16–4 ELECTROMAGNETISM FOR ROTATIONAL MOTION
Electrical rotating machines encompass both motors and generators. All of these ma- chines operate on the principle of electromagnetism.
Motors and generators are covered more completely in Chapters 19 and 21. At this time we merely introduce you to the concepts of generator action and motor effect.
The term generator action refers to the phenomenon that an electrical current can be generated simply by moving a wire through a magnetic field. The wire is moved across the magnetic field so as to cut lines of magnetic flux. This important principle, upon which all electrical generators work, is more fully explained in Chapter 18, where it is illustrated in Figures 18–1 and 18–2.
The term motor effect, or motor action, is used to describe the phenomenon that a current-carrying wire within a magnetic field will move. The reason for this motion is based on the fact that the current flowing through the wire produces its own magnetic field around the wire. This magnetic field will interact with the flux lines of the two poles between which the wire is situated. The direction of motion is entirely predictable and depends on
the direction of the current and the orientation of the magnetic field. The sketch in Figure 16–4 illustrates this motor effect. More will be said about this in Chapter 20.
16–5 OTHER APPLICATIONS OF THE MOTOR EFFECT
The term motor effect does not apply just to motors. Many other magnetic devices operate on the principle that a current-carrying wire will move within a magnetic field; for instance, (1) electrical measuring instruments, (2) loudspeakers, and (3) TV picture tubes.
Many electrical measuring instruments, or meters, depend on the interaction of magnetic fields to move a tiny coil of wire, thereby causing a deflection of the meter movement. The amount of deflection depends on the strength of the magnetic field produced by the flowing current. Chapter 17 is entirely devoted to this principle, and you are encouraged to look ahead at the introductory illustrations there.
Loudspeakers also operate on the motor effect by causing a tiny coil (known as the voice coil on a speaker) to vibrate within a magnetic field.
Mechanical vibration produces sound. Vibrations in the range from 20 to 18,000 vibrations per second can be heard by the human ear. As the frequency of vibration increases, the pitch increases, and as the amount of back-and-forth movement of the mechanical vibration increases, the sound produced by the vibration increases in loudness.
Figure 16–5 shows a coil of wire wound on a paper sleeve that is suspended so that it can move freely near the pole of a permanent magnet. If an alternating current is applied to the coil of wire, the coil is alternately attracted to the permanent magnet, as shown in
Figure 16–5A, and repelled by it, as shown in Figure 16–5B. The coil vibrates (moves back and forth) at the same frequency as the frequency of the electron vibration of the alternating current.
Many radio loudspeakers are constructed as in Figure 16–6. To obtain a uniform magnetic field in which the moving coil can vibrate, one pole of the magnet is located
just inside the moving coil. The second pole is constructed so that it surrounds the moving coil. The moving coil is attached to a cone made of composition paper. The vibration of the cone produces sound when an alternating current from an amplifier is applied to the movable voice coil.
Early telephone receivers, as illustrated in Figure 16–7, used a stationary coil consisting of many turns of fine wire wrapped around the poles of a permanent horseshoe mag- net. A receiver of this type operates on a much smaller current than is required by a loudspeaker; therefore, the coil must have a large number of wire turns. The alternating current in the coils strengthens and weakens the pull of the magnet. These variations in the strength of the magnet cause the flexible iron disk (diaphragm) to vibrate.
Television picture tubes also operate on the motor effect when they develop the picture on the screen. You may recall, from our discussion in Chapter 13, that the electron beam tra- versing a cathode-ray tube can be deflected by the field of an electromagnet. (For review, see Figure 13–12.) It really makes no difference whether the electrons travel through a metal conductor or move as part of a cathode ray through a gas or vacuum; the effect is the same. In either case, the deflection is caused by the interacting magnetic fields.
The picture on the fluorescent coating on the face of a TV picture tube is caused by an electron beam that sweeps the screen horizontally at 15,750 times per second and vertically at 60 times per second. This scanning motion of the beam is accomplished by two sets of electromagnetic coils wound on a core of magnetic material placed around the neck of the tube. Deflection of the beam occurs because electrons moving through a magnetic field experience a force at right angles both to their direction of motion and to the direction of the magnetic lines of force.
Figure 16–8 shows the vertical deflection coils. The current in this pair of coils controls the vertical position of the electron beam. A similar pair of coils, one above and one below the neck of the tube, controls the horizontal movement of the electron stream. A coil encircling the neck of the tube focuses the electron beam, again using a magnetic field to control electron movement.
