0195136047.pdf

13.4 DIRECT-CURRENT MACHINES 599

IL = Ia

Vt = Ea − Ia(Ra + Rs)

+

−

Vt

Ra

Ea

− Series field Rs

+

(a)

Diverter Ia(Ra + Rs)

A

B

C

Voltage

Vt

Ea

Load current

Residual

voltage

(b)

Figure 13.4.6Dc series generator.(a)Schematic diagram of connections.(b)Volt–ampere characteristic at
constant speed.

The schematic diagram and the volt–ampere characteristic of a dc series generator at constant

speed are shown in Figure 13.4.6. The resistance of the series-field winding must be low for

efficiency as well as for low voltage drop. The series generator was used in early constant-current

systems by operating in the rangeB–C,where the terminal voltage fell off very rapidly with

increasing current.

The volt–ampere characteristics of dc compound generators at constant speeds are shown in

Figure 13.4.7.Cumulatively compoundgenerators, in which the series- and shunt-field winding

mmfs are aiding, may beovercompounded, flat-compounded,orundercompounded,depending

on the strength of the series field. Overcompounding can be used to counteract the effect of a

decrease in the prime-mover speed with increasing load, or to compensate for the line drop when

the load is at a considerable distance from the generator.Differentially compoundedgenerators, in

which the series-winding mmf opposes that of the shunt-field winding, are used in applications in

which wide variations in load voltage can be tolerated, and when the generator might be exposed

to load conditions approaching short circuit.

Overcompounded

Flat compounded

Undercompounded

Differentially

compounded

1.0 (rated value)

(rated value)

1.0

Per-unit

terminal voltage

Per-unit armature current

Figure 13.4.7Volt–ampere characteristics of dc compound generators

at constant speed.