Page 82 - Instrumentation and Measurement

FIGURE 5.24 Shut down if air pressure fails.

5.5 Motors

Electric motors are frequently used as the final control element in position or speed control systems.
The basic principle on which motors are based is that a force is exerted on a conductor in a magnetic
field when a current pass through it. For a conductor of length L carrying a current I in a magnetic
field of flux density B at right angles to the conductor, the force F equals BIL.

There are many different types of motor. In the following, discussion is restricted to those types of
motor that are commonly used in control systems, this including d.c. motors and the stepper motor.
A stepper motor is a form of motor that is used to give a fixed and consistent angular movement by
rotating an object through a specified number of revolutions or fraction of a revolution.

5.5.1 D.C. Motors

In the d.c. motor, coils of wire are mounted in slots on a cylinder of magnetic material called the
armature. The armature is mounted on bearings and is free to rotate. It is mounted in the magnetic
field produced by field poles. This magnetic field might be produced by permanent magnets or an
electromagnet with its magnetism produced by a current passing through the, so-termed, field coils.
Whether permanent magnet or electromagnet, these generally form the outer casing of the motor and
are termed the stator. Figure 5.25 shows the basic elements of d.c. motor with the magnetic field of
the stator being produced by a current through coils of wire. In practice there will be more than one
armature coil and more than one set of stator poles. The ends of the armature coil are connected to
adjacent segments of a segmented ring called the commutator which rotates with the armature.
Brushes in fixed positions make contact with the rotating commutator contacts. They carry direct
current to the armature coil. As the armature rotates, the commutator reverses the current in each coil
as it moves between the field poles. This is necessary if the forces acting on the coil are to remain
acting in the same direction and so continue the rotation.

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