. But adding these mechanics lead to even higher build volume and mechanical losses. Furthermore it requires more maintenance, increases risk of contamination (oil, grease) and leads to less accuracy. So when build volume is restricted and high torque levels are required, outrunner motors are the best option. 

Advantages outrunner torque motor

Adventages of an outrunner motor, in short, are:

      • Maximum torque achieved for minimal build volume 
      • No need for mechanical transmission or gearboxes
      • Rotor can directly drive the application
      • Highly accurate
      • No risk of contamination

Visual representation of how an outrunner motor works

In the video you can see a visual representation of how an outrunner frameless torque motor works. On the inside is the stator, this is the static part of the torque motor, which contains the lam stack and copper wire. The lam stack contains “lam stack teeth” or “stator teeth”. On each of these teeth copper windings are wound. 

When three phase AC current is supplied to the copper windings, the stator turns into an array of electromagnets. As a result, an alternating rotating magnetic field is created around the stator, as shown in the video to the right. The orange and blue areas depict the south and north part, respectively. 

On the outside is the rotor with rare earth permanent magnets. These are consequently attracted and repelled by the alternating elec tromagentic field in the stator teeth, which drives the torque motor as shown in the video.

The copper windings on the stator can be configured in different ways to create different performance parameters that can be important for various applications in for instance semicon, robotics, food, and packaging industries.

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