Page 566 - The Mechatronics Handbook

0066_Frame_C20 Page 36 Wednesday, January 9, 2002 5:49 PM

Similarly Eq. (20.19) provides a proportionality relationship between the torque, the armature current,
and the magnetic flux, i.e.,

T ∝ I a Φ (20.25)

Equation (20.24) shows that the speed of the motor is directly proportional to the applied voltage and
inversely proportional to the magnetic flux. All methods of speed control for dc motors are based on
this proportionality relationship.
Equation (20.25) indicates that the torque of a given dc motor is directly proportional to the product
of the armature current and the flux per pole. It is obvious therefore that speed control methods which
are based on altering the magnetic flux will also have an effect on the output torque.

Methods of Connection
The Shunt-Wound Motor
The shunt-wound motor (Fig. 20.58) is wired such that the armature and field coils are connected in
parallel with the supply.
Under normal operating conditions, the field current will be constant. As the armature current
increases, the armature reaction effect will weaken the field and the speed will tend to increase. However,
the induced voltage will decrease due to the increasing armature voltage drop and this will tend to decrease
the speed. The two effects are not self cancelling and overall the motor speed will fall slightly as the
armature current increases.
The motor torque increases approximately linearly with the armature current until the armature
reaction starts to weaken the field. These general characteristics are shown in Fig. 20.59 where it can also
be seen that no torque is developed until the armature current is large enough to overcome the constant
losses in the machine. Figure 20.60 shows the derived torque-speed characteristic.
Since the torque increases dramatically for a slight decrease in speed, the shunt-wound motor is partic-
ularly suitable for driving equipment like pumps, compressors, and machine tool elements where the
speed must remain “constant” over a wide range of load conditions.
The Series-Wound Motor
The series-wound motor is shown in Fig. 20.61. As the load current increases, the induced voltage, E, will
decrease due to the armature and field resistance drops. Because the field winding is connected in series
with the armature, the flux is directly proportional to the armature current. Equation (20.24) therefore

I a

I f
R a

Supply
V
E M

FIGURE 20.58 The shunt-wound motor.

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