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direct-current generator[də¦rekt ¦kə·rənt ′jen·ə‚rād·ər]
a DC machine that functions as a generator. Its operation is described by the following equations: P = UIAr, where P is the output power, U the terminal voltage, and IAr the armature current; U = E – IArRAr, where E is the armature electromotive force and RAr is the resistance of the armature circuit; and RAr = rAr + rCo + rSe (Figure 1).
A terminal voltage that remains constant under changing load conditions is the principal requirement imposed upon a DC generator. The relationship between the terminal voltage and the load current U = f(I) is called the external characteristic of a DC generator and is determined by the type of field excitation used in the generator. Several methods of excitation are illustrated in Figure 1, and the external characteristics for various excitation methods are shown in Figure 2, a. The voltage of a DC generator decreases with an increasing load because of the voltage drop in the armature circuit and the demagnetizing effect of the armature field owing to saturation of the magnetic circuit. The optimal excitation system is exhibited by the compound generator, which permits the obtaining of the same voltage under no-load and rated-load conditions. An exact compensation for the voltage drop in the armature (rotor) circuit and for the armature field’s demagnetizing effect, which causes a decrease of the main magnetic flux under load conditions, is possible only for one value of the load current. In separately excited generators there is no compensation. The greater voltage decrease in self-excited generators is a result of the decrease in the exciting current with the increase of load. The range over which the exciting current must be regulated so as to keep the voltage constant under changing load is determined by the control characteristic Iex = f(I) of the DC generator (Figure 2,b).
Sparkless current commutation is another important requirement that must be satisfied by a DC generator. Sparking can be reduced if the stator of the machine is equipped with additional commutating poles. High-power DC generators are sometimes built with compensating windings, which are inserted in the slots of the pole shoes and are connected in series with the armature winding. The purpose of the compensating winding is to compensate for the armature field in the zone under the main poles. The winding provides an automatic compensation at all load conditions and a uniform distribution of magnetic flux density under the pole arc. The maximum voltage between adjacent commutator bars is thus lowered, and potential sparking outside of the commutation zone is eliminated.
In the USSR both general-purpose DC generators (series 2-P) and DC generators for special purposes are produced. Examples of special-purpose machines are DC generators for electric welding: series GSO and GD and also series PSU and PSG, which are driven by an induction motor and operated at currents of 125 to 500 amperes and at voltages of 60 to 70 volts. The amplidyne can also be classed as a special-purpose DC generator. DC tachometer generators are used in automatic control systems; they operate with greater accuracy than do AC tachometer generators.
REFERENCESSee references under
L. M. PETROVA
A rotating electric machine which delivers a unidirectional voltage and current. An armature winding mounted on the rotor supplies the electric power output. One or more field windings mounted on the stator establish the magnetic flux in the air gap. A voltage is induced in the armature coils as a result of the relative motion between the coils and the air gap flux. Faraday's law states that the voltage induced is determined by the time rate of change of flux linkages with the winding. Since these induced voltages are alternating, a means of rectification is necessary to deliver direct current at the generator terminals. Rectification is accomplished by a commutator mounted on the rotor shaft. Carbon brushes, insulated from the machine frame and secured in brush holders, transfer the armature current from the rotating commutator to the external circuit. See Commutation, Electric rotating machinery, Generator, Windings in electric machinery
The field windings of dc generators require a direct current to produce a magnetomotive force (mmf) and establish a magnetic flux path across the air gap and through the armature. Generators are classified as series, shunt, compound, or separately excited, according to the manner of supplying the field excitation current.
In the separately excited generator, the field winding is connected to an independent external source. Separately excited generators are among the most common of dc generators, for they permit stable operation over a very wide range of output voltages.
Using the armature as a source of supply for the field current, dc generators are also capable of self-excitation. Residual magnetism in the field poles is necessary for self-excitation. Series, shunt, and compound-wound generators are self-excited, and each produces different voltage characteristics. The armature winding and field winding of a series generator are connected in series. The field winding of a shunt generator is connected in parallel with the armature winding. A compound generator has both a series field winding and a shunt field winding. Both windings are on the main poles with the series winding on the outside. The shunt winding furnishes the major part of the mmf. The series winding produces a variable mmf, dependent upon the load current, and offers a means of compensating for voltage drop.