Permanent Magnets in Motors for Motion Control

There are many types of motors one can choose for motion control. The choice is guided primarily by the application and the environment in which it will work. One option is electric drive motors that use permanent magnets instead of the more typical rotor windings. A permanent magnet motor uses rare earth magnets to generate torque. Here are a few key things to understand about permanent magnet motors and how they could help your business.

Components

Permanent magnet motors incorporate what are known as rare earth magnets. The two main types used today are neodymium and samarium-cobalt. Rare earth magnets can be up to three times more powerful than similarly sized non-rare earth magnets such as ceramic or ferrite magnets.

Construction

The overall form of a permanent magnet electric motor remains unchanged from an induction motor. The rotor still nests in the stator and generates torque and rotation. However, older styles of induction motors use copper windings on both the rotor and the stator. Permanent magnet motors substitute rare-earth magnets in place of the windings. If the magnets are mounted to the stator, this motor is known as “brushless”. If the magnets are mounted to the rotor, this motor is known as “brushed”.

Speed

Depending on how the magnets are mounted, a permanent magnet motor is capable of very high speeds. In a “surface mounted” motor, the magnets are mounted to the surface of the rotor. These tend to cost less due to a simpler construction method but aren’t capable of very high speeds without the potential risk of the magnets becoming detached. In an “interior buried” motor, the magnets are incorporated into the structure of the rotor. This is a more robust construction method that can achieve higher speeds - but the extra speed can come with a higher price tag.

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Application Benefits

Compared to traditional induction motors, permanent magnet motors can have benefits when used in motion control applications. The torque curve of an induction motor tends to have flat and inconsistent peaks. The torque curve of a permanent magnet motor tends to be much smoother and sinusoidal. This can mean much smoother output, which is a huge benefit to production accuracy and quality control. 

Important Note

Permanent magnet motors can only be used with a permanent magnet drive. Permanent magnet drives use current-switching to control torque. In addition to controlling torque, they control flux current simultaneously. If upgrading from induction motors to permanent magnet motors, the motor and drive must be replaced as a pair.

Permanent magnet motors can offer a significant amount of power, control and customization options. They can help your manufacturing line become more efficient.

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