611 Permanent Magnets

Permanent magnets are a source of mmf much like a constant current source, with relative permeability |r just greater than air, i.e., |ar~1.05-1.07. PM characteristics

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FIGURE 6.1 Characteristics of commonly used permanent magnets.

are displayed in the second quadrant of the B-Hplot, as shown in Figure 6.1, confirming the fact that these are sources of mmf. The magnets remain permanent as long as the operating point is within the linear region of its B-H characteristics. However, if the flux density is reduced beyond the knee-point of the characteristics (Bd), some magnetism will be lost permanently. On removal of the demagnetizing field greater than the limit, new characteristics will include another straight line parallel to, but lower than, the original. The common type of magnets used in PM machines are the ferrites, samarium cobalt (SmCo), and neodymium-iron-boron (NdFeB). The features and properties of these three magnets are as follows.1

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