13.4 DIRECT-CURRENT MACHINES 597are usually employed only in machines designed for heavy overload or rapidly changing loads,
such as steel-mill motors subjected to reverse duty cycles or in motors intended to operate over
wide speed ranges by shunt-field control. The schematic diagram of Figure 13.4.2 shows the
relative positions of various windings, indicating that the commutating and compensating fields
act along the armature axis (i.e., the quadrature axis), and the shunt as well as series fields act
along the axis of the main poles (i.e., the direct axis). It is thus possible to achieve rather complete
control of the air-gap flux around the entire armature periphery, along with smooth sparkless
commutation.
DC Generator Characteristics
Figure 13.1.8 shows schematic diagrams of field-circuit connections for dc machines with-
out including commutating pole or compensating windings. Shunt generators can be either
separately excitedorself-excited,as shown in Figures 13.1.8 (a) and (b), respectively. Com-
pound machines can be connected eitherlong shunt,as in Figure 13.1.8(d), orshort shunt,in
which the shunt-field circuit is connected directly across the armature, without including the
series field.
In general, three characteristics specify thesteady-stateperformance of a dc generator:
- Theopen-circuit characteristic(abbreviated as OCC, and also known asno-load magne-
tization curve), which gives the relationship between generated emf and field current at
constant speed. - Theexternal characteristic, which gives the relationship between terminal voltage and
load current at constant speed. - Theload characteristic,which gives the relationship between terminal voltage and field
current, with constant armature current and speed.
All other characteristics depend on the form of the open-circuit characteristic, the load, and the
method of field connection. Under steady-state conditions, the currents being constant or, at most,
varying slowly, voltage drops due to inductive effects are negligible.
As stated earlier and shown in Figure 13.4.3, the terminal voltageVtof a dc generator is
related to the armature currentIaand the generated emfEaby
Vt=Ea−IaRa (13.4.6)whereRais the total internal armature resistance, including the resistance of interpole and
compensating windings as well as that of the brushes. The value of the generated emfEa,by
Equation (13.4.3), is governed by the direct-axis field flux (which is a function of the field current
and armature reaction) and the angular velocityωmof the rotor.
The open-circuit and load characteristics of a separately excited dc generator, along with
its schematic diagram of connections, are shown in Figures 13.4.5(a) and (b). It can be seen
from the form of the external volt–ampere characteristic, shown in Figure 13.4.5(c), that the
terminal voltage falls slightly as the load current increases.Voltage regulationis defined as the
percentage change in terminal voltage when full load is removed, so that, from Figure 13.4.5(c)
it follows that
Voltage regulation=Ea−Vt
Vt×100% (13.4.7)