Self-Excitation of Generators

a kind of excitation of generators where the magnetic field of the main poles is excited by a current supplied to the windings of the main poles from the armature (rotor) winding. A self-excited generator is thus distinguished from a separately excited generator, in which the windings of the main poles receive current from an external source.

Self-excitation is made use of most often in DC generators. When a self-excited generator is started, the initial current in the field winding is produced by the electromotive force (emf) induced in the armature winding by the residual magnetic field of the main poles. To sustain self-excitation, the initial current must reinforce this field. The additional magnetic flux increases the emf in the armature and, consequently, the current in the windings of the main poles. Because, however, of magnetic saturation in the magnetic circuit, the increments in magnetic flux corresponding to equal increments in current become smaller as the current builds up. The process of self-excitation continues as long as the emf in the armature exceeds the voltage drop in the field winding. At a certain magnitude of the magnetic flux, electric equilibrium is reached, and there is no further increase in magnetic flux, armature emf, and excitation current. Self-excitation can be achieved when the value of the resistance of the field winding does not exceed a certain limit, which depends on the electric parameters of the generator.

Shunt-wound, series-wound, and compound-wound self-excited generators are used. In shunt-wound machines, the armature winding and field winding are connected in parallel. In series-wound machines, the armature winding and field winding are connected in series. Compound machines have both a series field winding and a shunt field winding. If a self-excited generator for some reason loses the residual magnetization of its main poles, a residual magnetic flux can be created by letting a current from an external source flow through the field winding in the required direction.

AC self-excited generators are discussed in the article ELECTRICAL MACHINE EXCITER.