0195136047.pdf

598 ROTATING MACHINES

Ia = IL

Vt

IaRa

IaRa

Ea

Ra

Ea

If

+−Vf

(a)

Field current If

Drop due to Open circuit

armature reaction

Load

Terminal voltage

Vt

Terminal voltage

Vt

Vt at rated full load

External characteristic

Drop due to armature reaction

Rated load

Load current IL

−

+

−

+

(b)

(c)

Figure 13.4.5Open-circuit and

load characteristics of a separately

excited dc generator.(a)Schematic

diagram of connections.(b)Termi-

nal voltage versus field current rela-

tionships.(c)External volt-ampere

characteristic.

Because the separately excited generator requires a separate dc field supply, its use is limited to

applications in which a wide range of controlled voltage is essential.

A shunt generator maintains approximately constant voltage on load. It finds wide application

as an exciter for the field circuit of large ac generators. The shunt generator is also sometimes

used as a tachogenerator when a signal proportional to the motor speed is required for control or

display purposes.