One of the major applications of electrical energy is the production of mechanical energy. The motion desired may be in a straight line (lateral motion) or rotating (as in motors). Either way, the mechanical energy is produced by the attractive or repulsive forces of electromagnetism.
Solenoids are electromagnetic coils that use a movable plunger to translate the electrical energy into straight-line motion. The moving plunger, activated by the magnetism of the coil, can be used to operate valves, set brakes, or position an object. Solenoids are extensively used for control of hydraulic and pneumatic circuits.
16–2 THE ELECTROMAGNETIC RELAY
A relay can be described as a solenoid with switching contacts attached to a movable plunger. In other words, a relay is an electromagnetic switch, often with multiple switching contacts, that may open or close when the relay coil is energized.
For the better part of this century, millions of relays have been used in the following applications:
1. Remote control in locations that may be inaccessible or hazardous to the operator
2. Automated industrial processes, where the relay automatically responds to monitoring devices that can sense environmental changes, such as temperature, light, sound, or position of a machine
3. Controls for very strong currents or high voltages, with a relatively small voltage or current source Notice in Figure 16–1 that the relays have two distinctly separate circuits, namely:
1. The control circuit, which has a weak current flowing through the coil to energize relay M in order to attract armature A and close contact C. This, in turn, completes
2. The power circuit, in which a substantially stronger current is delivered to the con- trolled device.
Consider, for example, the circuit shown in Figure 16–2. This circuit will illustrate the use of a starting relay (sometimes called a starting solenoid ) in your automobile. No- tice that the starter motor is connected to the battery with a heavy-duty cable. (Starter motors often draw more than 200 amperes and thus require heavy cables.)
By contrast, the control circuit is shown with thin lines, representing a relatively small wire. (Only a weak current is going to flow through the coil.) It is important to see that the two circuits are electrically isolated from each other.
Furthermore, since an automobile is made from steel, it is customary to use its body as an electrical conductor, thereby eliminating almost 50% of the wiring. The battery, therefore, has its negative pole attached to the chassis. This is called a negative ground. Likewise, the other components of the circuit are also connected to the ground.
Relays have been used so extensively in industry that electrical blueprints are known as relay ladder diagrams or relay ladder logic. This concept of relay ladder logic plays an important role in the solid-state devices known as programmable controllers, which are rapidly replacing electromechanical devices for the control and operation of industrial machinery.
It is conceivable that electronic devices may some day supplant electromagnetic relays. For the time being, however, the multitude of relays still in use demand that electricians and technicians be familiar with their use and applications.
16–3 MAGNETIC VIBRATORS AND BELL
A relay circuit can be modified to produce vibratory motion, which may be utilized for different applications. The electric bell (or buzzer) demonstrates this principle well.
The flat spring and iron armature shown in Figure 16–3 comprise a movable as- sembly that pivots at the left end of the flat spring. When the bell is not in use, the free (unattached) end of the spring touches the stationary contact. When an external switch (push button) is closed, the bell is connected to a battery. The resulting current path is shown by the arrows in the figure. When the iron horseshoe is magnetized by the cur- rent, it attracts the armature and the spring is pulled away from the stationary contact,
breaking the circuit. When the spring leaves the contact, the current in the circuit stops and the magnet loses its magnetism. Since the magnet can no longer hold the armature, the elastic spring moves the armature and spring away from the magnet until the spring touches the contact again. Then the entire process can be repeated. Removal of the gong converts the bell to a buzzer.
Magnetic vibrators have been used in the past to rapidly switch the current on or off in a circuit to produce specific results. For instance, earlier models of car radios utilized vibrators to chop up the battery’s DC and produce current pulsations suitable for use with transformers. (Transformers normally do not work on DC.) Another example of magnetic vibrators is the example of a spark coil in the old Model-T automobile. Ignition coils, also called induction coils, produce a high voltage when a direct current through them is rapidly switched on or off. Most of these functions of magnetic vibrators are now accomplished more efficiently by the use of electronic devices.
Labels: Direct current fundamentals
