What kind of magnet is used in the inner rotor of the motor?

An inner rotor refers to the rotating part of the motor being located inside the stator. So, what kind of magnet is used on the inner rotor?

The primary function of the magnets on an internal rotor is to generate a magnetic field that causes the motor to rotate. As for the type of magnet used, this varies depending on the specific motor. Common magnets used in internal rotors include neodymium-iron-boron magnets, ferrite magnets and samarium-cobalt magnets, with radial external magnetisation.

Neodymium-iron-boron magnets are currently the most widely used. Their advantages include strong magnetic force, compact size and high motor efficiency; consequently, they are the preferred choice for many high-speed and miniaturised motors. For example, the inner rotors of drones, power tools and electric vehicle drive motors often utilise neodymium-iron-boron magnets.

Radial magnetisation is typically used for magnetising the inner rotor.

For products with lower cost requirements, ferrite magnets are typically used. Although their magnetic strength is not as high as that of neodymium-iron-boron magnets, they are inexpensive and offer good corrosion resistance. Applications such as toy motors and car windscreen wiper motors often utilise ferrite magnet tiles or multi-pole magnetic ring structures.

Another type is samarium-cobalt magnets. These magnets offer excellent high-temperature resistance and are suitable for high-temperature environments or equipment requiring high reliability, such as certain motors used in aviation, power tools and the defence sector. However, due to their high cost, they are not used as widely as neodymium-iron-boron magnets.

The method of mounting magnets in internal-rotor motors also varies. In some cases, they are attached directly to the surface of the rotor, known as surface-mounted. In others, they are embedded within the rotor core, known as an internal-embedded configuration. Many drive motors in modern new energy vehicles now utilise this internal-embedded magnet structure, which offers higher rotational speeds and improved efficiency.

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