5.5. Compound Wound Generator
In case of a series generator the voltage regulation is very poor but the ability of the series field to produce additional useful magnetisation in response to increased load cannot be denied. This useful characteristic of the series field, combined with the relatively constant voltage characteristic of the shunt generator, led to the compound generator. Fig. 42 shows connection diagrams for a compound generator.
Fig. 42. Connection diagrams for a compound generator.
Regardless of the method of connection, the terminal voltage V of the short shunt or long shunt compound generator in same,
V = Eg – (Ise Rse + IaRa).
The generated voltage, Egs if of a compound generator is the result of the combination of m.m.f.s .produced by the series (IseTse) and shunt (IshTsh) ampere-turns due to currents which flow in their field windings. In a compound generator, the shunt field predominates and is much stronger of the two. When the series field m.m.f. aids the shunt field m.m.f., the generator is said to be ‘cumulatively compounded’ [see Fig. 43 (a)]. When the series field m.m.f. opposes the shunt field m.m.f., generator is said to be ‘differentially compounded’ [see Fig. 43 (b)].
Fig. 43. Current directions in series and shunt-field coils of cumulative and differential-compound generators.
Fig. 44. External load voltage characteristics of cumulative and differential compound generators.
These types are called:
Over compound generator. An over-compound generator is one whose terminal voltage rises with the application: of load so that its full-load voltage exceeds its no-load voltage (negative regulation).
Flat-compound generator. A flat compound generator has a load-voltage characteristic in which the no-load and full-load voltages are equal (zero per cent regulations.
Under-compound generator. An under-compound generator has a load characteristic in which the full load voltage is somewhat less than no-load voltage, but whose aiding series-field ampere-turns cause its characteristic to have better regulation than an equivalent shunt generator.
- Most commercial compound D.C. dynamos, whether used as generators or motors, are normally supplied by the manufacturer as over-compound machines. The degree of compounding (over, flat or under) may be adjusted by means of divertor which shunts the series field.
Characteristic of Differential Compound Generator
- The differential compound generator is defined as that compounding produced when the series field m.m.f. opposes the shunt field m.m.f The difference in current direction of the two windings is shown in Fig. 43 (b), where for the sake of clarity, the series field winding is shown above (rather than directly around) the shunt field winding.
Fig. 45 shows the load characteristic of differential compound generator.
When the differential compound generator is without load it builds up and self-excites its shunt field in much the same manner as the shunt generator. However, when a load is applied, the generated voltage Eg is now reduced by the reduction in the main field flux created by the opposing m.m.f. of the series field. This reduction in Eg occurs in addition to the armature and series circuit voltage drop, the armature
reaction, and the reduction in field current produced by reduction of the armature voltage. The result is a sharp drop in the terminal voltage with load as shown in Fig. 45, and the field is below saturation and rapidly unbuilds.
- The differential compound generator is used as a constant-current generator for the same constant-current applications as the series generator.