16–6 ELECTROMAGNETISM AT WORK
We have surveyed the use of magnetism from the perspective of mechanical motion, either lateral or rotational. But there are numerous other applications where the only motion involved is that of a changing magnetic field, as encountered with alternating currents. Your future studies of electronics will reveal that electromagnetism enters into almost every aspect of electronic communication and industrial processes.
Some inventions of nearly 100 years ago, such as Joseph Henry’s telegraph and Alexander Graham Bell’s telephone, share a common element with the most sophisticated electronic devices of modern times, namely, electromagnetism. From sound and video equipment to computers, and from broadcasting stations to radar installations, electromagnetism has many modern-day uses.
Consider, for example, the magnetic tape recorders we enjoy for home entertainment. Audio and video recorders alike operate on the principle of storing electronic signals by producing variations in the strength of a magnetic field and storing these signals by magnetizing the red oxide particles deposited along the length of the tape.
As stated earlier, sound vibrations can be converted into corresponding electrical signals (by a microphone, for instance), which are then amplified and converted to electromagnetic variations in the recording head. As the tape is fed across the recording head, the needle-shaped oxide particles, which are about 1 micron long (1 micron 5 0.000001 inch), are rearranged in conformity with the magnetic variations.
In audio recorders, the tape head is generally stationary and the pattern of magnetization is longitudinal along the length of the tape; see Figure 16–9A. Many video recorders
employ rotating recording heads, producing an oblique recording pattern on a helically guided tape; see Figure 16–9B. Some earlier commercial-type recorders have successfully employed four tape heads positioned 90° apart on a rotating disk. This results in a trans- verse recording pattern on the magnetic tape; see Figure 16–9C.
Thus, the tape remembers; and when it is pulled across the playback head, the stored-up magnetism induces voltage variations in the electromagnetic coil of the playback head. The varying voltage signals so produced contain all the elements of speech or music, which then can be processed to activate the loudspeaker.
This is merely one example to demonstrate the widespread use of electromagnetism in modern electronics. Your future studies in this subject will introduce you to many more such applications.
Our discussion of electromagnetism would not be complete without mentioning one of the first applications of magnetic pull in lifting magnets, which are widely used for the transfer of scrap steel.
The lifting magnet shown in Figure 16–10A is constructed so that the coil is nearly surrounded by iron. One pole of the magnet is formed on the core inside the coil, and the other pole is formed on the shell that surrounds the coil, as shown in Figure 16–10B. This type of circular horseshoe magnet produces a strongly concentrated magnetic field.
SUMMARY
• Solenoids are electromagnets with a movable plunger, designed to change electrical energy into straight-line motion.
• Relays are electromagnetic switches that can be used for remote control, automation, or for control of high voltages and currents.
• Relays have two distinct circuits that are electrically isolated from each other.
• The concept of relay ladder logic carries over into modern applications of solid-state control.
• Electrical, rotating machinery operates on magnetic concepts known as generator action and motor effect.
• The concept of motor effect is applied in the operation of electrical meters, loudspeakers, and TV picture tubes.
• Electromagnetism finds extensive applications in electronics for communication and industrial processes.
Achievement Review
The Electric Bell
1. Finish the drawing that follows question 3 by connecting the parts of the bell to the push button and the battery. Be sure to notice the letters N and S in the draw- ing, indicating magnetic polarity.
2. Draw tiny arrowheads on the wires of the coil in the drawing following question 3 to show the direction of the current, proving the magnetic polarity by the left-hand rule.
3. Sketch with fine, dashed lines the path of the magnetic flux.
4. Write a brief but complete explanation of the theory behind the bell. Explain how it works.
Electrical Door Chimes
1. Finish the drawing below by connecting the solenoids in the chime to the push- buttons and to the step-down transformer.
2. Assuming that the top wire of the voltage supply is positive (as indicated), trace the current through the solenoids by drawing tiny arrows in the drawing below. Using the left-hand rule for coils, determine the north and south poles on the solenoids.
3. Write a brief but complete explanation of the theory behind the door chimes.
Explain how it works.
The Relay
Shown below is a relay with two sets of switching contacts. The abbreviation N.C. stands for normally closed and means that the contacts are in a closed position as long as the coil is de-energized. Similarly, N.O. means normally open and the contact remains open as long as there is no current flowing through the coil.
Finish the drawing by connecting all parts in such a manner that lamp A is burning all the time but turns off when the push button is depressed. Lamp B will turn on at the same time lamp A is extinguished.
