Chapter 1 Electrical Fundamentals Review 9 Copyright Goodheart-Willcox Co., Inc. In a generator, conductors form loops or cir- cuits that are moved through a magnetic fi eld. This creates a source of emf. See Figure 1-9. 1.3.2 Alternators An ac generator, or alternator, consists of sev- eral parts, Figure 1-10. The following is a brief description of the four basic parts: • A coil or armature rotates. Its conducting wire cuts across a magnetic fi eld. The electron fl ow begins in this part. • Stationary poles create the necessary magnetic fi eld. In some generators, the poles are magnetized by a portion of the electrical current generated. • Metal slip rings are always in contact with one of the terminals of the coil or armature. They transfer the current to the brushes. • The brushes transfer current from slip rings to the external circuit. One brush is in contact with each slip ring. Alternator Operation As the armature is turned, it cuts through the magnetic fi eld created by the fi eld magnets. Electrons fl ow through the wires of the arma- ture. The electrons move into one of the slip rings, then to one of the brushes and, fi nally, into the external circuit. See Figure 1-11. Every half-turn, the electron fl ow reverses. Then the electrons fl ow out to the external cir- cuit, through the opposite slip ring and brush. This creates the reversal of electron fl ow that accounts for alternating current. Magnets North pole South pole Magnetic field Wire moving through field Electron flow Goodheart-Willcox Publisher Figure 1-8. Moving a conductor up and down between the poles of a magnet will displace electrons in the conducting wire and cause them to move through the conductor. This is called electromagnetic induction. Field magnet Field magnet Field magnet Lines of magnetic field Rotating armature or wire loop Field magnet N A B N S S Goodheart-Willcox Publisher Figure 1-9. A simple generator of electricity. A rotating coil of wire has current induced in it as it cuts a magnetic field created between the poles of a magnet. A—Straight-on view of the generator. Voltage is low with the loop in this position. The wire is cutting the field at a sloping angle. B—Perspective view showing loop cutting the magnetic field at a right angle. Voltage is highest in this position. Slip rings Armature or wire loop Brushes to circuit Field magnet Field magnet Rotation Shaft S N Goodheart-Willcox Publisher Figure 1-10. Basic elements of an alternator. The wire loop carries induced current. Electrons flow out of one brush, through the circuit, and back in through the other brush.