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Exploring the Differences Between Outer Rotor Motors and Inner Rotor Motors

August 3, 2022

Miniature motors aren’t only classified by technology types like brush DC, brushless DC, and stepper; you can also classify them by their rotor configuration! There are two major types of motor configurations: outer rotor motors and inner rotor motors. In an outer rotor motor, the rotor is on the outside (external to the motor) and the stator is on the inside of the motor. Comparatively, in an inner rotor motor, the rotor is on the interior of the motor and the stator is on the outside of the motor. As a reminder, the stator's function is to supply a charge that will either repel or attract the magnets that are inside the rotor, which spins – thus causing the motor to rotate.

inner rotor motor construction                                 outer rotor motor construction

                                                                                  Construction of an Inner Rotor Motor                                                                                                 Construction of an Outer Rotor Motor

Key Differences Between Outer Rotor Motors vs. Inner Rotor Motors

Though both outer rotor motors and inner rotor motors are quite common, there are a few key differences distinguishing the two that go beyond their configurations described above. Examples of these characteristics include:

  • Topology. Outer rotor topology provides a greater amount of physical space for a higher number of magnetic poles as compared to inner rotor technology. Because of the higher number of magnet poles, there is relatively less flux per pole; this means that either a thin back iron (also known as the motor casing or motor tube) can be used, which will reduce the overall diameter of the motor, or higher-grade magnets can be used, which can increase the torque.
  • Torque. Outer rotor motors can deliver higher torques as compared to inner rotor motor technologies. Due to a high pole count, the electrical frequency is quite high; this may result in high eddy current losses in the stator core. To control these losses, very thin laminations can be used.
  • Inertia. Inner rotor motors have a lower moment of inertia compared to outer rotor motors, which translates into higher acceleration, faster response, and a lower mechanical time constant. In outer rotor motors, the material selection and weight reduction features are critical to reducing the moment of inertia.There is also a need for rotor balancing to deliver less noise, lower vibration, and a desired bearing life in outer rotor motors.
  • Axial Length. For a given power, inner rotor motors are axially longer than outer rotor motors. Outer rotor motors have a lower length to diameter (L/D) ratio, which makes them compact and flat; thus, outer rotor technology can deliver a higher torque density compared to inner rotor technology.
  • Heat Dissipation. The inner rotor motor has an exterior winding, which results in better heat dissipation than with outer rotor motors. In outer rotor motors, heat dissipation varies with speed, but can be improved by adding heat dissipation features.
  • Ingress Protection. Inner rotor topology has the advantage over outer rotor topology for ingress protection. With inner rotor topology, the motor is almost enclosed from all sides; there is very little gap present for foreign particles like dust and water to enter the motor, and it can also be protected from these particles by adding additional features. Conversely, with outer rotor motors, the rotor is rotating outside, which makes it relatively complex to close the motor from all sides and thus protect it from foreign particles.
  • Applications. Applications featuring outer rotor motors are typically high torque and low speed, such as E-bikes, drones, fans, pumps, surgical robots, electrical grippers, and premium tattoo machines. Conversely, applications utilizing inner rotor motors are relatively high speed and low torque, including medical infusion pumps, respirators, ventilators, surgical hand tools, and industrial power tools.

Searching for your next miniature motor solution? Portescap is a global motor supplier that has decades of experience in developing both inner rotor and outer rotor technologies for our portfolio, including inner rotor motors in brush DC, stepper, and brushless DC slotted & brushless DC slotless technologies ranging from 8mm to 60mm; we’ve also recently introduced the 20ECF and 32ECF, our newest flat outer rotor slotted motors. Let us know how we can help you develop your next motion control solution!