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commutator continues to rotate, the direction of the rotor field will
◦
◦
again change from −30 to +30 , and it will switch again when the
brushes switch to the next pair of segments. In this machine, then,
◦
the torque angle γ is not always 90 , but can vary by as much as
◦
±30 ; the actual torque produced by the machine would fluctuate
by as much as ±14 percent, since the torque is proportional to sin
γ.
As the number of segments increases, the torque fluctuation
produced by the commutation is greatly reduced. In a practical
machine, for example, one might have as many as 60 segments,
◦
◦
and the variation of γ from 90 would be only ±3 , with a torque
fluctuation of less than 1 percent. Thus, the DC machine can
produce a nearly constant torque (as a motor) or voltage (as a
generator).

Configuration of DC Machines
In DC machines, the field excitation that provides the
magnetizing current is occasionally provided by an external
source, in which case the machine is said to be separately excited
[Figure 3.5 (a)]. More often, the field excitation is derived from
the armature voltage, and the machine is said to be self-excited.
The latter configuration does not require the use of a separate
source for the field excitation and is therefore frequently preferred.
If a machine is in the separately excited configuration, an
additional source Vf is required. In the self-excited case, one
method used to provide the field excitation is to connect the field
in parallel with the armature; since the field winding typically has
significantly higher resistance than the armature circuit (remember
that it is the armature that carries the load current), this will not
draw excessive current from the armature.
Further, a series resistor can be added to the field circuit to
provide the means for adjusting the field current independent of
the armature voltage. This configuration is called a shunt-
connected machine and is depicted in Figure 3.5 (b). Another
method for self-exciting a DC machine consists of connecting the
field in series with the armature, leading to the series-connected
machine, depicted in Figure 3.5 (c); in this case, the field winding
will support the entire armature current, and thus the field coil
must have low resistance (and therefore relatively few turns).

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