Chapter 6 Electromechanical Systems 127 Copyright Goodheart-Willcox Co., Inc. fl owing through the conductors. As either the magnetic field fl or current increases, fi the amount of torque also increases. All motors have several parts in common. The stator is the stationary por- r tion of a motor and includes the permanent magnets, a frame, and other station- ary components. The rotor is the rotating component of a motor and includes the r armature, shaft, and associated parts. Movement of the rotor creates torque. Aux- iliary devices for dc motors and ac motors are discussed in the following sections. 6.4.1 DC Motors DC motors are used when speed control is a critical factor. The basic parts of a dc motor are shown in Figure 6-7. A 7 7 commutator is a part or assembly used to switch r the direction of current fl ow alternately to produce a one-directional direct current fl (dc) through the windings (coils of wire) of the armature. The brushes are carbon devices that rub against the commutator in an electric motor and allow current to fl ow through them. The flow fl of current through the brushes to the commuta- fl tor and armature windings transfers current from the stationary power source to energize the rotating armature conductors. Most dc motors use electromagnetic windings rather than permanent mag- nets to create the magnetic field of the stator. The coil wires wrapped around the fi electromagnets in the stator are called fi eld windings. fi The interaction of the sta- tionary electromagnetic fi eld of the stator coils and the electromagnetic field fi of the fi rotating armature produces rotation of the dc motor. The operational characteristics of dc motors are shown in Figure 6-8. As the armature of a dc motor rotates, it generates its own voltage called the counter electromotive force (cemf). This voltage fl ows against the voltage coming fl into the motor. The amount of cemf depends on the number of rotating conductors and the speed of rotation. As the mechanical load (amount of work effort demand S N + – DC source Rotor (armature) Stator Field pole/ permanent magnet Brush-commutator assembly Mounting Field pole/ permanent magnet Goodheart-Willcox Publisher Figure 6-7. The basic parts of a dc motor include a brush-commutator assembly for switching the current fl ow. fl