DAVOG ELECTRICAL COMPANY

Broadcasting, Radio and Television, primary means by which information and entertainment are delivered to the public in virtually every nation around the world. The term broadcasting refers to the airborne transmission of electromagnetic audio signals (radio) or audiovisual signals (television) that are readily accessible to a wide population via standard receivers.

Broadcasting is a crucial instrument of modern social and political organization. At its peak of influence in the mid-20th century, national leaders often used radio and television broadcasting to address entire countries. Because of its capacity to reach large numbers of people, broadcasting has been regulated since it was recognized as a significant means of communication. (For more information, see the section "The Regulation of Broadcasting.")

Beginning in the early 1980s, new technologies–such as cable television and videocassette players–began eroding the dominance of broadcasting in mass communications, splitting its audiences into smaller, culturally distinct segments. Previously a synonym for radio and television, broadcasting has become one of several delivery systems that feed content to newer media.

Basic AC Motor Operation

An AC motor has two basic electrical parts: a "stator" and a "rotor" as shown in Figure 8. The stator is in the stationary electrical component. It consists of a group of individual electro-magnets arranged in such a way that they form a hollow cylinder, with one pole of each magnet facing toward the center of the group. The term, "stator" is derived from the word stationary. The stator then is the stationary part of the motor. The rotor is the rotating electrical component. It also consists of a group of electro-magnets arranged around a cylinder, with the poles facing toward the stator poles. The rotor, obviously, is located inside the stator and is mounted on the motor's shaft. The term "rotor" is derived from the word rotating. The rotor then is the rotating part of the motor. The objective of these motor components is to make the rotor rotate which in turn will rotate the motor shaft. This rotation will occur because of the previously discussed magnetic phenomenon that unlike magnetic poles attract each other and like poles repel. If we progressively change the polarity of the stator poles in such a way that their combined magnetic field rotates, then the rotor will follow and rotate with the magnetic field of the stator.

In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then turned by external means producing a rotating magnetic field, which induces a 3-phase voltage within the stator winding.

•Field windings are the windings producing the main magnetic field (rotor windings

•armature windings are the windings where the main voltage is induced (stator windings)

Construction of synchronous machines

The rotor of a synchronous machine is a large electromagnet. The magnetic poles can be either salient (sticking out of rotor surface) or non-salient construction.

Non-salient-pole rotor: usually two- and four-pole rotors.

Salient-pole rotor: four and more poles.

Rotors are made laminated to reduce eddy current losses

Construction of synchronous machines

Two common approaches are used to supply a DC current to the field circuits on the rotating rotor:

1.Supply the DC power from an external DC source to the rotor by means of slip rings and brushes;

2.Supply the DC power from a special DC power source mounted directly on the shaft of the machine.

Slip rings are metal rings completely encircling the shaft of a machine but insulated from it. Graphite-like carbon brushes connected to DC terminals ride on each slip ring supplying DC voltage to field windings