Motor magnetic wedges

They are used primarily on very large generator units to trim the last fractions off unwanted losses and provide a more efficient design. However, due to coil weight and rotational speed, the physical and magnetic properties have to be well matched with the design intent. For original equipment, wedges of specific magnetic materials can be used because the manufacturer understands the physical and magnetic properties of the material well AND has the process to accommodate them.

Most of the "bad press" for magnetic wedges comes from rewind shops, actually. Either the incorrect material is used (ranging from a poor magnetic profile to one that is not magnetic at all), or a bad match regarding mechanical strength is made (shape is wrong, or grain orientation is incorrect leading to poor retention of the coil, or the magnetic properties are damaged by the machining process, etc.)

The amount of loss in an induction machine due to stray/eddy effects is pretty tiny on the smaller equipment so the "gains" made from using magnetic material are correspondingly smaller as well. In the bigger picture, it's mostly not worth the effort to use magnetic wedges in smaller machines ... there's far more benefit to be gained by improving the mechanical efficiency of the remainder of the drive train and the electrical efficiency of the distribution system.