Page 83 - Instrumentation and Measurement

FIGURE 5.25 Basic elements of a d.c. motor.
For a d.c. motor with the field provided by a permanent magnet, the speed of rotation can be changed
by changing the size of the current to the armature coil, the direction of rotation of the motor being
changed by reversing the current in the armature coil. Figure 5.26 shows how, for a permanent
magnet motor, the torque developed varies with the rotational speed for different applied voltages.
The starting torque is proportional to the applied voltage and the developed torque decreases with
increasing speed.

FIGURE 5.26 Permanent magnet motor characteristic.
D.C. motors with field coils are classified as series, shunt, compound and separately excited
according to how the field windings and armature windings are connected.
1. Series-wound motor
With the series-wound motor the armature and field coils are in series (Figure 5.27A). Such a motor
exerts the highest starting torque and has the greatest no-load speed. However, with light loads there
is a danger that a series-wound motor might run at too high a speed. Reversing the polarity of the
supply to the coils has no effect on the direction of rotation of the motor, since both the current in the
armature and the field coils are reversed.

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