Page 79 - Mechanical Engineers Reference Book

2/20 Electrical and electronics principles

T-

Over-compounded
shunt I

Level
Cumulative
under
compounded

Figure 2.33 The shunt-wound motor
1 - Differentially
I compounded Figure 2.34(a) shows that no torque is developed until the
I armature current is large enough to supply the constant losses
in the machine. Since the torque increases significantly for a
slight decrease in speed, the shunt-wound motor is particularly
Full load
suitable for driving equipment such as pumps, compressors
Load current and machine tool elements, where the speed must remain
‘constant’ over a wide range of load.
Figure 2.32 Load characteristic for the compound-wound generator
2.2.9 The series-wound motor
equally well to the d.c. motor. Since the number of poles and The series-wound motor is shown in Figure 2.35. As the load
number of armature conductors are fixed, a proportionality current increases, the induced voltage, E, will decrease due to
relationship can be derived to relate speed as a function of reductions in the armature and field resistance voltages.
induced e.m.f. and flux, i.e. Because the field winding is connected in series with the
N = El4 (2.72) armature the flux is directly proportional to the armature
current. Equation (2.74) therefore suggests that the speed/
or, using equation (2.71), armature current characteristic will take the form of a rectan-
N = (V - IaRa)/@ (2.73) gular hyperbola. Similarly, equation (2.75) indicates that the
torquelarmature current characteristic will be approximately
The value of I,R, is usually less than about 5% of the parabolic. These general characteristics are illustrated in Figure
terminal voltage such that, to a reasonable approximation, 2.36 along with the derived torque-speed characteristic.
The general characteristics indicate that if the load falls to a
N = VI@ (2.74) particularly low value then the speed may become dangerously
high. A series-wound motor should therefore never be used in
Similarly, equation (2.69), which gives the armature torque on situations where the load is likely to be suddenly relaxed.
a d.c. generator, also applies to the d.c. motor. A proportion- The main advantage of the series-wound motor is that it
ality relationship for the d.c. motor torque is therefore provides a large torque at low speeds. These motors are
T = Ia@ (2.75) eminently suitable, therefore, for applications where a large
Equation (2.74) shows that the speed of a d.c. motor is starting torque is required. This includes, for example, lifts,
hoists, cranes and electric trains.
approximately proportional to the voltage applied to the
armature and inversely proportional to the flux. All methods
of controlling the speed of d.c. motors are based on these 2.2.10 The compound-wound motor
proportionality relationships. Equation (2.75) indicates that
the torque of a given d.c. motor is directly proportional to the Compound-wound motors, like compound generators, are
product of the armature current and the flux per pole. produced by including both series and shunt fields. The
resulting characteristics of the compound-wound motor fall
somewhere in between those of the series- and the shunt-
2.2.8 The shunt-wound motor wound machines.
The shunt-wound motor is shown schematically in Figure 2.33.
Under normal operating conditions the field current will be 2.2.11 Starting d.c. motors
constant. As the armature current increases, however, the
armature reaction effect will weaken the field and the speed With the armature stationary, the induced e.m.f. is zero. If,
will tend to increase. The induced voltage will decrease due to while at rest, the full voltage is applied across the armature
the increasing armature voltage drop, and this will tend to winding, the current drawn would be massive. This current
decrease the speed. The two effects are not self-cancelling, would undoubtedly blow the fuses and thereby cut off the
and, overall, the motor speed will fall slightly as the armature supply to the machine. To limit the starting current, a variable
current increases. external resistance is connected in series with the armature.
The motor torque increases approximately linearly with the On start-up the full resistance, is connected in series. As the
armature current until the armature reaction starts to weaken machine builds up speed and increases the back e.m.f.; the
the field. These general characteristics are shown in Figure external resistance can be reduced until the series resistance is
2.34, along with the derived torque-speed characteristic. disconnected at rated speed.