Permanent-magnet ac motors are polyphase synchronous motors with permanent-magnet rotors.
Thus they are similar to the synchronous machines discussed up to this point in this chapter with the exception that the field windings are replaced by permanent magnets.
Figure 5.29 is a schematic diagram of a three-phase permanent-magnet ac machine. Comparison of this figure with Fig. 5.2 emphasizes the similarities between the permanent-magnet ac machine and the conventional synchronous machine. In
fact, the permanent-magnet ac machine can be readily analyzed with the techniques of this chapter simply by assuming that the machine is excited by a field current of constant value, making sure to calculate the various machine inductances based on the effective permeability of the permanent-magnet rotor.
Figure 5.30 shows a cutaway view of a typical permanent-magnet ac motor. This figure also shows a speed and position sensor mounted on the rotor shaft. This sensor is used for control of the motor, as is discussed in Section 11.2. A number