self excited generator procedure?

1 ANSWERS

1. amomipais82
   

   Hi There ,
   FIELD EXCITATION
   
   When a dc voltage is applied to the field windings of a dc generator, current flows through the windings and sets up a steady magnetic field. This is called FIELD EXCITATION.
   
   This excitation voltage can be produced by the generator itself or it can be supplied by an outside source, such as a battery. A generator that supplies its own field excitation is called a SELF-EXCITED GENERATOR. Self-excitation is possible only if the field pole pieces have retained a slight amount of permanent magnetism, called RESIDUAL MAGNETISM. When the generator starts rotating, the weak residual magnetism causes a small voltage to be generated in the armature. This small voltage applied to the field coils causes a small field current. Although small, this field current strengthens the magnetic field and allows the armature to generate a higher voltage. The higher voltage increases the field strength, and so on. This process continues until the output voltage reaches the rated output of the generator.
   
   CLASSIFICATION OF GENERATORS
   
   Self-excited generators are classed according to the type of field connection they use. There are three general types of field connections - SERIES-WOUND, SHUNT-WOUND (parallel), and COMPOUND-WOUND. Compound-wound generators are further classified as cumulative-compound and differential-compound. These last two classifications are not discussed in this chapter.
   
   Series-Wound Generator
   
   In the series-wound generator, shown in figure 1-15,the field windings are connected in series with the armature. Current that flows in the armature flows through the external circuit and through the field windings. The external circuit connected to the generator is called the load circuit.
   
   Figure 1-15. - Series-wound generator.
   
   32NE0409.GIF (2275 bytes)
   
   A series-wound generator uses very low resistance field coils, which consist of a few turns of large diameter wire.
   
   The voltage output increases as the load circuit starts drawing more current. Under low-load current conditions, the current that flows in the load and through the generator is small. Since small current means that a small magnetic field is set up by the field poles, only a small voltage is induced in the armature. If the resistance of the load decreases, the load current increases. Under this condition, more current flows through the field. This increases the magnetic field and increases the output voltage. A series-wound dc generator has the characteristic that the output voltage varies with load current. This is undesirable in most applications. For this reason, this type of generator is rarely used in everyday practice.
   
   The series-wound generator has provided an easy method to introduce you to the subject of self-excited generators.

   Report (0) (0)  |   12 years, 8 month(s) ago