Permanent-magnet dc machines are widely found in a wide variety of low-power applications. The field winding is replaced by a permanent magnet, resulting in simpler construction. Permanent magnets offer a number of useful benefits in these applications.

Chief among these is that they do not require external excitation and its associated power dissipation to create magnetic fields in the machine. The space required for the permanent magnets may be less than that required for the field winding, and thus permanent-magnet machines may be smaller, and in some cases cheaper, than their externally-excited counterparts.

Alternatively, permanent-magnet dc machines are subject to limitations imposed by the permanent magnets themselves. These include the risk of demagnetization due to excessive currents in the motor windings or due to overheating of the magnet.

In addition, permanent magnets are somewhat limited in the magnitude of air-gap flux density that they can produce. However, with the development of new magnetic materials such as samarium-cobalt and neodymium-iron-boron (Section 1.6), these characteristics are becoming less and less restrictive for permanent-magnet machine design.

Figure 7.16 shows a disassembled view of a small permanent-magnet dc motor.

Notice that the rotor of this motor consists of a conventional dc armature with commutator segments and brushes. There is also a small permanent magnet on one end which constitutes the field of an ac tachometer which can be used in applications where precise speed control is required.

Unlike the salient-pole field structure characteristic of a dc machine with external field excitation (see Fig. 7.23), permanent-magnet motors such as that of Fig. 7.16 typically have a smooth stator structure consisting of a cylindrical shell (or fraction thereof) of uniform thickness permanent-magnet material magnetized in the radial direction.

Such a structure is illustrated in Fig. 7.17, where the arrows indicate the direction of magnetization. The rotor of Fig. 7.17 has winding slots and has a commutator and brushes, as in all dc machines.

Notice also that the outer shell in these motors serves a dual purpose: it is made up of a magnetic material and thus serves as a return path for magnetic flux as well as a support for the magnets.

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