358 Electrical Power Systems Technology
Good speed regulation (low % SR) results when a motor has nearly
constant speeds under varying load situations.
Types of DC Motors
The types of commercially available DC motors basically fall into
four categories: (1) permanent-magnet DC motors, (2) series-wound DC
motors, (3) shunt-wound DC motors, and (4) compound-wound DC mo-
tors. Each of these motors has different characteristics that are due to its
basic circuit arrangement and physical properties.
Permanent-Magnet DC Motors—The permanent-magnet DC motor,
shown in Figure 14-4, is constructed in the same manner as its DC genera-
tor counterpart, which was discussed in Chapter 7. The permanent-mag-
net motor is used for low-torque applications. When this type of motor is
used, the DC power supply is connected directly to the armature conduc-
tors through the brush/commutator assembly. The magnetic field is pro-
duced by permanent magnets mounted on the stator.
Figure 14-4. Permanent-magnet DC motorThis type of motor ordinarily uses either alnico or ceramic perma-
nent magnets, rather than field coils. The alnico magnets are used with
high-horsepower applications. Ceramic magnets are ordinarily used for
low-horsepower, slow-speed motors. Ceramic magnets are highly resis-
tant to demagnetization, yet they are relatively low in magnetic-flux level.
The magnets are usually mounted in the motor frame, and then magne-
tized prior to the insertion of the armature.
The permanent-magnet motor has several advantages over conven-
tional types of DC motors. One advantage is a reduced operational cost.
The speed characteristics of the permanent-magnet motor are similar to
those of the shunt-wound DC motor. The direction of rotation of a perma-