An outer rotor motor has a magnetic rotor that rotates around a stator. The rotor radius of an outer rotor motor is large, so it can produce a larger amount of drive torque than an inner rotor motor with the same diameter, giving it a superior constant velocity. On the other hand, countermeasures for vibration and noise that occur during rotation are vital as well.
The electromotive force is a cause of the vibration that occurs when a motor rotates. Additionally, when this electromotive force resonates with the motor’s eigenmodes, it causes even larger vibrations and noise. Countermeasures such as changing the motor’s eigenfrequency through processes like setting a hole in the rotor core have been taken with the objective of preventing resonance. In order to carry out these kinds of studies, it is necessary to get a precise, definite grasp of the electromotive force’s spatial distribution, frequency analysis, and natural frequency.
This note presents the use of a magnetic field analysis and structural analysis to obtain the sound pressure caused by electromagnetic vibrations in an outer rotor motor with holes fabricated in the rotor core.

The eigenmode for a rotor core with and without fabricated holes are indicated in Fig. 3. This analysis confirms the minimum frequency for the mode that is deformed in the radial direction, which was 649 Hz with holes and was 677 Hz without them. A large amount of vibration is generated by the resonance phenomenon of the eigenfrequency and the electromagnetic force.