Environmental Engineering Reference
In-Depth Information
speeds, brushes increase difficulty in maintaining contact, creating sparks, because
of some irregularity in the collector surface. This sparking limits the maximum
speed of the machine, as too-rapid sparking will overheat, erode, or even melt the
collector. The current density per unit area of the brushes, in combination with the
resistivity of the material, limits the performance of the motor also causing
electrical noise. Brushes eventually wear out and require replacement, and the
collector itself is subject to wear and maintenance or replacement.
Other types of DC motors are classified according to their specific stator
excitation, based on four standard methods of supplying current to the excitation
windings for brushed DC motor: parallel DC machines, series DC machines,
compound DC machines and separately excited DC machines (Fig.
5.3
a-d) [
6
-
8
].
The parallel DC motor has the ability to self-regulate its speed, but it is particularly
difficult to control, because in some circumstance, a reduction in supply voltage
have very little effect on the speed. The main advantage of the series DC motor is
that the torque is very high at low speed and falls off while the speed is increasing.
This characteristic is useful in some applications, for example, the starter device of
internal combustion engines, but it is not what a road vehicle generally requires.
The compound DC motor is designed to combine the advantaged of the motors
from which is derived.
In separately excited DC machines (Fig.
5.3
d) the excitation is obtained by
means of field windings which have a supply separated from the rotor windings,
and this gives a separated control of speed and torque, according to what is
required in an electric vehicle. Separately excited brushed DC motors were widely
+
+
DC
MOTOR
DC
MOTOR
E
s
E
s
-
-
(a)
(b)
+
+
DC
MOTOR
DC
MOTOR
E
s
E
s
E
f
-
-
(c)
(d)
Fig. 5.3 Different methods of supplying current to the excitation windings for brushed DC
motor. a parallel DC machines, b series DC machines, c compound DC machines, d separately
excited DC machines
