170 Electrical Power Systems Technology
is an alternating current. However, it is possible to convert the alternat-
ing current that is induced into the armature into a form of direct current.
This conversion of AC into DC is accomplished through the use of a split-
ring commutator. The conductors of the armature of a DC generator are
connected to split-ring commutator segments. The split-ring commutator
shown in Figure 7-12 has two segments, which are insulated from one an-
other and from the shaft of the machine on which it rotates. An end of each
armature conductor is connected to each commutator segment. The pur-
pose of the split-ring commutator is to reverse the armature-coil connection
to the external load circuit at the same time that the current induced in the
armature coil reverses. This causes DC of the correct polarity to be applied
to the load at all times.
Voltage Output of DC Generators
The voltage developed by the single-coil generator shown in Figure
7-9 would appear as illustrated in Figure 7-10. This pulsating DC is not
suitable for most applications. However, using many turns of wire around
the armature and several split-ring commutator segments causes the volt-
age developed to be a smooth, direct current like that produced by a bat-
tery. This type of output is shown in Figure 7-10b. The voltage developed
by a DC generator depends upon the strength of the magnetic field, the
number of coils in the armature, and the speed of rotation of the armature.
By increasing any of these factors, the voltage output can be increased.
Sample Problem: Voltage Output of a DC Generator
Voltage output of a DC generator can be expressed as:
Z × n × Φ
Vo = ————
60
where:
Vo = voltage developed across the generator brushes in volts,
Z = total number of armature conductors,
n = speed of rotation in r/min, and
Φ = magnetic flux per pole in webers.
Given: a four-pole DC generator rotates at 1200 r/min. The armature
has 36 slots, and each coil has four turns of wire. The magnetic flux per
pole is 0.05 webers.
Find: the voltage output of the generator.