Most brushless motors are "inner rotor motors", with a stator with coils that are fixed in place; The middle rotor rotates under the action of the magnet on the shaft. In the "outer rotor motor", the stator is located on the inner side, and the rotor includes a bell shaped housing that rotates externally, on which the magnet is mounted.
The advantage of brushless motor internal rotor motor is that the rotational inertia of the rotor is low and the heat dissipation is very fast. On the contrary, in an external rotor motor, due to the presence of a rotor housing and magnets, the heating coil is isolated from the environment, resulting in relatively slow heat dissipation. Due to the large rotational inertia torque of the rotor and the difficulty in controlling the balance of the rotor housing, the outer rotor motor is not suitable for high rotational speed modes. Therefore, internal rotor motors are widely used in most industrial applications.
External rotor motors have significant advantages in mass production applications because this mode can reduce production costs. External rotor motors can also have shorter structures and typically have smaller stopping torque, and due to their larger rotor diameter under the same magnetic force, their torque is also greater.
For very small brushless motors, a very important special design form in the inner rotor is the cogless BLDC motor. Its stator is only composed of a ring-shaped metal plate, with a flat winding adhered or encapsulated inside. Because there is no iron core, the inductance of the motor is very low, and the current in the winding increases very quickly. In addition, the iron loss is significantly reduced, so brushless motors have higher efficiency levels. In slow operation, the lack of torque fluctuations can bring positive effects.

This design type is very important for brushless motors with a diameter less than 40mm, as their power density is significantly higher compared to slotted motors. This is because a large part of the stator between the windings in a cogged motor is empty. In cogless motors, this installation space can be completely filled with copper windings. The smaller the diameter of the brushless motor, the greater the advantages exhibited by the cogless motor.

Editor: ZiYu