Permanent Magnet Motor
Permanent magnets are attached to the rotor in place of rotor windings to generate magnetic field. PM motors are limited to lower power motors, usually less than 50KW.
In general, permanent magnets are beneficial because they reduce the total motor weight and increase efficiency. In a PPM rotor the magnetic field is produced by PMs.
The advantages of PM motors are usually the following:
- less motor weight
- simplification of rotor structure
- higher efficiency
- low rotor inertia
- less rotor losses
Two main types:
- Interior Permanent Magnets Motor, IPM
- Surface Permanent Magnet Motor, SPM
They are usually more expensive due to the use of rare earth magnets such as neodymium and samarium. Latest discoveries have shown ways to create such powerful magnets without using rare earth materials. Below is a configuration of a PM rotor
Usually there is a set of small and large magnets. The small magnets help to minimize eddy current losses
Assembly Process of Magnets
PM are added to the rotor with the following methods
- mechanical retaining
- chemical retaining
- physical retaining
A common technique is to use a special resin and glue the magnets inside the magnet pockets of the rotor core. Thus the magnets become load bearing structures and they are subject to centrifugal loading that can exceed their limits. Depending on the design of the rotor core the magnets can be positioned on the surface of the rotor or inside the rotor and thus can be called surface or interior permanent magnet motors accordingly. Figure below illustrates a surface permanent magnet rotor together with the stator.