Armature Reactance
IR drop lag the terminal voltage by angle
In this
example, the armature IZ drop is more nearly in phase
The armature current of an ac generator varies
with the terminal voltage and the generated voltage.
approximately as a sine wave. The continuously
Hence, the terminal voltage is approximately equal to
varying current in the generator armature is
the generated voltage, less the armature IZ drop.
Because the IZ drop is much greater than the IR drop,
accompanied by an IXL voltage drop in addition to the
IR drop. Armature reactance in an ac generator may be
the terminal voltage is reduced that much more. The
from 30 to 50 times the value of armature resistance
volt age vectors for a leading power-factor load are
because of the relatively large inductance of the coils
shown in figure 3-30, view C. The load current and IR
compared with their resistance.
drop lead the terminal voltage by angle
This
condition results in an increase in terminal voltage
A simplified series equivalent circuit of one phase
above the value of EG. The total available voltage of
of an ac generator is shown in figure 3-30. The voltage
the ac generator phase is the combined effect of EC
generated in the phase winding is equal to the vector sum
(rotational y induced) and the self-induced voltage (not
of the terminal voltage for the phase and the internal
shown in the vectors). The self-induced voltage, as in
voltage loss in the armature resistance, R, and the
any ac circuit, is caused by the varying field
armature reactance, XL, associated with that phase. The
(accompanying the varying armature current) linking
voltage vectors for a unity power-factor load are shown
the armature conductors. The self-induced voltage
in figure 3-30, view A. The armature IR drop is in phase
always lags the current by 90; hence, when I leads ET,
with the current, I, and the terminal voltage, ET.
the self-induced voltage aids EG, and ET increases.
Because the armature IXL drop is 90 out of phase with
the current, the terminal voltage is approximate] y equal
Armature Reaction
to the generated voltage, less the IR drop in the armature.
The voltage vectors for a lagging power-factor load
When an ac generator supplies no load, the dc field
are shown in figure 3-30, view B. The load current and
flux is distributed uniformly across the air gap. When
Figure 3-30.--The ac generator voltage characteristics.
3-23
