Page 77 - Mechanical Engineers Reference Book

211 8 Electrical and electronics principles

If

Field Armature
(a) Separately excited (b) Shunt-wound

(c) Series-wound (d) Compound-wound
Figure 2.27 Methods of field connection

the magnetic hysteresis causes the terminal voltage to have a
slightly greater value than that obtained when the field current
Saturation was being increased. When the field current is reduced to
zero, a ‘residual voltage’ remains. On increasing the field
0)
+- - current once more, the curve follows the broken line to merge
!2
- with the original lower curve. These curves are termed the
‘open-circuit characteristics’ of the machine.
m
.- K If the generator is now connected to a variable external load
E and driven at constant speed with a constant field current; I,,
&
+ the terminal voltage variation with armature current is as
shown in Figure 2.29. The decrease in terminal voltage with
increase in load is due mainly to the voltage drop across the
armature resistance, R,. Additionally, the decrease in ter-
minal voltage is attributed to a decrease in flux caused both by
--- the de-magnetizing ampere-turns of the armature and the
magnetic saturation in the armature teeth. These effects are
collectively known as ’armature reaction’. Figure 2.29 is
referred to as the ‘load characteristic’ of the generator.
Field current The separately excited generator has the disadvantage inhe-
rent with a separate source of direct current required for the
Figure 2.28 Open-circuit characteristics of a separately excited field coils. They are, however. used in cases where a wide
generator range in terminal voltage is required.