PERMANENT MAGNETS
Permanent magnets are a common excitation source for rotating machines. The performance of a permanent magnet depends on how the magnet is installed in the machine and whether it was magnetized before or after installation. Most permanent magnets, except for the new neodymium-iron-boron magnet, are not machinable. They must be used in the machine as obtained from the manufacturer. Table 2.3 lists the main characteristics of com- mon permanent magnets.
Figure 2.8 illustrates the demagnetization curve which is a portion of the hysteresis loop of alnico V. The coercive force Hc (the intersection of the curve with the horizontal H axis) rep- resents the ability of the metal to withstand demagnetization from external magnetic sources. A second curve known as the energy product is often shown on this figure. It is the product of B and H plotted as a function of H. It represents the energy stored in the permanent magnet.
Figure 2.9 illustrates the B–H characteristics of several alnico permanent magnets. The characteristics of several ferrite magnets are shown in Fig. 2.10. The neodymium- iron-boron (NdFeB) permanent magnets are superior to most permanent magnets.
They also have a lower cost than samarium-cobalt (SmCo) magnets. Their machining characteristics, strength, and hardness are similar to those of iron and steel. Figure 2.11 shows a comparison of the NdFeB magnet characteristics with those of other common magnets. The energy product [product of B in gauss (G) and H in oersteds (Oe)] and the permeance ratio (ratio of B/H) are also shown on these figures. Permanent magnets are most efficient when operated at conditions that result in max- imum energy product. The permeance ratios are useful in designing magnetic circuits. The flux density Bd and field intensity Hd are used to designate the coordinates of the demagnetization curve.