TABLE OF CONTENTS Torque is produced by the interaction between the axial current carrying conductors on the rotor and the radial magnetic flux produced by the stator. The flux or excitation can be furnished by permanent magnets (Figure 3.2(a)) or by means of field windings (Figures 3.1 and 3.2(b)).
Permanent magnet versions are available in motors with outputs from a few watts up to a few kilowatts, while wound field machines begin at about 100 W and extend to the largest (MW) outputs. The advantages of the permanent magnet type are that no electrical supply is required for the field, and the overall size of the motor can be smaller. On the other hand, the strength of the field cannot be varied, so one possible option for control is ruled out.
Ferrite magnets have been used for many years. They are relatively cheap and easy to manufacture but their energy product (a measure of their effectiveness as a source of excitation) is poor. Rare earth magnets (e.g. neodymium iron boron or samarium cobalt) provide much higher energy products, and yield high torque/volume ratios: they are used in high performance servo motors, but are relatively expensive and difficult to manufacture and handle. Nd Fe B magnets have the highest energy product but can only be operated at temperatures below about 150 C, which is not sufficient for some high performance motors.
