Chapter 6 Electromechanical Systems 129 Copyright Goodheart-Willcox Co., Inc. As noted earlier, an important feature of dc motors is the ability to control speed. By changing the applied dc voltage, speed can vary from zero to the maxi- mum rpm. Some types of dc motors have more desirable speed characteristics than others in terms of the motor’s ability to maintain a constant speed under varying load conditions. The difference between no-load motor speed (rpm) and the rated full-load motor speed (rpm) is used to determine the percentage of speed regulation for different dc motors. Lower speed regulation values mean better speed regulation capabilities. Thus, a motor that operates at nearly constant speeds under varying load situations provides good speed regulation. There are four basic categories of commercially available dc motors: perma- nent-magnet, series-wound, shunt-wound, and compound-wound. Each type has different characteristics that result from the basic circuit arrangement and physi- cal properties. The permanent-magnet dc motor is used when a low amount of torque is r needed for applications, such as motor-driven timers and printers connected to computer systems, Figure 6-9. The dc power supply is connected directly to the conductors of the rotor through the brush-commutator assembly. The magnetic field is produced by permanent magnets mounted in the stator. fi In a series-wound dc motor, the armature and field r r circuits are connected in a fi series arrangement, Figure 6-10. This is the only type of dc motor that also can be operated using ac power. For this reason, series-wound motors are sometimes called S N + – DC source Permanent-magnet pole (Pictorial) + – DC source (Schematic) PM Goodheart-Willcox Publisher Figure 6-9. A permanent-magnet dc motor is used for applications in which low torque is required. (Pictorial) + – DC source (Schematic) Field Armature Low-resistance field windings – + DC source Goodheart-Willcox Publisher Figure 6-10. A series-wound dc motor produces high torque, but it has poor speed regulation.